What numbers off the tire itself do I put into the hypertech programmer to adjust the speedo? I just messed up a rear tire (hit a curb ) and had to put on a smaller tire (same width, smaller sidewall) on the rear wheels.

 

You need to calculate the tire diameter, or go to a site like Tire Rack, and look up the diameter in the specs for the specific tire and size you bought.

You can calculate the diameter:

Divide the section width (e.g. 245 or 255) by 25.4. That will give you the tire width in inches.

Multiply that by the "aspect ratio"... (eg a 245/50 has a 50% aspect ratio, so multiply the tire width in inches by 0.50). That gets you the height of the sidewall.

Multiply the sidewall by 2, and add it to the wheel diameter. That gives you the overall diameter.

Or, post the tire size and we'll tell you the diameter.

 

ok.....using your formula, Fred, for 245/50/16 tires I got -
25.6456692913 and about 500 other numbers I don't want to type out

 

Thanks. I never knew it involved anything like that. I figured it was either the 245 or the 50, learn something new everyday .

 

Feel free to "round" to 2 figures after the decimal point.....

That gives you 25.65, which is what GM puts in the stock PCM for the 245/50-16. That means with the HPP, which only offers you numbers in 1/4-inch increments... 25.50, 25.75, 26.00..... you've got another major decision to make........

 

umm for some reason i cannot figure out your formula but hey i never was good with math at all, if i have 245/45/ZR17 's what is my tire height? I couldnt find it on tire rack.

 

bump _______

 

I use 24.75" as the height for a stock 245/50/16 tire.
Thats what my tape measure tells me is the height. My speedo is right on so I don't understand why people use 25.65". That number will make the speedo read fast.

 

why not put the smaller tire on the front

 

My 275/50/15 nittos and my old 315/35/17 nittos were both 25.6", so I just used 25.75". Stock height in the HPP is 25.75". My speedo is dead on also.

 

You keep posting your 24.75" dimension, but I've never seen anyone agree with you.

GM uses 25.65" in the stock PCM programming with 245/50-16's.... they must be wrong.

Virtually every manufacturer of a 245/50-16, 275/40-17 or 315/35-17 uses 25.5" to 25.8" as the diameter, in their published tire dimensions.... they must be wrong. In fact, a quick check of Tire Rack shows 36 different tires in 245/50-16. The published dimensions are:

25.5" - 1 tire model
25.6" - 4 tires
25.7" - 19 tires
25.8" - 3 tires

..... they must be wrong.

I've run 245/50-16 GS-C's, 275/40-17 GS-C's and now 315/35-17 BFG DR's, and the smallest diameter I have ever measured.... doing it the correct way, is 25.58".... I guess I'm wrong

Might want to think about it.

To answer cz28s question, a 245/45-17 would be 25.68". But according to Tire Rack, they do not carry any tires in that size. Hoosier makes that in their Sports Car DOT - Radial series, and uses 25.3", but that is a "shaved" tire for road racing.

 

Tires are rarely the diameter the mfg. claims. 245/50-16 BFG DR's are a good example. Besides, "advertised" diameter (even if it's correct) isn't the most accurate way to set your programed tire height. What you really need is the rolling diameter (with the weight of the car on it).

If you've got a Hypertech Programer the book that comes with it tells you how to do it. If you don't have the book, or you're just someone else who would like to know, here's what you do:

1) Put a chalk mark on the tire where it contacts the pavement (on the side wall pointing straight down). Put a corresponding mark on the pavement.

2) Roll the car in a straight line until the chalk mark on the tire makes one complete revolution and is pointing exactly straight down again. Mark the pavement at this point.

3) Measure the distance between the two marks on the pavement (in inches) and divide the measurement by 3.1416. This will give you the rolling tire height.

BFG DR's inflated to 20 PSIG have a rolling diameter of almost exactly 25.50 inches, not the 26.1 they are advertised to be.

 

I'm not suggesting that you blindly use the manufacturer's published data..... I was simply making the point that the preponderance of the evidence would suggest that 24.75 is not necessarily the correct diameter for all 245/50-16's. And, yes in a previous post with this same individual, I went all through the chalk mark and tape measure procedure... and that was what I meant when I said "doing it the correct way".

Its entirely possible that 2cub49's tire diameter measures out to 24.75", although in the other thread he explained that he measured the height directly with a ruler, not using the chalk mark method. My point is simply that there are numbers other than 24.75" to use when programming.

 

Sorry, I didn't see your explanation for the chalk mark method. My apologies And I know you weren't "suggesting that you blindly use the manufacturer's published data".