ok guy's here's the problem. I've got a 07 Gt Convertible auto, cold air, SCT XCalkibratior2 and i had to buy some new rear tires the other night as one of my drag radials went out and the only street tires i could get was slightly smaller than my DR's. i had p295-40-zr 18's on there. i now have p275-40zr-18's on there and my speedo is about 4 miles per hr fast. i used my GPS to see my speed and it shows i'm just about 4 miles fast. i went into my SCT unit and my axel ratio is 373 which is correct for this car and added 500 revs per mile to the settings (it was on stock before) and at 42 mile per hr the speedo showed i was doing 80 so wrong way. and i can't subtract from the tire rev so how do i correct this?
I thought about settng my axel ratio to 355's and then adding tire rev's from there but without some data as how many to add this could take sometime to do.
am i even correct here or do i need to do something else?
I need some help here guys. anyone?
Thanks, Charles

 

you need to know the Revs Per Mile of the new tire. once known then put that in as the parameter in the SCT and reload the tune.

check the tire co. website for RPM

 

Cooper doesn't list it on the website that i can find and i work for them.
I oculd ask one of our tire engineers but that will be after the first of the year.
Charles

 

Tire size calculator or if a mere 4 MPH
is bugging you that much, get a tape
measurer outand wrap a tire loaded
with the vehicle weight.
Divide 63360 by the diameter in inches.
Thats revs/mile.

 

Delete with a bad attitude!

 

From a quick check on TireRack.com that size tire from various manufacturers runs from 773 to 786 revs per mile. Most of them are between 780 and 783. So, set the tuner to 780, or 785 and you should be as close as you can get. And set your axle ratio back to 3.73 if that's what you're running.

 

i wonder if i shouldn't set the ratio back to stock and not select 3.73?
what do you guys think?

 

yes, you adjust your speed w/ RPM not the axle ratio

 

I think if you select the stock rear end ratio, you'll have to do a bunch of math to figure out the Revs per mile that will give you the proper speed, whereas if you set your axle ratio to 3.73, which you have, and your RPMs to 780ish, which is what you have, then you'll be pretty much spot on and have an accurate speedo without having to do a bunch of guesswork.

 

IMHO, it's best to measure the effective radius of the tire from the road surface to the wheel centerline with the tire at its normal pressure, and then multiplying by 2. This will give the tire's effective diameter, which is more accurate than either measuring the tire's circumference or its overall height. Then, divide 20,168 by the effective diameter(inches) to get rev/mile.