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Any way to know what my final drive ratio is? [message #371348]

Sun, 21 May 2023 09:35

boybach

Messages: 566
Registered: December 2020
Location: Vancouver Island

Karma: 4

Senior Member

Hi all, I just got back from a trip and once again all was well with the coach. Now that all things are in order of course I have to be concerned about SOMETHING Razz

-so I was wondering whether or not my PO had changed the final drive.
The coach is a 77 with a 403 and seems to have lots of oomph on hills, rolling along pretty good at speed too ...so my question is can a person tell if the coach has the original 3.07 simply from the performance -or- in other words where does the 3.07 fall flat compared to other options?

Larry

Larry - Victoria BC - 1977 ex-Palm Beach "Ol' Leaky" 40,000 miles, PO said everything working but forgot the word NOT. Atwood helium fridge, water heater & furnace. SS exhaust system, Onan, Iota Converter, R134A, New fuel lines & heat exchange hoses

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Re: Any way to know what my final drive ratio is? [message #371349 is a reply to message #371348]

Sun, 21 May 2023 10:43

lqqkatjon

Messages: 2324
Registered: October 2010
Location: St. Cloud, MN

Karma: 5

Senior Member

You can try to check rpm and compare to mph. But my experience is pretty tricky to be accurate with that method.

Otherwise the only guarantee way is to pull cover and count teeth.

Sometimes paint(applied i think painted some blue?). But really still guessing.

If someone switched to a 3.21 final, that has 8 bolt cover. But pretty rare.

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: Any way to know what my final drive ratio is? [message #371350 is a reply to message #371348]

Sun, 21 May 2023 11:00

seeburg220

Messages: 18
Registered: April 2009

Karma: 0

Junior Member

I also have a '77 403 w/stock 3.07, and it's a pig. So, I'm betting you have something better installed, or perhaps a Caspro chain drive set that changes the overall ratio. I had a Caspro 3.55 chain set in my '78 before I sold it, and it was great. Unfortunately, they no longer make chain sets, otherwise, I probably would have gone that route again. Final drives are very expensive. I have a 3.70 I'm getting ready to install and look forward to actually moving down the road Very Happy

Currently '77 403 Palm Beach. Formerly '78 403 Kingsley.

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Re: Any way to know what my final drive ratio is? [message #371353 is a reply to message #371350]

Sun, 21 May 2023 15:20

Carl S.

Messages: 4186
Registered: January 2009
Location: Tucson, AZ.

Karma: 13

Senior Member

If you have a tach, this chart from Applied might help:

https://www.gmcrvparts.com/product-p/gm1-1.001.1.htm

Carl Stouffer '75 ex Palm Beach Tucson, AZ. Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles, Solar battery charging. GMCSJ and GMCMI member

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Re: Any way to know what my final drive ratio is? [message #371354 is a reply to message #371353]

Sun, 21 May 2023 16:35

boybach

Messages: 566
Registered: December 2020
Location: Vancouver Island

Karma: 4

Senior Member

Carl S. wrote on Sun, 21 May 2023 13:20

If you have a tach, this chart from Applied might help:

https://www.gmcrvparts.com/product-p/gm1-1.001.1.htm

I'll have to get a tach hooked up, thanks!

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Re: Any way to know what my final drive ratio is? [message #371355 is a reply to message #371350]

Sun, 21 May 2023 16:38

boybach

Messages: 566
Registered: December 2020
Location: Vancouver Island

Karma: 4

Senior Member

seeburg220 wrote on Sun, 21 May 2023 09:00

I also have a '77 403 w/stock 3.07, and it's a pig.

Can you elaborate what you mean by "pig"? Slow off the line? Top speed lacking? no uphill pulling power? Just wondering.

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Re: Any way to know what my final drive ratio is? [message #371358 is a reply to message #371348]

Sun, 21 May 2023 18:59

Tilerpep

Messages: 404
Registered: June 2013
Location: Raleigh, NC

Karma: 7

Senior Member

Two things I can think of
If your gps speed does not match your indicated speed, then some math might tell you how off, then the ratio of available options that would make you that much off.

And in that vein, if you have a speedometer fixer box it would indicate someone put in final. It is a little gear box inline where speedo goes in tranny on mine.

1975 Glenbrook, 1978 Royale rear bath Raleigh, NC

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Re: Any way to know what my final drive ratio is? [message #371359 is a reply to message #371355]

Sun, 21 May 2023 20:57

seeburg220

Messages: 18
Registered: April 2009

Karma: 0

Junior Member

boybach wrote on Sun, 21 May 2023 17:38

Can you elaborate what you mean by "pig"? Slow off the line? Top speed lacking? no uphill pulling power? Just wondering.

Mostly just really slow off the line. Merging onto a highway was interesting. (I will say that part of the problem until recently, was the gas pedal rod was bent, so if you floored it, the carb wasn't opening all the way. Bending the gas pedal rod back up helped quite a bit.) Regardless, it just feels like you can feel all 12k lbs it's trying to pull. With my 3.55 coach, it got up and went.

Currently '77 403 Palm Beach. Formerly '78 403 Kingsley.

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Re: Any way to know what my final drive ratio is? [message #371360 is a reply to message #371359]

Sun, 21 May 2023 22:47

boybach

Messages: 566
Registered: December 2020
Location: Vancouver Island

Karma: 4

Senior Member

seeburg220 wrote on Sun, 21 May 2023 18:57

boybach wrote on Sun, 21 May 2023 17:38

Can you elaborate what you mean by "pig"? Slow off the line? Top speed lacking? no uphill pulling power? Just wondering.

Mostly just really slow off the line. Merging onto a highway was interesting. (I will say that part of the problem until recently, was the gas pedal rod was bent, so if you floored it, the carb wasn't opening all the way. Bending the gas pedal rod back up helped quite a bit.) Regardless, it just feels like you can feel all 12k lbs it's trying to pull. With my 3.55 coach, it got up and went.

Well mine is pretty slow getting away at the lights so I guess I probably do have the 3.07 ...however, that isn't a deal breaker for me -plus I very rarely would have to merge into fast traffic or tow anything at the moment. I guess I'll just drive it like the 46 yr old thing it is and suck it up.

thanks!

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Re: Any way to know what my final drive ratio is? [message #371362 is a reply to message #371348]

Mon, 22 May 2023 05:54

RF_Burns

Messages: 2276
Registered: June 2008
Location: S. Ontario, Canada

Karma: 3

Senior Member

The OEM 3.07 was the Toronado's trailing option ratio (standard was 2.7 something). Its still pretty tall for moving 12,000 lbs.

The 3.70 seems to be the favorite of 403 owners. The engine rpms is in it's torque band and most report the same MPG since the engine is not lugging.

GM had a 3.42 in the works for the Motorhome, but it never had it into production. When Cinnabar bought the rights to the motorhome, they did make the 3.42. I purchased a used 3.42 when that owner went to a higher ratio for his 403. I have a 455 and it is no longer a lumbering slug off the line and up hills. I only got it installed last fall so I don't know if I lost on MPGs yet.

Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.
1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that

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Re: Any way to know what my final drive ratio is? [message #371364 is a reply to message #371348]

Mon, 22 May 2023 10:27

JohnL455

Messages: 4447
Registered: October 2006
Location: Woodstock, IL

Karma: 12

Senior Member

I had a 10 hour drive Sat (traffic and construction around Chicago and N In). Had originally had a 3.07. Only great above 70 with no wind or tail wind. I now have Cinnabar USA 3.42 and with a 455 think it’s the best (least downsides) option. Better in stop and go. Better on hills up and down (important as I use gears) in mountains. I cruised at 75 and at times did 80 due to truck traffic speeds. I would not want higher 3 gears as this is how driving is now.
If you have a friend with a known ratio, and are musical, could do side by side 60 MPH and find the note on a guitar and fine tune to it. Then take it to your coach, do same speed and if pitch is same you have same gears (if same size tires). No tach needed.

John Lebetski
Woodstock, IL
77 Eleganza II

[Updated on: Mon, 22 May 2023 10:28]

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Re: Any way to know what my final drive ratio is? [message #371365 is a reply to message #371364]

Mon, 22 May 2023 11:25

boybach

Messages: 566
Registered: December 2020
Location: Vancouver Island

Karma: 4

Senior Member

I don't find the coach slow or lugging on hills however when things get steeper and it drops down to about 45mph I put it into second and it roars up quite contentedly often changing back up to third. Also I rarely go over 65 although I feel the coach would certainly go faster if I floored it.

Mostly drive around 55 and have no problems keeping up with traffic. On trips up the west coast on rural and dirt roads where the average speed is probably around 35-40mph, I'm the envy of friends that drive C-class rigs and have to wait for them the top of the big climbs (or pretty much any hills).

Hwy speed limits in my neck of the woods are in the 80-90 kph (50-55mph) range with the odd stretch at 110 (68mph).

I guess if I was traveling on the interstate or towing perhaps I'd need to change something - for now though, I can't think how things could be improved enough to warrant the swap. And who knows, maybe I do have a different ratio because there's no way I'd describe the performance as sluggish or like a pig! Laughing

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Re: Any way to know what my final drive ratio is? [message #371374 is a reply to message #371365]

Thu, 25 May 2023 14:27

Keith V

Messages: 2337
Registered: March 2008
Location: Mounds View,MN

Karma: 0

Senior Member

Look for a speedo correction box on the speedo cable down by the transmission.
If it has one you know it's not stock. Maybe it says what the correction is?

I have 3.70's behind my 455, it's awesome. I like to drive the secondary roads and it's very happy at 50/55
Of course even on the freeway I rarely exceed 60.

3.42 MIGHT be better if you do all freeway and like to go 70.

Keith Vasilakes
Mounds View. MN
75 ex Royale GMC
ask me about MicroLevel
Cell, 763-732-3419
My427v8@hotmail.com

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