Home » Public Forums » GMCnet » 3000 RPMs at 70- a little much, 3.7 final drive ?
| 3000 RPMs at 70- a little much, 3.7 final drive ? [message #360333] |
Sat, 28 November 2020 14:25  |
hertfordnc
Messages: 1164
Registered: September 2009
Location: East NC
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The final drive had been upgraded but he PO was not sure to what.
FInally got enough things right for a short high speed run- 70MP by GPS app was about 3100 on the tach (and the speedo was reading a little low)
So that comes to 3.7 with 225/75 tires
That seems a bit excessive.
I'm thinking for a 23 footer not towing anything heavy 3.21 or 3.55 might be a better option.
A lot of my travel involves I95 in the South, lots of flat cruising.
What's the point of having a 70's muscle car if you can't enjoy 70's freeway cruising?
Dave & Ellen Silva
Hertford, NC
76 Birchaven, 1-ton and other stuff
Currently planning the Great american Road Trip Summer 2021
It's gonna take a lot of Adderall to get this thing right.
[Updated on: Sat, 28 November 2020 14:26] Report message to a moderator |
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| Re: 3000 RPMs at 70- a little much, 3.7 final drive ? [message #360334 is a reply to message #360333] |
Sat, 28 November 2020 14:31  |
Scott Nutter
Messages: 782
Registered: January 2015
Location: Houston/San Diego
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I really like my 3:21 final drive. 2700 rpm’s @70 mph.
Scott.
Scott Nutter
1978 Royale Center Kitchen, Patterson 455, switch pitch tranny, 3.21 final drive, Quad bags, Dave Lenzi super duty mid axle disc brakes, tankless water heater, everything Lenzi. Alex Ferrera installed MSD Atomic EFI
Houston, Texas
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| Re: 3000 RPMs at 70- a little much, 3.7 final drive ? [message #360335 is a reply to message #360333] |
Sat, 28 November 2020 14:48 |
lqqkatjon
Messages: 2324
Registered: October 2010
Location: St. Cloud, MN
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I would only want 3.7 with a 455 if mountains. Or towing. I think it makes all the difference in easing the burden in those cases.
I like my 3.21 in my 26 foot coach on the non towing flat ground. It is perfect.
In a 23’ coach. I would guess even better.
It is amazing the load on a coach going up 6% grades or dragging 3500 pounds. Lower gearing is important there.
Jon Roche
75 palm beach
EBL EFI, manny headers, Micro Level, rebuilt most of coach now.
St. Cloud, MN
http://lqqkatjon.blogspot.com/
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| Re: [GMCnet] 3000 RPMs at 70- a little much, 3.7 final drive ? [message #360336 is a reply to message #360333] |
Sat, 28 November 2020 14:33 |
Randy Hecht
Messages: 93
Registered: March 2019
Location: Roswell, GA
Karma: -5
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I would say most 1970s muscle cars didn’t weight close to 10,000 pounds either.
Randy Hecht
Roswell, GA
1974 Canyon Lakes GMC Motor-coach
> On Nov 28, 2020, at 15:26, dave silva via Gmclist wrote:
>
> The final drive had been upgraded but he PO was not sure to what.
>
> FInally got enough things right for a short high speed run- 70MPH by GPS app was about 3100 on the tach (and the speedo was reading a little low)
>
> So that comes to 3.7 with 225/75 tires
>
> That seems a bit excessive.
>
> I'm thinking for a 23 footer not towing anything heavy 3.21 or 3.55 might be a better option. A lot of my travel involves I95 in the South, lots of
> flat cruising.
>
> WHat's the point of having a 70's miscle car if you can't enjoy 70's freeway cruising.
> --
> Dave & Ellen Silva
>
> 1972 Revcon Olds 455, toro drive train. All Stock
>
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
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| Re: 3000 RPMs at 70- a little much, 3.7 final drive ? [message #360342 is a reply to message #360333] |
Sat, 28 November 2020 17:23 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
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My 23' with the stock gears pulled Monteagle and US 23 into Sylva NC with the toad behind without any sweat. Once I got a clean radiator and good fan clutch in it. Those are the steepest hills I see in normal use, most of the rest is rolling hills and flatland in the South. The 26 I have now has a 3.7 and to me it's overkill. It came from northern CA, and the PO pulled a closed trailer with a pair of Harleys in it, I suppose it was worthwhile there.
--johnny
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
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| Re: 3000 RPMs at 70- a little much, 3.7 final drive ? [message #360355 is a reply to message #360333] |
Sun, 29 November 2020 09:46 |
Tilerpep
Messages: 404
Registered: June 2013
Location: Raleigh, NC
Karma: 7
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I had my first big run with the new engine this week, and I was watching the gauges a lot...I was seeing 3000 rpm at 70, and by my PO notes, I think I have a 3.42 from Cinnabar. There use to be a chart of gears to speed, anyone know where that is?
I don't understand your comment about having a muscle car and not being able to use it at cruise - are you trying to avoid higher revs for noise, wear, or ?
My old setup got 7 mpg on a good day - this trip appears a solid 10 mpg with same tranny, FD, new gaskets carb, different vac advance can and one lighter spring in the mechanical advance on the dizzy. I include it here because final drive didn't change, but it sure ran happy.
1975 Glenbrook, 1978 Royale rear bath
Raleigh, NC
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| Re: [GMCnet] 3000 RPMs at 70- a little much, 3.7 final drive ? [message #360358 is a reply to message #360355] |
Sun, 29 November 2020 10:17 |
Rob
Messages: 651
Registered: November 2013
Location: Victoria, BC
Karma: 3
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I think the chart you’re looking for is here:
https://cdn3.volusion.com/mjjms.kmgtp/v/vspfiles/photos/GM1-1.001.1-3T.jpg?v-cache=1567085816
The second pic from this page (if that link is temporary in nature):
https://www.gmcrvparts.com/product-p/gm1-1.001.1.htm
Rob
Victoria, BC
76 Royale - Rear Twins/Dry Bath
> On Nov 29, 2020, at 7:47 AM, Tyler via Gmclist wrote:
>
> I had my first big run with the new engine this week, and I was watching the gauges a lot...I was seeing 3000 rpm at 70, and by my PO notes, I think I
> have a 3.42 from Cinnabar. There use to be a chart of gears to speed, anyone know where that is?
> --
> 1975 Glenbrook, 1978 Royale rear bath
> Raleigh, NC
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Rob -
Victoria, BC -
76 Royale - Rear Twins/Dry Bath
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| Re: 3000 RPMs at 70- a little much, 3.7 final drive ? [message #360364 is a reply to message #360355] |
Sun, 29 November 2020 11:20 |
hertfordnc
Messages: 1164
Registered: September 2009
Location: East NC
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Tilerpep wrote on Sun, 29 November 2020 09:46
I don't understand your comment about having a muscle car and not being able to use it at cruise - are you trying to avoid higher revs for noise, wear, or ?
Yeah, mostly noise. 2500 RPMS feels pretty good and it's a level i can tolerate for many many hours.
Also, i don't see this coach getting heavily loaded down. It's so easy t oget round i'm thinking we might travel without a TOAD.
Dave & Ellen Silva
Hertford, NC
76 Birchaven, 1-ton and other stuff
Currently planning the Great american Road Trip Summer 2021
It's gonna take a lot of Adderall to get this thing right.
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| Re: [GMCnet] 3000 RPMs at 70- a little much, 3.7 final drive ? [message #360372 is a reply to message #360366] |
Sun, 29 November 2020 18:23 |
jimk
Messages: 6734
Registered: July 2006
Location: Belmont, CA
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Yourn not seeing the actual road driving where the speed drops down around
50mph and the endine is out of the torque curve.
Drop the ratio to 3.55 and you'll notice a difference.
It is about 70% aerodynamic drag that the coach is challenged with.
For your info, the 23Foot has more drag than the 26
On Sun, Nov 29, 2020 at 9:28 AM Dave Stragand via Gmclist <
gmclist@list.gmcnet.org> wrote:
> On the subject of final drives, does anyone have a handy guide in
> physically identifying the final drive ratios from external
> stampings/markings from both GM and our vendors, without opening the cover
> and counting teeth? And on the same topic, any way externally to tell if a
> Power Drive or other modified chain/sprocket has been installed?
>
> Thanks!
>
> -Dave
> 1978 Transmode near Pittsburgh
> _______________________________________________
> GMCnet mailing list
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--
Jim Kanomata
Applied/GMC, Newark,CA
jimk@appliedairfilters.com
http://www.gmcrvparts.com
1-800-752-7502
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Jim Kanomata
Applied/GMC
jimk@appliedairfilters.com
www.appliedgmc.com
1-800-752-7502
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| Re: [GMCnet] 3000 RPMs at 70- a little much, 3.7 final drive ? [message #360375 is a reply to message #360372] |
Sun, 29 November 2020 19:35 |
hertfordnc
Messages: 1164
Registered: September 2009
Location: East NC
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jimk wrote on Sun, 29 November 2020 18:23Yourn not seeing the actual road driving where the speed drops down around
50mph and the endine is out of the torque curve.
Drop the ratio to 3.55 and you'll notice a difference.
It is about 70% aerodynamic drag that the coach is challenged with.
For your info, the 23Foot has more drag than the 26
RIght, for most of my travel the speed is around 70. That's what i want to optimise for. Weight and climbing will not be a big part of this coaches life.
Do you think 3.55 is right? THat still seems high.
Dave & Ellen Silva
Hertford, NC
76 Birchaven, 1-ton and other stuff
Currently planning the Great american Road Trip Summer 2021
It's gonna take a lot of Adderall to get this thing right.
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| Re: [GMCnet] 3000 RPMs at 70- a little much, 3.7 final drive ? [message #360376 is a reply to message #360336] |
Sun, 29 November 2020 19:39 |
Mike Kelley
Messages: 467
Registered: February 2017
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Scott N.:
I am w/ you Man - have 3:21’s in both my 26’ and my 28’ and they serve me well. Of course we spend 6 months in Tx. and 6 months in Midwest so very few steep hills and no mountains!
To each his own!!!
Mike/the Corvair a holic
Sent from my iPhone
> On Nov 28, 2020, at 2:33 PM, Randy Hecht via Gmclist wrote:
>
> I would say most 1970s muscle cars didn’t weight close to 10,000 pounds either.
>
> Randy Hecht
> Roswell, GA
>
> 1974 Canyon Lakes GMC Motor-coach
>
>
>> On Nov 28, 2020, at 15:26, dave silva via Gmclist wrote:
>>
>> The final drive had been upgraded but he PO was not sure to what.
>>
>> FInally got enough things right for a short high speed run- 70MPH by GPS app was about 3100 on the tach (and the speedo was reading a little low)
>>
>> So that comes to 3.7 with 225/75 tires
>>
>> That seems a bit excessive.
>>
>> I'm thinking for a 23 footer not towing anything heavy 3.21 or 3.55 might be a better option. A lot of my travel involves I95 in the South, lots of
>> flat cruising.
>>
>> WHat's the point of having a 70's miscle car if you can't enjoy 70's freeway cruising.
>> --
>> Dave & Ellen Silva
>>
>> 1972 Revcon Olds 455, toro drive train. All Stock
>>
>>
>>
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
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| Re: [GMCnet] 3000 RPMs at 70- a little much, 3.7 final drive ? [message #360380 is a reply to message #360372] |
Sun, 29 November 2020 19:52 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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You guys that live in flat country have not a clue to the GMC's
requirements for climbing grades. You can cover 6 states in a day and not
break a sweat.
Try running the Tehachapi grade out of either California or
Arizona/Nevada, or the Grapevine coming North out of Los Angeles on
Interstate 5, or the Siskiyou summit a bit farther North. You will run out
of gears pretty darn quickly. I have a heavy 78 Royale with a 403 and 3:70
final drive, and it works really well. And I run it at 70 mph + most of the
time. 3400 rpm and that 403 is a happy camper. Over 130,000 miles on an
unmolested engine. Works for me.
Jim Hupy
Salem, Oregon
On Sun, Nov 29, 2020, 5:40 PM Mike Kelley via Gmclist <
gmclist@list.gmcnet.org> wrote:
> Scott N.:
> I am w/ you Man - have 3:21’s in both my 26’ and my 28’ and they serve me
> well. Of course we spend 6 months in Tx. and 6 months in Midwest so very
> few steep hills and no mountains!
> To each his own!!!
> Mike/the Corvair a holic
>
> Sent from my iPhone
>
>> On Nov 28, 2020, at 2:33 PM, Randy Hecht via Gmclist gmclist@list.gmcnet.org> wrote:
>>
>> I would say most 1970s muscle cars didn’t weight close to 10,000 pounds
> either.
>>
>> Randy Hecht
>> Roswell, GA
>>
>> 1974 Canyon Lakes GMC Motor-coach
>>
>>
>>> On Nov 28, 2020, at 15:26, dave silva via Gmclist gmclist@list.gmcnet.org> wrote:
>>>
>>> The final drive had been upgraded but he PO was not sure to what.
>>>
>>> FInally got enough things right for a short high speed run- 70MPH by
> GPS app was about 3100 on the tach (and the speedo was reading a little
> low)
>>>
>>> So that comes to 3.7 with 225/75 tires
>>>
>>> That seems a bit excessive.
>>>
>>> I'm thinking for a 23 footer not towing anything heavy 3.21 or 3.55
> might be a better option. A lot of my travel involves I95 in the South,
> lots of
>>> flat cruising.
>>>
>>> WHat's the point of having a 70's miscle car if you can't enjoy 70's
> freeway cruising.
>>> --
>>> Dave & Ellen Silva
>>>
>>> 1972 Revcon Olds 455, toro drive train. All Stock
>>>
>>>
>>>
>>> _______________________________________________
>>> GMCnet mailing list
>>> Unsubscribe or Change List Options:
>>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>>
>>
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
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| Re: [GMCnet] 3000 RPMs at 70- a little much, 3.7 final drive ? [message #360381 is a reply to message #360380] |
Sun, 29 November 2020 20:16 |
hertfordnc
Messages: 1164
Registered: September 2009
Location: East NC
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James Hupy wrote on Sun, 29 November 2020 19:52You guys that live in flat country have not a clue to the GMC's
requirements for climbing grades. You can cover 6 states in a day and not
break a sweat.
Try running the Tehachapi grade out of either California or
Arizona/Nevada, or the Grapevine coming North out of Los Angeles on
Interstate 5, or the Siskiyou summit a bit farther North. You will run out
of gears pretty darn quickly. I have a heavy 78 Royale with a 403 and 3:70
final drive, and it works really well. And I run it at 70 mph + most of the
time. 3400 rpm and that 403 is a happy camper. Over 130,000 miles on an
unmolested engine. Works for me.
Jim Hupy
Salem, Oregon
Yup, not a clue and it is an ignorance i expect to maintain for a couple more years.
Dave & Ellen Silva
Hertford, NC
76 Birchaven, 1-ton and other stuff
Currently planning the Great american Road Trip Summer 2021
It's gonna take a lot of Adderall to get this thing right.
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| Re: [GMCnet] 3000 RPMs at 70- a little much, 3.7 final drive ? [message #360384 is a reply to message #360381] |
Sun, 29 November 2020 20:47 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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Dave, glad to hear that you have met Covid, and emerged intact. Some do
not. Sure hoping those vaccines work as advertised. We have one of our
members in the. Willamette Valley Street Rods contracted it, and spent a
month on a ventilator in intensive care. All told over 6 weeks in the
hospital. Still weak as a kitten. He is a lot younger than I am. Scary
deal. Stay well out there.
Jim Hupy
Salem, Oregon
On Sun, Nov 29, 2020, 6:16 PM dave silva via Gmclist <
gmclist@list.gmcnet.org> wrote:
> James Hupy wrote on Sun, 29 November 2020 19:52
>> You guys that live in flat country have not a clue to the GMC's
>> requirements for climbing grades. You can cover 6 states in a day and not
>> break a sweat.
>> Try running the Tehachapi grade out of either California or
>> Arizona/Nevada, or the Grapevine coming North out of Los Angeles on
>> Interstate 5, or the Siskiyou summit a bit farther North. You will run
> out
>> of gears pretty darn quickly. I have a heavy 78 Royale with a 403 and
> 3:70
>> final drive, and it works really well. And I run it at 70 mph + most of
> the
>> time. 3400 rpm and that 403 is a happy camper. Over 130,000 miles on an
>> unmolested engine. Works for me.
>> Jim Hupy
>> Salem, Oregon
>
> Yup, not a clue and it is an ignorance i expect to maintain for a couple
> more years.
>
> --
> Dave & Ellen Silva
>
> 1972 Revcon Olds 455, toro drive train. All Stock
>
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
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| Re: [GMCnet] 3000 RPMs at 70- a little much, 3.7 final drive ? [message #360387 is a reply to message #360384] |
Sun, 29 November 2020 22:07 |
jimk
Messages: 6734
Registered: July 2006
Location: Belmont, CA
Karma: 9
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Flat places still need the torque to overcome wind drag.
My knowledge comes from people that understand what our coaches
benifits from .
Back in 1990 there were people like Marvin Peck( Gear Master }, Clarence
Buskirk, Alex Sirum ,CO Richards, and others that knew what works for the
coaches.
Some have tested a 2.73 and found there was no economy advantage,
I have traveled at least 42-45 states and 3,70 has shown me to be the best
overall ratio,
All the ford, chev, and dodge use 4,11-4.56 ratio running 16" rim and tires
size same as ours.
I drive from 60-80mph and know when you hit hilly interste speed will fall
in 60.
On Sun, Nov 29, 2020 at 6:48 PM James Hupy via Gmclist <
gmclist@list.gmcnet.org> wrote:
> Dave, glad to hear that you have met Covid, and emerged intact. Some do
> not. Sure hoping those vaccines work as advertised. We have one of our
> members in the. Willamette Valley Street Rods contracted it, and spent a
> month on a ventilator in intensive care. All told over 6 weeks in the
> hospital. Still weak as a kitten. He is a lot younger than I am. Scary
> deal. Stay well out there.
> Jim Hupy
> Salem, Oregon
>
> On Sun, Nov 29, 2020, 6:16 PM dave silva via Gmclist gmclist@list.gmcnet.org> wrote:
>
>> James Hupy wrote on Sun, 29 November 2020 19:52
>>> You guys that live in flat country have not a clue to the GMC's
>>> requirements for climbing grades. You can cover 6 states in a day and
> not
>>> break a sweat.
>>> Try running the Tehachapi grade out of either California or
>>> Arizona/Nevada, or the Grapevine coming North out of Los Angeles on
>>> Interstate 5, or the Siskiyou summit a bit farther North. You will run
>> out
>>> of gears pretty darn quickly. I have a heavy 78 Royale with a 403 and
>> 3:70
>>> final drive, and it works really well. And I run it at 70 mph + most of
>> the
>>> time. 3400 rpm and that 403 is a happy camper. Over 130,000 miles on an
>>> unmolested engine. Works for me.
>>> Jim Hupy
>>> Salem, Oregon
>>
>> Yup, not a clue and it is an ignorance i expect to maintain for a couple
>> more years.
>>
>> --
>> Dave & Ellen Silva
>>
>> 1972 Revcon Olds 455, toro drive train. All Stock
>>
>>
>>
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
--
Jim Kanomata
Applied/GMC, Newark,CA
jimk@appliedairfilters.com
http://www.gmcrvparts.com
1-800-752-7502
_______________________________________________
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Jim Kanomata
Applied/GMC
jimk@appliedairfilters.com
www.appliedgmc.com
1-800-752-7502
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| Re: 3000 RPMs at 70- a little much, 3.7 final drive ? [message #360390 is a reply to message #360333] |
Mon, 30 November 2020 06:26 |
hertfordnc
Messages: 1164
Registered: September 2009
Location: East NC
Karma: 0
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Senior Member |
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So there is consensus among some of you that 3.7 is the optimal all-around gear ratio?
Help me understand.
If it's optimal for a 26 footer pulling a car up and down the West Coast how is not overkill for a 23 footer pulling nothing?
Isn't that like a 6,000 lb difference?
Dave & Ellen Silva
Hertford, NC
76 Birchaven, 1-ton and other stuff
Currently planning the Great american Road Trip Summer 2021
It's gonna take a lot of Adderall to get this thing right.
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| Re: 3000 RPMs at 70- a little much, 3.7 final drive ? [message #360392 is a reply to message #360333] |
Mon, 30 November 2020 07:29 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
Karma: -74
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Dave, I expect your use is like mine. As long as it pulls Monteagle and 23 to Silva from here, that's all we're gonna see. Same grade as the western stuff, but for a mile or so instead of for the afternoon, and not nearly as thin air here when you top out.
JimK, what is the reasoning which says the 23' shows greater wind drag than the 26'?
--johnny
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
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| Re: [GMCnet] 3000 RPMs at 70- a little much, 3.7 final drive ? [message #360394 is a reply to message #360390] |
Mon, 30 November 2020 08:24 |
Ray Erspamer
Messages: 1707
Registered: May 2007
Location: Milwaukee Wisconsin
Karma: -3
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Senior Member |
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I have the 403 engine, 3.70 final drive and run 245 tires. At 70 mph I'm right at 3,000 rpm. The 403 just purrs right along and seems very content there.Sent from my U.S.Cellular© Smartphone
-------- Original message --------From: dave silva via Gmclist Date: 11/30/20 6:27 AM (GMT-06:00) To: gmclist@list.gmcnet.org Cc: dave silva Subject: Re: [GMCnet] 3000 RPMs at 70- a little much, 3.7 final drive ? So there is consensus among some of you that 3.7 is the optimal all-around gear ratio? Help me understand.If it's optimal for a 26 footer pulling a car up and down the West Coast how is not overkill for a 23 footer pulling nothing? Isn't that like a 6,000 lb difference? -- Dave & Ellen Silva1972 Revcon Olds 455, toro drive train. All Stock_______________________________________________GMCnet mailing listUnsubscribe or Change List Options:http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
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Ray Erspamer
78 GMC Royale Center Kitchen
403, 3.70 Final Drive
Holley Sniper Quadrajet EFI System,
Holley Hyperspark Ignition System
414-484-9431
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