Tire Size Change effects on odometer and speedo...

[Akira]

I am new to customizing so bare with me...

I got a 2001 camaro v6 last september and I needed tires put on it recently, so I went to get some. The guy said that he could up grade my tires to 245\50 R-16 cheaper than buying 235\55 R-16 tires. So I had him do that but he didn't do anything with recalibrating my speedo and odometer.

Do I need them calibrated? And how much are they off by?

Thanks Yal!

[TraceZ]

Go to www.tirerack.com and look up the overall heights of both the original tires and the new ones. If it is almost the same, no problem. If it is considerably different you'll need to have the PCM reprogrammed or get a claibration box to plug into the transmission. You could also adjust it by changing the speedo gears, I believe.

Get a stopwatch and time yourself between mile markers to be sure.

Here is an example....

235/55 height on a goodyear eagle #1 is 26.1"
255/50 height on a goodyear Eagle HP is 26"

I wouldnt worry about that too much.

BUT... the 245/50 is a bit more of a difference at 25.6.
I'm not so sure that would work.

[Akira]

When I went to Discounttiredirect.com and Tirerack.com and put in my model year and model and make of car and all, Discount tire direct listed my 245/50-16 tires as being Plus Zero Size tires. When I went to Tirerack.com they listed them under their optional tire sizes for 16 inch rims, they list them...

So do I need to find someone to recalibrate my speedo? Or am I okay? AND WHAT THE HECK DOES Plus Zero Size MEAN?

[wolfdogs]

you wont find somebody that can recalibrate you speedo cheap. you need a special program to do that. its computerized and the dealer cannot do that function with thier gm tech tool.

your new tire is only a half inch taller than the old one. you dont need to do anything. the design rim width is the same also.

[Christopherrr]

This is what I came up with....


Nitto Tires

235-55-16 height 26.14" rpm 795
245-50-16 height 25.59" rpm 811

that's about 1.97% more rpms versus stock...or 16 rotations more on the 245s per mile. So your speedo is saying 60, but you're really doing 61.182 mph (I think I did my math right) so it's not a biggie.

that's only .55" difference...is that enough to worry about???

I don't think it's enough to effect anything.

[Christopherrr]

Originally posted by Akira
When I went to Discounttiredirect.com and Tirerack.com and put in my model year and model and make of car and all, Discount tire direct listed my 245/50-16 tires as being Plus Zero Size tires. When I went to Tirerack.com they listed them under their optional tire sizes for 16 inch rims, they list them...

So do I need to find someone to recalibrate my speedo? Or am I okay? AND WHAT THE HECK DOES Plus Zero Size MEAN?

Plus Zero means, you didn't move up a rim size....

So, Plus One means you moved up to 17 and 255s (I know the rim goes up one inch...but not sure about tire size)

Plus Two 18's and 275

Get the idea???

Hey there is only one man on this forum that knows for sure...and that's KenC

[Injuneer]

The math is almost right..... his new tire is 0.55" SHORTER than the stock 235/55. When you put a smaller tire on than the speedo is calibarted for, the tire rotates more times to cover the same distance. Hence, when the speedo says you are going 60mph (based on the 235/55), you will actually be going 58.8mph.

Your odometer will also be affected.... when you drive 1000 actual miles, the odometer will record 1,037 miles.

Not sure if they ever made a Hypertech Power Programmer for the 2001 V6.... if they did, you could correct the tire diameter with that. Also not sure why Ken said the dealer couldn't correct the tire diameter.... I would think the Tech 2 could access it and change it.

The 255/50-16 might have been a better choice.

[Akira]

So I may need it recalibrated but it is not really manditory, right?

Like Wolfman said that I shouldn't worry about it, then if the speedo is only off by about a mph then I shouldn't worry. But the odometer of by 37 what could I do about that or could I not worry about it too?

Thanks though for all the information thus far.

And the reason I didn't get the 255/50-16 is because they cost about $240 more than the set of 4 I have currently on my car which cost a little more than $400.

Thanks again...

[Akira]

Oh and does the speed rating and load rating have an effect as well because I notice while on tirerack.com that two tires where basically the same for some small car. I think it was like 205/60-15 94T and a 205/60-15 94V or something like that. And their revolutions per mile were 813 and 817 respectively.

Why is that possible...?

Thanks...

[TraceZ]

Originally posted by Injuneer
The 255/50-16 might have been a better choice.

Didn't someone else say that earlier??

[Injuneer]

Originally posted by Akira
Oh and does the speed rating and load rating have an effect as well because I notice while on tirerack.com that two tires where basically the same for some small car. I think it was like 205/60-15 94T and a 205/60-15 94V or something like that. And their revolutions per mile were 813 and 817 respectively.

Why is that possible...?

Thanks...

Tire sizes are "nominal"..... they need to be "close" but they don't have to be exact. Maybe Ken could tell us what the acceptable diameter tolerance is for any particular size?????

Maybe the two manufacturers made the casing the same size, and one has thicker tread..... a 2/32" tread depth difference would account for that small diff in revs/mile. Or maybe one manufacturer measured it on a 6.5" rim, and one measured it on a 6.0" rim. Or, maybe the air pressure affects it. Or maybe the allowable load deflects the one with the higher rating less.

Ken........ ????????

[gimp19]

Heres a tire size calculator http://www.miata.net/garage/tirecalc.html

[wolfdogs]

Originally posted by Injuneer
Tire sizes are "nominal"..... they need to be "close" but they don't have to be exact. Maybe Ken could tell us what the acceptable diameter tolerance is for any particular size?????

Maybe the two manufacturers made the casing the same size, and one has thicker tread..... a 2/32" tread depth difference would account for that small diff in revs/mile. Or maybe one manufacturer measured it on a 6.5" rim, and one measured it on a 6.0" rim. Or, maybe the air pressure affects it. Or maybe the allowable load deflects the one with the higher rating less.

Ken........ ????????

correct. the tolerance is .5" in o.d. for passenger car tires. thats why its not a good idean to ever mix different brand designations on the same axle.

the T speed rated tire's belt package is not as thick as the V speed rated tires, but..the v's tread surface remains more "flat" due to less circumfrance exspansion during revolutions, hence forth the 3 more rotations a mile. tread thickness may very well be more on the T, which is designed to provide longer tread life for the average tire consumer, unlike the V and higher speed rated tires.

air pressure will have more effect in rolling o.d. on the T than the V due to casing structure integrity.

[Akira]

Hay thanks yal...

It took me a while to find a place around here where I live to do a measured mile but I did eventually find a place. I tried driving 60 mph on my speedo up and down the measured mile for 3 tries. At the end of the 3 tries, my stopwatch said something like 1:00.34, 1:00.48, and 1:00.24. So I'm assuming that it didn't throw my car off any. My reactions time before at a drag strip where about .450 to about .600... I have a tendency to red light for some stupid reason.

I also tried the measured mile for my odometer. So I reset it and drove up the measured mile and there was no difference in my odometer. I also want and got me some gas and drove home to see if my odometer gave me the exact thing that my mom's odometer gave her, and it did.

We called a dealership around here and they said it wasn't off any, but to do that test.

So thanks for telling me about that test. But how is that possible, I mean even miata's tire size calculator thing said it should be off by 60mph on dash, really doing 58.8mph. And I wasn't.