Change Differential Gears to new Ratio?

[Dstrahm]

Has anyone changed the differential gears in their MH to a different ratio? Our 1999 Brave is geared way too low for how we use it and my thought was that I'd get better mileage with a higher ratio. Now at highway speeds the engine is running around 2000 RPM and it has enough low end torque to pull stumps easily. I'd gladly trade a slower acceleration off the light for better gas mileage and a quieter running engine. The axle is a Dana model, 80 I believe.

Thanks!

[SoCalDesertRid1]

The 454 and especially the V10 both need to rev to produce their optimum power. Lowering the engine rpms by reducing the gearing is not what you want, with these gas engines.

While diesels are comfortable cruising at 1500-1800 rpms, gas engines are comfortable cruising at 2000-2500 rpms and won't see an increase in fuel economy by making them cruise at a diesel rpm range.

The gas engines don't have the low rpm torque that diesels have, to be able to cruise efficiently at the diesel rpm range.

[SoCalDesertRid1]

smkettner wrote:

pianotuna wrote:
Then how come I use less throttle at 50 mph than at 60 mph? (and way less fuel)

Jagtech wrote:
Lower engine speed equals larger throttle opening, which effectively negates any fuel savings.

I believe we are talking same speed less RPM.

Yes, same road speed, less engine rpm, due to higher ratio gearing (numerically lower), while the engine is also under load.

It is proven that operating at low rpm with foot deep into the pedal produces lower fuel economy, than operating at higher rpm with less thottle, when the engine is under load.

[pianotuna]

So why would the throttle be farther open at lower RPM?

[time2roll]

pianotuna wrote:
Then how come I use less throttle at 50 mph than at 60 mph? (and way less fuel)

Jagtech wrote:
Lower engine speed equals larger throttle opening, which effectively negates any fuel savings.

I believe we are talking same speed less RPM.

[pianotuna]

Then how come I use less throttle at 50 mph than at 60 mph? (and way less fuel)

Jagtech wrote:
Lower engine speed equals larger throttle opening, which effectively negates any fuel savings.

[Jagtech]

Lower engine speed equals larger throttle opening, which effectively negates any fuel savings.

[Dstrahm]

Sam Spade wrote:

lenr wrote:
Would be shocked if fuel savings would pay for gear change.

And I would be shocked if some layman's gut feeling turned out to be better overall that all of the mechanical engineers who designed and built the thing to start with.

If you think you MUST.....wait until you need tires anyway and get some slightly taller ones.

If you do the gears you are likely to OVER do it and the result will be worse mileage and engine and transmission strain and a MUCH bigger bill in the end than you bargained for.

Sam Spade, it's a bit disappointing to see a Senior Member disrespecting another member, a recent member at that, with such comments.

My dad was a degrees mechanical engineer and worked for GM as an automotive engineer on drivetrains before he moved on to aerospace. I'm a degreed engineer also and have torn down to individual parts and rebuilt just about every automotive system in just about every type of passenger vehicle, including drive axles. To back the axles out and drop the pumpkin and replace with a different ratio part is not a big project.

I am also quite familiar with the engineering work performed during the design and component selection for any vehicle, but I also understand that the choices are based on trying to best meet the needs of a very diverse population of potential users.

My question was to see if there were any others on this board that knew of, or had direct experience with this mod. The tires approach was considered, and may well be the chosen path. My concern was adding height, road to axle and road to frame, and also having to "mental math" the speedo.

I've read quite a few great posts on this board and value the information shared. Let's all remember to keep it positive and respectful. If I misinterpreted your response, please accept my apologies.

[SoCalDesertRid1]

2000 rpm cruise with a 454 or V10 is plenty low enough rpm. There is no reason to change to a higher (numerically lower) axle ratio in that vehicle.

[jhilley]

smkettner wrote:
2000 rpm is pretty good for gas or is this diesel?

That year Brave was only available with a Chevrolet 7.4l V8 or Ford 6.8l V10. My 1999 Brave with V10 turns 2300 rpm at 55 mph.

[sjholt]

If you have a 5.9L diesel and are running at 70 mph @2000 rpm- Leave it alone because you'll need the gearing for climbing the hills.(6 speed trans). If you have a 4 speed trans- good luck with that.
If you have an 8.3L diesel then yes a gear change will help with that.

As far as my MH- 4.88 gears and a 6 speed trans puts me at 1900 rpms @ 65mph- Ideal for the 5.9L cummins and gives me 10.4 to 11.1 mpg.

[Bruce_Brown]

Sam Spade wrote:

lenr wrote:
Would be shocked if fuel savings would pay for gear change.

And I would be shocked if some layman's gut feeling turned out to be better overall that all of the mechanical engineers who designed and built the thing to start with.

If you think you MUST.....wait until you need tires anyway and get some slightly taller ones.

If you do the gears you are likely to OVER do it and the result will be worse mileage and engine and transmission strain and a MUCH bigger bill in the end than you bargained for.

I our case even the Cummins engine optimization calculator said we were geared wrong, Spartan said it was spot on. Cummins was right, Spartan was making excuses.

[time2roll]

2000 rpm is pretty good for gas or is this diesel?

[Sam_Spade]

lenr wrote:
Would be shocked if fuel savings would pay for gear change.

And I would be shocked if some layman's gut feeling turned out to be better overall that all of the mechanical engineers who designed and built the thing to start with.

If you think you MUST.....wait until you need tires anyway and get some slightly taller ones.

If you do the gears you are likely to OVER do it and the result will be worse mileage and engine and transmission strain and a MUCH bigger bill in the end than you bargained for.

[lenr]

Would be shocked if fuel savings would pay for gear change.

[pianotuna]

Hi,

I put oversize tires on my E-450 class C. That had the effect of lowering the differential ratio. I gained about 1 mpg. It was not cheap. I did it for more ground clearance and better tire load capacity. I would do it again.