Rear axle ratios

D

dick kennedy

New member
Jun 1, 1998
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There are a lot of mini motorhomes running with the 400 ci small block
chevy and duals on the back. These use the same size wheels and the same
transmission as the GMC uses but I don't know what rear end gears these
had? I have owned two of these and liked the way they drove. I think the
GMC is quite a lot heavier but it's a real dog by comparision.

Does anyone have information on weights and gear ratios to add some numbers
to this impression? My guess is that those small blocks are running about a
370 gear or more but I don't know.

I'm listening

Dick Kennedy
 
Hi, Dick, I have such a beast even as we speak. It is an 82 GMC G30 (one ton
van) chassis with a motorhome body from the doors back. Second one I've had;
the other was a 79 model.

This one has a 3.73 rear ratio and the 79 had a 4.10 ratio. Both used 16.5
wheels with 8.75 tires, same as the GMC. They both have 350 small block
Chevy engines and run decently.

To the best of my knowledge, GM didn't offer a numerically lower ratio than
3.73 on the one-ton rear end; I believe only 3.73, 4.10, and 4.56 were
offered.

But the 455 Olds, besides being larger in displacement, is a longer stroke
engine and makes its power at a lower RPM than the small block does. I doubt
that the 455 would benefit from the 3.73, and higher than that would almost
certainly be a mistake.

Also worth noting is that the two SOB's that I've had felt like they were
running with their tongues hanging out past about 65 MPH, whereas the GMC's
I've driven were just giggling at 70 and faster. 3.42 is probably a good
idea for those who want to tow and especially in mountainous regions, but my
guess is that the 3.07 is pretty near optimum for most of us if used with
the 8.75x16.5 or 225/75R16 tires. The 9.50's or 235/85 tires are about 10%
larger and thus will make a 3.42 very nearly equivalent to a 3.07 with stock
tire size...note that 3.42 is pretty close to 10% more than 3.07.

Travis

- -----Original Message-----
From: Dick Kennedy
To: gmcmotorhome
Date: Monday, August 30, 1999 9:40 PM
Subject: Re: GMC: Rear axle ratios

>There are a lot of mini motorhomes running with the 400 ci small block
>chevy and duals on the back. These use the same size wheels and the same
>transmission as the GMC uses but I don't know what rear end gears these
>had? I have owned two of these and liked the way they drove. I think the
>GMC is quite a lot heavier but it's a real dog by comparision.
>
>Does anyone have information on weights and gear ratios to add some numbers
>to this impression? My guess is that those small blocks are running about a
>370 gear or more but I don't know.
>
>I'm listening
>
>Dick Kennedy
>
>
>
 
More than likely they are running 4:10 or better. Some of the 350 ci's are
running 5:07 or better in some cases. I also think they are using 400 trannys
not 425's. They also have a different final drive configuration. Somebody shoot
me down if I am incorrect. Darren

> There are a lot of mini motorhomes running with the 400 ci small block
> chevy and duals on the back. These use the same size wheels and the same
> transmission as the GMC uses but I don't know what rear end gears these
> had? I have owned two of these and liked the way they drove. I think the
> GMC is quite a lot heavier but it's a real dog by comparision.
>
> Does anyone have information on weights and gear ratios to add some numbers
> to this impression? My guess is that those small blocks are running about a
> 370 gear or more but I don't know.
>
> I'm listening
>
> Dick Kennedy

- --
Darren Paget
76 Experimental
Another Fab Day
paget
 
Yep, TH400 transmissions on most, 350 on a few. The TH400 is the same thing
as the 425, though...the 425 is for all practical purposes a 400 that is
adapted to front wheel drive instead of longitudinal.

I haven't seen ratios higher than 4.56, but I haven't gone looking for them
either. I kinda doubt it though...they would be screaming past about 55-60
MPH unless they had some mighty big tires.

Travis

- -----Original Message-----
From: Darren Paget
To: gmcmotorhome
Date: Monday, August 30, 1999 10:03 PM
Subject: Re: GMC: Rear axle ratios

>More than likely they are running 4:10 or better. Some of the 350 ci's are
>running 5:07 or better in some cases. I also think they are using 400
trannys
>not 425's. They also have a different final drive configuration. Somebody
shoot
>me down if I am incorrect. Darren
>

>
>> There are a lot of mini motorhomes running with the 400 ci small block
>> chevy and duals on the back. These use the same size wheels and the same
>> transmission as the GMC uses but I don't know what rear end gears these
>> had? I have owned two of these and liked the way they drove. I think the
>> GMC is quite a lot heavier but it's a real dog by comparision.
>>
>> Does anyone have information on weights and gear ratios to add some
numbers
>> to this impression? My guess is that those small blocks are running about
a
>> 370 gear or more but I don't know.
>>
>> I'm listening
>>
>> Dick Kennedy
>
>--
> Darren Paget
> 76 Experimental
> Another Fab Day
> paget
>
>
 
I had 3.70 gears in a chev truck - carried a car and pulled a trailer
with another car on it - 350 engine - loved that combination - great
gear for highway cruising - took the pass between chatanooga and
nashville in 3rd gear fully loaded - cruise all day long in 4th with
good mileage

- --

ron keener near Austin, Texas http://MidAmericaBreedingTech.com
**************************************************************
*
* proud owner of classic 1978 GMC Birchaven motorhome
**************************************************************
 
"The 9.50's or 235/85 tires are about 10% larger and thus will make a
3.42 very nearly equivalent to a 3.07 with stock tire size...note that
3.42 is pretty close to 10% more than 3.07."

I can tell you exactly - 9.50s are 5.5% larger than 8.75s according to
my odometer - i just made the switch recently - i like the 9.50s on the
front - more stable on groves and ridges - still have 8.75s on rear

- --

ron keener near Austin, Texas http://MidAmericaBreedingTech.com
* proud owner of classic 1978 GMC Birchaven motorhome
**************************************************************
 
Dick,

I have a 1978 Midas built on a GMC 1 ton van chassis. Following are the specs:

Engine: Chevy small block 400
Trans: Turbo 400
Length: 23 1/2 feet
Empty Weight: 7,955 lb
Gross Vehicle Weight Rating: 10,500 lb
Tires (originally specified): 8.00X16.5
Tires (now on vehicle): 8:00X16.5RLT steel
Rear Axle Ratio: 4.10

As far as performance is concerned: from a dead stop it will stay with
traffic; it will cruise comfortably between 70-75 mph but will run 80 mph
without signs of stress; steep, long hills really knock the speed down,
especially when you have to drop down to second gear; gas mileage goes down as
the speed goes up with 8-9 being about average if the speed is in the 70-75 mph
range and the terrain is moderate.

Lorry

> There are a lot of mini motorhomes running with the 400 ci small block
> chevy and duals on the back. These use the same size wheels and the same
> transmission as the GMC uses but I don't know what rear end gears these
> had? I have owned two of these and liked the way they drove. I think the
> GMC is quite a lot heavier but it's a real dog by comparision.
>
> Does anyone have information on weights and gear ratios to add some numbers
> to this impression? My guess is that those small blocks are running about a
> 370 gear or more but I don't know.
>
> I'm listening
>
> Dick Kennedy
 
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