4x4chvyman

I have a 2001 Silverado 4x4. I have 35/12.50 tires on it with stock 3:73 gears. A friend of mine said I need to change the gears and recommended putting 4:10 gears in it. Is this needed? What will it do to my gas mileage? Any help is appreciated.

therewolf

Depends if you're having low end power and torque issues. 373s are very respectable gears, and a good aftermarket intake will give you improved response if you're just a little logy. 410s have great low end power, but your highway driving goes into the toilet. Expect a pickup in power, also expext your ABS sensor to go nutz and your speedometer to be telling you the wrong speed(It'll say you're going @3-7 MPH faster than you are) until you get your PCM reprogrammed.

Just did a gear change in my 04 recently, is how I know. Good luck, amigo.

4x4chvyman

Thanks for the info.

BassinProf

I have to disagree with Rivereye. You say you're running 35" tires? That's not stock, right? I don't know about your 2001, but my 2006 came with 31" tires. Assuming stock was 31" tires and there hasn't been any mods to the drive train, running 35" tires gives you an effective gear ratio of about 3.3, instead of the original 3.73 ratio. Right now, with the bigger tires your RPMs are too low and your speedo is off. Here's a table that I'm going by.

http://www.4wheelparts.com/aux_incl/gear_ratio.html

According to the table, your stock RPM at 65 mph was 2628. Now with 35" tires and the same 3.7 gears your RPMs are about 2328. So changing to 4.11 gears will bring your RPMs back up to around 2600. Instead of changing gears you could get a programmer that will allow you to adjust your speedometer for the bigger tires, but it will still be a dog out of the hole. Going to 4.1 gears will probably hurt your mileage a bit on the highway, but it might even improve your city mileage.

ehhget

You dont have to change anything. Just get a programmer like a hypertech and it lets you adjust your computer for the new tire size. Do not do 4:10's your truck will be screaming at 65 mph!

BassinProf

No offense, but I dont understand your reasoning Ehhget.

Can we agree that his truck didn't come with 35" tires? Assuming his truck came with 31" tires, moving up to 35s is an increase in tire diameter by 13%.

35/31 = 1.13

This means that the tires are spinning 13% faster than before at the same RPM, so RPMs will be down relative to stock. The way to makeup for the bigger tires is to increase the gears by an equivalent 13%.

3.75 * .13 = .49 (necessary gear increase)

3.75 + .49 = 4.24 (equivalent gear ratio with 35" tires).

According to the fomula, going to a mythical gear ratio of 4.24 will produce the exact same RPM as stock. Since he sounds like he's concerned about mileage, 4.1 gears would provide a good compromise between power and mileage. If anything the RPMs will still be slightly less than they were with the stock 31" tires and 3.75 gears.

Am I overlooking something? If so, tell me what. By the way, my truck came with 4.1 gears and runs just great with 35" tires.

Also, won't a programmer just mess with the shift points and speedometer? Programmers can't change the physical gear ratio, right?