Gearing for HD

[DMAX_AZ]

Hello all,

New to te board and in need of some tech advise regarding gearing.

 

It is my understanding that if I re-gear my truck to say 4.56 from 3.73, since I run 35" tires on a DMax truck that I could expect the following:

 

1 Speedo will be corrected (about 8mph slow above 50) no need to eletronically adjust speedo.

 

2 New 4.56 gearing will put the engine in the proper rpm for the speed and mileage will increase from my 16.5 to possibly 18-19.

 

3 The resulting RPM increase will be about 400 rpm. from about 1700@70mph to 2100@70mph. Used Precision Gear website calculator that assume 1:1 final while the 5th gear in the allison is .74:1 I believe.

 

Are these assumptions correct? What are your experiences with regearing the HD trucks, specifically diesles as gearing affect them differently.

 

Thanks in advance.

Leo

[whitesnake01]

Hello all,

New to te board and in need of some tech advise regarding gearing.

 

It is my understanding that if I re-gear my truck to say 4.56 from 3.73, since I run 35" tires on a DMax truck that I could expect the following:

 

1 Speedo will be corrected (about 8mph slow above 50) no need to eletronically adjust speedo.

 

2 New 4.56 gearing will put the engine in the proper rpm for the speed and mileage will increase from my 16.5 to possibly 18-19.

 

3 The resulting RPM increase will be about 400 rpm. from about 1700@70mph to 2100@70mph. Used Precision Gear website calculator that assume 1:1 final while the 5th gear in the allison is .74:1 I believe.

 

Are these assumptions correct? What are your experiences with regearing the HD trucks, specifically diesles as gearing affect them differently.

 

Thanks in advance.

Leo

I am no expert, but acording to my back-of-the-envelope calculation your engine is currently running 13% lower RPMs than stock at the same speed. This is also the amount your speedometer is under-representing actual speed. Going from 3.63 gears to to 4.56 gears would be overdriving it by 6.4% instead. Going to 4.11 would under drive by 4.2%.

 

Exact gearing depends on many individual factors. As Dmax fuel economy appears to worsen considerably above 70mph, you might be better off with a slight under drive than a slight over drive, assuming you are looking for fuel economy. If you are looking for performance, overdriving is probably a better idea. If you go from 3.73 to 4.56 gears, engine RPM will increase by 22%, assuming the same gear is used. But with that wide of a gap, you might not be running in the same gear for a given speed anyway. That is, in your current configuration the computer might be more inclined to hold you back at a lower gear than it would be with 4.56 gears because the engine behaves like it is under load.

 

I am not so sure you'll get good mileage with 35 inch tires, no matter what ratio.. the best you can do is "less bad mileage". The choice of gears is a function of how you intend to use the truck.

[DMAX_AZ]

Hmm interesting observations. The whole point of this excersice is to get the engine rpm/ vehicle speed back in line with a stock config, in which it is my understanding that the fuel economy is sginificantly bettery that what I am getting.

 

Now I do most of my driving between 60-70 mph for 32 mile trips twice a day. I'm not looking for miracles here. I do undertand that with 35's and a 6" lift my fuel economy will suffer slightly.

 

It is my understanding that diesles act differently with relation to load and rpm. In my current configuration I am told that the lower engine rpm is actually worse do to the lower boost and the higher work per rpm required. It is my understanding that if I can regain the 400 rpm or so back at a given speed that due to the boost level, and reduced work per rpm that the fueling will actually be less or at the least more efficient.

 

I have read/been told of individuals regearing to lower gears and actually improving performance and economy both or at least matching the stock performance and economy. With the Allison trany it always puts you in the highest possible gear and will lock it up if the load isn't to great for the highest gear.

 

I just wich I could find someone out there that has first hand experience with my type of set up that has re geard.

 

Thanks for your impressions! Anyone else?

[Shotgun]

Your current actual gear ratio with the 35's is 3.24 which is horrible. You would need to install 4.29' s to get back to an actual 3.73 ratio with the 35" tires .Installing 4.10's would yeild an actual ratio of 3.56. If you install 4.56's you will have an actual ratio of 3.96 which in my opinion is perfect. I am running 4.56's with 33" tires and it's great .I know you are trying to get as close to an actual 3.73 as possible an I believe 4.56's are your best optiion.

[DMAX_AZ]

Thanks shotgun, Could you share your formula that equated to a ratio of 3.24?

 

Based on the numbers you provide it seems that the 4.56 is the way to go. PG lists a set of 4.88 avail but I don't need to go there.

[Shotgun]

Use the tire size change. Also you may want to look into yukon gears too. calculator------http://www.4lo.com/

[jelisfc]

I have 35's but green keys not a tall lift. The truck sits 4.5" higher than stock. I lost about 1 to 1.5 mpg after the tires. Realize I have a 6spd so I may not lose as much mpg. Where I really lose mileage is in town. It takes a lot more to get 35's rolling. Interestingly I took the same 800 mile trip this year to Canada with my 5K trailer as last year. My mileage with trailer was almost identical, 13-13.2 mpg. Same route, same time, everything.

 

I wish my truck had 4.10's but I'm happy with my setup. The 6% grade hill I pull on my trip is easier because with 245's I'd top out at 70 in 5th gear. Now I top out around 80mph so now I drop to 5th and let'er roll. It just walks up the hill.

[2500HD4X]

Just my 2 cents on Item #2 of your post- with 35" tires on there you will be lucky to get anywhere close to stock mileage no matter what the gearing change (even if close or a little lower than stock). It takes a lot more effort for your engine to accelerate those tires up to rolling speed due to their "unsprung weight". Even though the tires are taller than stock and lower your engine speed under cruising conditions, you usually will not see any benefits from this due to the higher rolling resistance of a wider tire (more friction from the road). Such a drastic step up from stock will usually drop gas mileage significantly, but the gearing change is worth doing just to return performance/speedo back up closer to stock. It is also a lot easier on your transmission to get em rolling with more help from your axle(s)....

 

35" Tires and a 6" lift = Lower gas mileage but a lot more cool factor.

 

I wish my truck got 16.5 mpg-best tank with speedo corrected-14.6 mpg

Usual tank (around town)-10 mpg; long trips on 55mph road 13.4mpg