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!

[Kayteg1]

Something must be wrong with engineering.
My Ford trucks would take the grades in my area on highest gear just fine, even with 2-3,000 lb load.
Stepmom's Toyota sedan had to downshift with me alone inside.
I start RVing when lot of transmissions have been 3-sp and Gear Vendor was very popular.
I had it in one RV that was converted from gasoline to diesel engine.
Changing the gear vendor on the move without a clutch took some practice, but worked just fine.

[BigRabbitMan]

Jagtech wrote:
All engines have an optimal rpm range, and MOST manufacturers will take that into consideration when building a chassis. That's what you want to see at cruising speed if you are considering a gear change.

This is the important and correct fact. ALL engines are designed for an optimal rpm range and have an optimal rpm for economy within the larger range. If you normally drive slower than the designers/engineers anticipated, you should increase the rpm for better mileage. Conversely, if you drive faster than anticipated by the designers/engineers then you should decrease the rpm for optimal mileage.

You have questions to answer to yourself, not this group, before you do anything. Another's experience with a different coach means nothing as it applies to your coach and your driving style.

All of those were considered when I made my conversion from gas to a Duramax diesel. The net change for me was to go from 7.5 mpg to 14.5 mpg. The only thing that indicates is that if you want a major increase in fuel mileage, you need to change the engine instead.

[Jagtech]

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

on edit, I should have been more clear here. Given the same load, the same road speed, the same fuel, wind, etc, a gear change producing a significantly lower engine rpm speed would require a wider throttle opening in order to overcome the increased engine load. Thus higher fuel consumption.... UNLESS the rpm drop puts your rig into a higher power range of engine speed. All engines have an optimal rpm range, and MOST manufacturers will take that into consideration when building a chassis. That's what you want to see at cruising speed if you are considering a gear change.

[harold1946]

Dstrahm wrote:
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!

www.drivetrain.com Has all the information on identifying you axle and gear ratios. There is also a full list of available ring and pinion options.
I suggest contacting them. They will be able to give the best advice.

[Sam_Spade]

Dstrahm wrote:

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

Let's all remember to keep it positive and respectful. If I misinterpreted your response, please accept my apologies.

It is YOU who is interpreting the comments as disrespectful.
There are no words in there that are inherently so.

AND....it is not always possible to express ones honest opinion and not have it come out a little negative.....especially for someone who is overly sensitive and is looking for an insult where none exists.

I thought this "always keep it positive" thing started about Gen Y and it makes it very hard to carry on an intelligent conversation.

[usersmanual]

SoCalDesertRider wrote:
Whatever the axle ratio and tire size are, the engine is cruising at 2000 rpm in high gear at normal highway speeds and that is just fine for a gas engine. The coach is geared appropriately. There's no reason to raise (numerically lower) the gear ratio or go to taller tires (which accomplishes the same).

I agree assuming its 2k@60-65 and not 50 mph

[usersmanual]

DATRAHN QUOTE: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.

1) you claim its a dana 80? there is no drop out section re dana 60-70-80 series and installing new gears is a lengthy job and requires special tools for reading pinion depth etc
2)one can change the speedo to read correctly depending on year and model there are a few different methods and weather changing gears or tires the same mod would be required

[SoCalDesertRid1]

Whatever the axle ratio and tire size are, the engine is cruising at 2000 rpm in high gear at normal highway speeds and that is just fine for a gas engine. The coach is geared appropriately. There's no reason to raise (numerically lower) the gear ratio or go to taller tires (which accomplishes the same).

[Hikerdogs]

Are you sure the torque converter is locking in for overdrive? As I recall if the motorhome is built on a Ford 20,500 lb. chassis it should have a 5.38:1 differential ratio, and when in overdrive the driveshaft speed should be .7 that of the engine.

The 5.38 was the tallest gear ration offered in 1999. If the motorhome is on a lighter chassis it could have a final gear ratio as low as 4.30:1

ON EDIT:
Also keep in mind the motorhome has the first version of the V10. It only has 275 HP. The following year they raised the HP to 310. The 3 valve version from the 2006 model year to the 2015 model year has 362 HP. I believe the same engine was derated to 320 HP in 2016 to meet more stringent EPA standards.

[Mike_Hohnstein]

Some good responses here but it would be good if the OP gave up some information related to the drive train. Engine/transmission for one, and existing gear ratio. Tire diameter would be helpful too.
Let's not forget the fact that increased speed creates increased air drag and tire friction. I believe the correct term is 'exponentially'.

[time2roll]

It is possible to get a measured improvement.
Much better chance with electronic controls today vs the older stuff with a carburetor.
Sure a little extra for tires that you are already purchasing might pay off. Still not sure if a gear change will be money ahead or how long to pay off the cost.

What was the tire size change? Even so every set up can react differently.

[pianotuna]

My calculations were based on routes I've traveled before multiple times in both directions with gas receipts.

[jhilley]

The odometer would also record fewer miles for distance traveled.

[pianotuna]

Then, why did I see a 1 mpg improvement when I went to larger taller tires? I use the cruise control when at all possible and I'm a slow driver.

SoCalDesertRider wrote:
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.