[student's tundra]

Originally Posted by imjin

I have 33's with stock gearing and I get between 12 and 14, sometimes 15 (rare) if I drive like granny and coast on the downhills. It's not a simple answer to compare to two sizes outright unless you factor in what else you've done to be able to run 35's, which usually means you've regeared to accommodate them and regearing will have a direct impact on your mpg since it affects rpm. For a 2001, we have the Tacoma 8" (210mm) rear and 7.5" (190mm) front differential, so aftermarket gearing choices are usually limited to 4.11, 4.56, 4.88, etc... There are no aftermarket 4.30 gears for the early year 1st gen Tundra, but we can use the Tacoma 4.30's which are insanely priced at the dealer so finding a used set is usually the way to go on those.

I have a set of used Tacoma 4.30 gears that I'm waiting to put in to accommodate the change in tire size, but I don't tow anything either, so 4.30's are what I'm looking for. If I did, I'd go lower with 4.56 or even 4.88 and especially if I went to a 35" tire, 4.56 or a 4.88 would be the way to go. Just going from a stock 30.6" tire to 33's while keeping stock gearing, my mpg and a noticeable but slight amount of low-end torque dropped off. What you're looking for is a good mid-range spot that balances your tire size with your gear ratio to keep it as close as possible to stock mpg performance. If you go with larger tires and don't regear, it's difficult if not impossible to keep mpg up because the motor and drivetrain are tuned to the factory tire setup, so you have to do additional things to accommodate that change and regearing is usually the way to do it. Also keep in mind that regearing will affect city versus highway mpg at certain speeds as well since you're moving the powerband in a different direction.

In my case, a 4.30 gear puts the ratio at about a 9% increase over stock gearing (3.91:1), but adding 285/75/16 (32.83" diam) tires drops it to about a 7% increase over stock which puts rpms at around 2400 running at 72mph. Stock 265/70/16 tires are 30.60", so with larger tires comes more weight and mass that the drivetrain and motor have to be able to move.

You can use the calculation below to figure up what your axle gear ratio should be close to. Falling below the New Axle Ratio value means you'll be slightly under-powered at the rear wheel and falling above the number means you'll be slightly over-powered and both directions affect mpg.

New Tire Diameter
------------------------- x Old Axle Ratio = New Axle Ratio
Old Tire Diameter

Using my numbers as an example below, stock tires were 30.60" (diam) and new tires are 32.83" and my stock gear ratio is 3.91. 32.83 divided by 30.60 = 1.07, then multipled by 3.91 comes out to 4.19. Well, you can't get a 4.19 gear and 4.11 falls slightly below what would be required, so 4.30 is the next one in line. It's a little over what I'd need, but it will give a little bit more rear wheel power than I have currently

32.83
-------- x 3.91 = 4.19
30.60

Well said, I will just add a couple of things in that not only the tacomas but also the V-6 4x4 tundras will also match up the 4.30s (as mine bought new were listed for a V-6 4x4 tundra). In addition, I will add that the Superlift Truspeed calibrator should be on your list as well unless you like doing math calculations
TruSpeed™ Speed Sensor Recalibrator · Superlift · www.superlift.com