Change gear ratio on 454 class C

jrbelk wrote:
I've got 98 coachman class C 454 with 4:11 rears. I want to improve mileage by going to 3:23 gears. Will this work? Thanks

I think you would probably end up with worse mileage in the end if you did that, in additional to terrible performance and short transmission life. Don't do it, especially if you have overdrive. I'd switch to 4.56 if anything.

Improve the mileage with a good solid tune up new filters and ignition wear items. I admit I don't know a lot about 98 454 in the van chassis, is it TBI? If so make sure you have good coolant temp sensors and it is showing it is warmed up correctly as per the computer. With TBI everything is based on coolant temp. Make sure you have 195 tstat. Make sure no vac leaks. Make sure fuel pressure is to spec. If you need to do anything with fuel or ignition make sure to use OEM GM parts not cheap ebay stuff.

enblethen wrote:
Maybe 3.73 but not 3.23. 3.23 will cause engine with the weight of MH will lug down.

More specifically, it will have to downshift to first gear and rev up very high just to get up small hills, well below posted speed limits. Potentially dangerous.

jrbelk wrote:
I've got 98 coachman class C 454 with 4:11 rears. I want to improve mileage by going to 3:23 gears. Will this work? Thanks

The next time you put tires on it, just install tires with a larger diameter. This changes the overall drive-train ratio just as if you had lowered the differential gears down from 4.11.

As a bonus, you also get longer tread life and better ground clearance with the larger diameter tires.

jrbelk wrote:
I've got 98 coachman class C 454 with 4:11 rears. I want to improve mileage by going to 3:23 gears. Will this work? Thanks

IMO, it will work, but you will not save much of anything overall as far as fuel mileage. IMO, here's why. Going from 4.11 to 3.23 is a BIG jump. The engineers pick the axle ratio in an attempt to run the engine close to the max torque point (this is a curve) while at cruising speed (this is a range) while in top gear. Dropping the ratio to 3.23 will cause the transmission to run in lower gears (higher RPM's) while at cruising speed a lot of the time. Even while at cruise speed when you are on level ground. The transmission prefers to be in top gear while cruising which is what most motorhomes do most of the time. If you are like most Class C motorhomes, you will be at or close to max gross weight most of the time. So, the engine needs to spin fast enough to generate enough torque. (horsepower too) If you want to improve fuel economy, slow down, lighten the load, and travel with the wind if you can

Chum lee

As an afterthought and in agreement with what Phil says below:

For example: If you are now running P225x70 R16 tires and change to P245x70 R16 tires this will have the effect of dropping your current 4.11 gears about 5% or down to 4.11 x .95 = 3.90. The corresponding RPM drop will occur in cruise as long as the engine/transmission can handle it.

Provided there is clearance in the wheel wells for larger tires, IMO this is a much more conservative, safer approach.

As mentioned, you will likely never save enough gas to justify the "upgrade"...that's assuming it works...which I suspect it won't with such an extreme jump.

Playing with gears used to work on pickups because they went from say 6000lb running empty to 14000lb towing heavy. A low numerical gear ratio would help MPG because it didn't take a lot of torque at the wheels to move an empty pickup so the new gear ratio allowed you to keep the RPM down in a more efficient range while not lugging the engine...Of course, you would wind struggling to hold 4th gear while towing but if 95% of miles were empty, it was a good trade off. (most actually would move in the opposite direction to a higher numerical gearing because they would be stuck in 3rd gear running too high of RPM while towing with the original low numerical gear ratio)

With a MH, you are always running heavy, so there isn't the same advantages to be had.

If you really want to pursue this anyway...you need to get the output curves for you motor and back calculate a rear end ratio that will put your motor at it's ideal RPM when cruising in top gear at your desired cruise speed.

Ok what speed are you cruising at?
What RPM are you running at that speed?
Does OD kick out on small rolling hills or moderate overpass?

3.23 could actually reduce your economy by lugging the engine and downshifting at the slightest breeze.

It's possible and the coin isn't that bad for something in that age group. Whether it will help with fuel economy, who is to say. Heavy duty vehicles of that era are not required to report fuel economy estimates. Based on a quick look at the chevy brochure from 2002 for the cutaway chassis, chevy put 3.73's in the 12000 gvr, 4.10's in 15000 gvr, and 4.56's in 17000 gvr commercial cargo vans. So if the ready to go weight of your '98 is way less than the gvwr, it could probably pull 3.73 gears.

Next thing to figure out is how long it will take that 1-2 mile per gallon improvement to pay for the gear replacement.

We had an '01 E-350 Class C mh. We go to antique tractor shows and caravans towing tractor on a flat bed trailer, and sometimes our car, not all at once.
I would set the cruse at 60mph on the express ways.
One time we drove US-2, 2 lane and a lot of small towns to slow down at and sometimes a stop light, for a few hundred miles.
I figured that our gas milage would drop, because of the slow downs and stops. But it was better than express way driving. So now I drive 55 on the express ways also.
The wind resistance is what is hard on gas milage, the faster you drive the higher the resistance.

You would be very unhappy after the 1st hill.
m
Mark

midnightsadie wrote:
. I bought a sprinter diesel and get 20mpg mines a 5cyc.

Good for you. How is it relevant?