07 silverado tire size/ gear ratio change

[white Z24]

ok so im gonna do my buddies 07 silverado classic 2wd. he lifted his truck and got a 410 gearing sitting on 20's with 33in tires..

ok so my question is as for the tire size do i measure the exact height of tire/wheel and put it in on the commit? and for the gears do i just put 410 on the gearing? i never did a truck or tire size change.. any help would be greatly appreciated

[93camaro_zzz]

If you want to do it 100% right, mark his tire on the sidewall right where it would meet the tread. Then have him drive forward until that mark comes around again and hits the ground. However long that line is that it left, just divide it by 6.283, and that will be the exact radius of the tire. Then, just like you said, enter 4.10 for the gear ratio.

[white Z24]

i don't get what you mean sidewall?

[JamesLinder]

you mark the sidewall of the tire with chalk and continue the mark on the driveway so you can see where you are starting from.
then you move the truck forward until the mark on the sidewall of the tire goes one revolution and mark the driveway again
where the mark on the sidewall is after this one revolution. The distance on the driveway between the two marks you made is
the exact circumference of your tire as it is riding on the road and with the weight of the truck on it. Dividing by 3.1416 just
gives you the diameter of the circle with the circumference of your tire. This diameter number is what you enter into the PCM
as tire height.

You can also check your calibration if you have a radar detector that gives you MPH readout (GPS is used) while you are driving.
You can then compare your speedometer reading to the radar detector reading while driving at constant speed, and see how far off your
speedometer is from the radar detector (the later being considered correct). This typically gives you a percentage error that you
can use to correct tire height in your PCM for fine tuning your speedometer to match your GPS readings.

[JamesLinder]

sidewall is the part of the tire sticking out of the rim - from the rim to the ground. Not the tread, but the SIDE of the tire.

[white Z24]

nice thanks a lot I'm gonna have to try it out now

[JamesLinder]

oh, and just divide by 3.1416 because this will give you the diameter which is equal to tire height.

The other guy gave you a divisor twice this large and that would give you the radius, only one-half
the tire height.