37" tires and 3:73 gears
 

will i regret this? will the trans be short lived even after a rebuild with billet convertor, hd clutches and shift kit? will the truck be sluggish?

trying to get feed back on this before buying the tires. currently running 35's which are closer to a 34" tall tire. the 37's will be 36.9" tall new

thanks

IMO, I'd say yes, you need to either keep the same size of tire or change the gear ratio to 4:10.

I run 37's on my 08 and while the engine made plenty of power to pull any load the transmission really wasn't happy. I got tired of having to watch egt's like a hawk and running in 5th everywhere under a load got old.

I put 4.56's in the rear and the truck is much happier all around. Egt's are down because rpm is up, and having more available gears when towing just makes sense. Fuel mileage was not effected in any way.

SpeedyWS6, how well do your brakes work with the 37s? We can change gears to support the taller tires, but the brakes will be less effective.

I've never run anything taller than 33" so it may not be any big deal.

The brakes don't worry me a bit. I have made emergency stops pulling my enclosed trailer with a full weight car inside and it does fine.

My co-worker went from 4.10 to 4.56 with 33" tires on his V10 2nd gen. Big improvement he says. No mpg penalty because OD is so tall anyways (.69?) He's certain his auto trans is happier.

I'd do 37's with 3.73, but then again, I'm running a cummins, and I don't have an auto trans to keep happy.

I am planning on 35s and going to the 4.10 ratio front and back. That should net out at 3.75 which is darn near stock. That will keep the same powerband as stock which is important as I sized the new twin turbos for low to mid-range spool and keeping the sweet spot around 1600 to 2000 rpm. If I was going to 37s would likely go to 4.56 ratio but I think 35s are as big as I want to go to maintain good braking and overall performance of the truck.

Seems like a big cost to change the gear ratio one step. Most rear end shops say its not worthwhile unless you go two steps.

I would probably suck it up and watch the trans temps till I had some other reason to do major axle work.

Well that is a 4" larger tire which is 12.9% so it is a pretty big change in tire size. If I was just going to a 33" tire then probably not a big enough jump in tire size to validate the gear change. Besides I was talking to a guy at work who went to 35s and said he lost 2 mpg but then had the 4.10s installed and got the 2 mpg back. And it would allow towing like before without worrying about lugging the engine and burning up the transmission. So yep it's gonna get done.

I actually kinda wish I had not gotten the twin turbo package and concentrated on the suspension / wheels / tires to get my truck desert ready. Oh well.

Most tuners change the trans shift points so you aren't dealing with 'stock' anyways.

You are also dealing with more rolling mass and resistance with the bigger tires, so it can be argued you need to overshoot the ratio to get closer to 'stock'.

Most guys I know that re-geared their auto trucks wish they'd gone a step further. My 2 cents.

Agreed...

my 35" suck with 3.73

going to 37" with 4.56 but is about as good as I can get

Braking,engine and transmission strain would be evident with 37's and 3.73's.
4.10's would help to re-establish the stock power curve and towing ability.

4.30 is supposed to be really good for 37s. Or 35s if towing heavy a lot. I recently tried a set of 255-80-17 Cooper ST MAXX and it runs OK with 3.73 but I can really feel the difference. Main culprit is that darn auto transmission. Ratios too tall and not enough gears. Manual trucks can work with the 3.73 much better. A ratio change in the diffs is much easier to do than a transmission swap to manual. I should have waited to find a manual transmission truck.