Not sure if this is the right place for this question.

I have 31x10.5x15's on my '01 Blazer. I know that the speedo is 5 mph slower than what I am actually driving. My question is about the odometer.

I would have to assume that this is going to be off also. Is there any type of calculation that can determine how wrong it is? I would like to be able to calculate my ( I am sure terrible) mpg's and also for my maintenance schedule.

I know Swartlkk is the local math genius and I am just the opposite.

 

The speedo isn't a fixed number off, it's off by a percentage so it will vary depending on how fast you're going. You can use the same percentage to figure out how many miles different your odometer should be.

(((new tire size - old tire size) / old tire size) + 1 ) * mileage = corrected mileage

That's a modified version of the percent change formula. Just write down the number you get from part in bold and keep it handy. Then every time you need to figure out the corrected mileage or speed you can just multiply by that.

If you're looking at actually fixing this problem, you have a number of options. You can use a hand held tuner like the ones from hypertech (look on ebay for used ones), you can have a mail order tune done (one place that does them), or you could have a local shop take care of it for you.

I bought a used hypertech off ebay as it had some other options I like. Hope that helps!

 

Thanks. I was looking into the hypertech but it is just not in the budget right now. I did check with my regular mechanic and a speed/custom shop and neither one could do it. The speed shop just wanted to sell me the Hypertech. I may try a local tranny shop next. I figured if anyone could do it they could.

 

For that formula, am I using the 235 or the sidewall height?

 

Dont waste your time on a handheld tuner. Get a custom tune from wait4me, pcmforless...etc. They will correct your tires size, firm up your shifts along with give you more power

 

To give yourself a ballpark figure just get on a highway, when you go past a mile marker glance at your odometer (put it in trip mode with tenths) and drive 1 mile or more. If you lose a tenth in one mile it's off by 10%. If you lose a tenth in 2 miles it's off by 5%. Remember that percentage applies to the odometer and speedometer.

 

Technically you could use the tire height or the circumference as long as you're consistent.

If you had 235/75/15 tires to being with, then it should have been about 28.9" tall. Of course you're 31" tall tires might not quite be 31" tall... that will very a little bit from one manufacturer to the next. For instance my BFG All Terrains are actually 30.7" according to their specs. So either check manufacturer's site for specs, or just use 31" and know you're really close.

 

Thanks for the help. I can't afford to have my PCM retunes. Wish I could.

On the highway I drive to work everyday, half the markers are in the woods so it is tough to use that method.

I will go with the tire size formula and see how that turns out.

I appreciate the help.

 

No problem.

I'm not going to push the issue if you're really not interested, but I got my hypertech power programmer III from ebay for just over $100. But it looks like they discontinued that one so they don't have the model number on the site, and I'm not finding any of the old ones on ebay right now. I can try to get my model number for you if you want to try to search by that.