| How hard to determine final drive ratio (Revcon) [message #343223] |
Thu, 09 May 2019 11:00  |
hertfordnc
Messages: 1164
Registered: September 2009
Location: East NC
Karma: 0
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The information regarding the final drive ratio on the Revcon is sketchy.
In one place i read that it is stock Toronado. But when i look into what that number might be i get different answers.
Also, mine is a 72 which makes it unclear what generation Tornado they used. It may have been sourced much earlier.
My calculation watching the tach and measuring the tires makes it close to 4.10 but that seems doubtful.
More likely is my tach and/or speedometer are not accurate.
Do i have the math right?
engine RPM at 60 equals 5280/tire circumference X final drive ratio ??
So, how much trouble is it to unbolt something and count the teeth?
Thanks
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] How hard to determine final drive ratio (Revcon) [message #343224 is a reply to message #343223] |
Thu, 09 May 2019 12:46  |
Richard Denney
Messages: 920
Registered: April 2010
Karma: 9
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Depending on the tire size, you’ll have 670-690 revolutions per mile (5280
/ tire circumference in feet).
670 is close the original GMC tire size of 16.5x8.75R78. 690 is closer to
the 225/75R16.
At 60 mph, RPM = tire rotations per mile X final drive ratio, but there
will be some slippage in the fluid coupling and some difficulty precisely
messing tire rolling circumference.
But it is unlikely that an electronic tach, if it reads at all, will read
only slightly off.
I run 70 mph down the highway at 2600 RPMs with a 3.21 final drive. That’s
with 225/75R16’s. That calculates to 694 tire rotations/mile.
What is your tach reading?
But from the factory, GM never had a gear shorter than 3.21 (used in
late-60’s Cadillacs). The GMC MH used a 3.07 from the factory, and I think
the standard Toro gear was 2.78 or similar.
Rick “not quite understanding your math notation” Denney
On Thu, May 9, 2019 at 12:12 PM dave silva via Gmclist <
gmclist@list.gmcnet.org> wrote:
> The information regarding the final drive ratio on the Revcon is sketchy.
>
> In one place i read that it is stock Toronado. But when i look into what
> that number might be i get different answers.
>
> Also, mine is a 72 which makes it unclear what generation Tornado they
> used. It may have been sourced much earlier.
>
> My calculation watching the tach and measuring the tires makes it close to
> 4.10 but that seems doubtful.
>
> More likely is my tach and/or speedometer are not accurate.
>
> Do i have the math right?
>
> engine RPM at 60 equals 5280/tire circumference X final drive ratio ??
>
>
>
>
> So, how much trouble is it to unbolt something and count the teeth?
>
> Thanks
>
>
> --
> 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
>
--
'73 X-Glacier 230 "Jaws"
Northern Virginia
Offlist email: rick at rickdenney dot com
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| Re: How hard to determine final drive ratio (Revcon) [message #343225 is a reply to message #343223] |
Thu, 09 May 2019 12:50 |
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Matt Colie
Messages: 8547
Registered: March 2007
Location: S.E. Michigan
Karma: 7
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Senior Member |
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hertfordnc wrote on Thu, 09 May 2019 12:00The information regarding the final drive ratio on the Revcon is sketchy.
In one place i read that it is stock Toronado. But when i look into what that number might be i get different answers.
Also, mine is a 72 which makes it unclear what generation Tornado they used. It may have been sourced much earlier.
My calculation watching the tach and measuring the tires makes it close to 4.10 but that seems doubtful.
More likely is my tach and/or speedometer are not accurate.
Do i have the math right?
engine RPM at 60 equals 5280/tire circumference X final drive ratio ??
So, how much trouble is it to unbolt something and count the teeth?
Thanks
Dave,
I do not have the time to resolve your equation right now, but will remind you that the tire circumference has to be in feet.
Here is a link that will take you to the final drive page of one of the GMC sites.
<http://gmcmotorhome.info/front.html#RATIO>
Yes, 4.10 is realistic as that is a very available ratio. The only issue would be that it is only common in a hypoid gear set. Revcon had to have figured out that none of the common Toro gears would be right and maybe they couldn't get the tow-package set that GMT&C elected to use in the GMCMH. Or, maybe they knew it was a wrong as it is.....
In our coaches, opening the final to count teeth is a major PITA, but it can be done.
Matt - With a coach up in the air for a while
Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit
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| Re: How hard to determine final drive ratio (Revcon) [message #343226 is a reply to message #343223] |
Thu, 09 May 2019 13:06 |
Hal StClair
Messages: 971
Registered: March 2013
Location: Rio Rancho NM
Karma: -12
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Someone may jump in on figuring out the ratio other than using the RPM/MPH/tire size but I'm not familiar with any other likely method. The problem is using this method is you MUST have accurate data. The RPM is the hardest to get perfect without a good instrument. Tire size is pretty easy and MPH using GPS is a snap. That all being said, GM only had a few choices in gear ratios, 3.07 being the probable choice. Of course it could have been changed to a 3.21 or even a 2.73 or higher (lower number) by sourcing through salvage yards, etc. Aftermarket sets have been available through our vendors in later years but that would probably be an unlikely guess.
As for checking the teeth, I don't think I'd go that way-way to much work.
Good luck, Hal
"I enjoy talking to you. Your mind appeals to me. It resembles my own mind, except you happen to be insane."
1977 Royale 101348,
1977 Royale 101586, Diesel powered,
1974 Eagle Bus 45',w/slideout,
Rio Rancho, NM
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| Re: How hard to determine final drive ratio (Revcon) [message #343228 is a reply to message #343226] |
Thu, 09 May 2019 13:22 |
hertfordnc
Messages: 1164
Registered: September 2009
Location: East NC
Karma: 0
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Senior Member |
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I'm sorry, i think i put the question o=in the wrong order.
The key parts was more mechanical than mathematical:
"how much trouble is it to unbolt something and count the teeth? "
I only included the math part to confirm that I was not missing something.
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: How hard to determine final drive ratio (Revcon) [message #344366 is a reply to message #343223] |
Thu, 20 June 2019 11:16 |
bwevers
Messages: 597
Registered: October 2010
Location: San Jose
Karma: 5
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Senior Member |
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I get:
RPM@60 = {[mile/minute * (5280 feet/mile)]/ 7.72 feet/revolution}* 3.07 * 1.05 = 2210 RPM for 3.07
RPM@60 = {[mile/minute * (5280 feet/mile)]/ 7.72 feet/revolution}* 3.55 * 1.05 = 2550 RPM for 3.55
; with 5% torque converter slippage
Bill Wevers GMC49ers, GMC Western States
1975 Glenbrook - Manny Powerdrive, OneTon
455 F Block, G heads
San Jose
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| Re: [GMCnet] How hard to determine final drive ratio (Revcon) [message #344368 is a reply to message #344366] |
Thu, 20 June 2019 11:46 |
Dolph Santorine
Messages: 1236
Registered: April 2011
Location: Wheeling, WV
Karma: -41
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Senior Member |
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Is 5% slippage typical?
Dolph
DE AD0LF
Wheeling, West Virginia
1977 26’ ex-PalmBeach
Howell EFI & EBL, Reaction Arms, Manny Transmission
“The Aluminum and Fiberglass Mistress"
> On Jun 20, 2019, at 12:16 PM, Bill Wevers via Gmclist wrote:
>
> I get:
> RPM@60 = {[mile/minute * (5280 feet/mile)]/ 7.72 feet/revolution}* 3.07 * 1.05 = 2210 RPM for 3.07
> RPM@60 = {[mile/minute * (5280 feet/mile)]/ 7.72 feet/revolution}* 3.55 * 1.05 = 2550 RPM for 3.55
> ; with 5% torque converter slippage
> --
> Bill Wevers GMC49ers, GMC Western States
> 1975 Glenbrook - Manny Powerdrive, OneTon
> 455 F Block, G heads
> San Jose
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
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