The guys in the shop were talking about ratio's today. They said its cheap horsepower. I figured that changing the ratio would hurt my Allison or make other problems arise..( they said it would not) I need the low down on all of this. I want to step it up a notch with a ratio change, but I want a dependable truck that is safe to drive the distance as well..

What do you mean by "step up"? Chances are you've got a 3.73 ratio right now... not as good for accelleration, but better mileage and lower rev's at the same speed (mph). Going to 4.10 gears would give you better accelleration, but worse mileage. Going to 3.43 gears would do the opposite. Having larger tires (even your 305's) is effectively like having "lower" gears; aka less than 3.73.

You won't hurt the alli by changing the ratio as long as your also change the front diff too, and update the settings in the computer.

As per your PM,

Your Dmax has a 11.5" 14bolt rear end. The easiest to find ring&pinion sets are 3.42, 3.73, 4.11, 4.56, 4.63, 4.88, 5.13, and 5.38.

Your truck almost definitely has a 3.73 in it right now. If you went from a 265/70R16 tire (factory) to a 305/50R20 tire, you effectively changed your rear end ratio by 0.95, or 3.73 x 0.95 = ~3.54. This means you're about 5% slower in acceleration but are running about 5% lower rev's on the highway.

Lets assume you're staying with your 305/50R20 tires. These have a diameter of 32.0". You can use this equation to calculate RPM differences with changes in rear end...

(336 x MPH x Gear Ratio) / (Tire Diam) = RPM

So, for a 3.73 (factory) ratio...

(336 x 70mph x 3.73) / (32) = 2742~RPM

Now, if you were to switch out to a 4.11 ratio...

(336 x 70mph x 4.11) / (32) = 3021~RPM

Now, those are both assuming a 1:1 transmission ratio, which would be drive. Overdrive would be different; on a 5sp allison, OD is 0.61... this means that at 70mph with 3.73 rears in OD you'd be running ~1947RPM, which is fine. Running 70mph with 4.11 rears in OD you'd be running ~2145RPM, which is also fine.

Even maxing out at 96mph, you'd be running ~2941RPM with 4.11's (which is below redline).

Basically, 4.11's aren't unreasonable on your engine/tranny with your current tires. If you want to go higher than that, you can use the formula's above to calculate the change...

wow..too many equations for me....ha

schulte pretty much covered the facts and common views of regearing. That being said, the trend right now seems to be moving towards trying to get less ratio,things like installing 3.42s or doing TCM programming to lock out 1st gear are currently being experimented with.

Also, whoever told you that regearing was cheap was not being 100% honest with you. Regearing can be very costly, especially if you don't know how to setup gears. If you have a 4wd then you can at least double the cost, if not more since you have quite a bit of disassembly to get the the front gears on our truck.

Cheap power is getting a programmer and a programmer alone will give you better power than say going to 4.10s will.

Moving to a 3.42 will slow my truck down though.. It would be understandable for pulling or fuel mileage but that does not concern me. My truck is not for toeing, and I do not go very many places. I am looking for fast times. Something that will do the job without having to upgrade my ally (yet), and still leave me with a reliable truck..

Where is a good place to find a good ring and pinion? I want to do some price checking to see what the damage is..

i would look for yukons, there were some in the for sale section here. about $200 i believe

3.73 gears are the ONLY factory ratio available with a Duramax/Allison.

3.73 gears are the ONLY factory ratio available with a Duramax/Allison.

I could be wrong, but I thought in the earlier years a 4.11 could be ordered?

I could be wrong, but I thought in the earlier years a 4.11 could be ordered?I had a HD with a 6.0 and it had 4.10's.I've never heard of a Duramax with 4.10's out of the factory though.;)However,I could be wrong.:eek: