Tire size change check please

[MMGT1]

I've put the summer rubber back on the car and was hoping someone could confirm what I have for the calculations? Tire is a 235/45/17(new one) Stock size was 225/50/16

Attachment 11581

Paul

"Just in case some of you that think accuracy in tire caliration does not matter. If there is a 15% difference between rear wheel speed and front wheel speed (or even side to side on the front) the abs will throw a code. Big deal you say, 15% is a lot, well not really, that is only .7 of an inch diameter difference on a passenger rated tire (read as soft side wall). If your abs lamp is kicking on while your cruise around at 50 to 60 kmh for say 3 minutes then you probably have a tire sizing issue. If one were to actually take the time and measure a stock tire on vehicle, then calculate that same tires size and additionally compare it to the calibrated tire size you will find a substantial trend toward the calibrated tire size being smaller than the calculated size.

Get out your measuring tape and measure from the exact centre of your axle to the floor, thats your real radius, now double it-thats your measured diameter, now calculate the the diameter from the tire size on the side wall, now average the two numbers and you will most likely have a number that matches the calibrated number."*--copied this from another post--*

Is this the right way to go?

[MMGT1]

Here is what I have:
Measured from center to ground= 11 13/16" Double that to get the Radius and am left with 23 5/8" or 23.625" actual
From the listed size (235/45/17) = 25.326"
Add 'em and average I get 24.475" to place into the VCM
*I think this is right*

[Russ K]

The formula for a different dia tire is; new advertised tire dia/old advertised tire dia* the stock current tire dia value. Then enter this new value into the new tire size field. Also enter the stock gear ratio.

Russ Kemp

[MMGT1]

Thanks Russ. You *fixed* my tire size for me last year but couldn't figure out how you got the size you did to put in the VCM. Had the winters on and just used the stock values from another car that had the same size in stock form for those. Again, many,many thanks....
Paul

[krunchss]

I plan on using a GPS to make mine accurate soon. But I don't expect if will be off much. Using the calculator will get you pretty close.

Krunch

[louvered97gtp]

I hate math.
So stock tire (ST)= 225/60-R16.
The new tire (NT)= 225/55-R16

So first we calculate stock tire dia. ST=225x.60x2/25.4+16= 26.6299?

Then we calculate new tire dia. NT=225x.55x2/25.4+16 = 25.7441?

Are these formula's even correct? Finally, the formula as stated should be :

The formula for a different dia tire is; new advertised tire dia/old advertised tire dia* the stock current tire dia value. Then enter this new value into the new tire size field. Also enter the stock gear ratio.

Russ Kemp

25.7441/26.6299 x 26.6299???

Magic number, aka new tire size is 25.7441 ??<--same value as new calculated tire dia. so what gives? Why all the extra math to come to same number? I dun't getit. Are different values supposed to be used?
Did I happen to mention I hate math?

[krunchss]

If what you have done so far is correct, you have found out that your new tire is 25.7441 / 26.6299 X 100 = 96.674% of the original size. Take you rcurrent pulses per mile and multiply by .96674 and that should be your new pulses per mile....I think.

Wait for a couple more replies to confirm.

Krunch

[MMGT1]

I just can't get this one right. Man this is driving me stupid, as if I wasn't already. I have done the calculations and the thing still doesn't shift right. Russ did one for me last year and it worked perfectly, now with the summer tires back on, it just won't shift right. Anyone able to have a look at this? I'll post what I have later but my first question is, Does the HP Tuner not automatically set the vss pulse for you when you change tire size?

original tire size is 225/50/16
new tire 235/45/17

entered 24.51 as the overall height and a 3.29 gear.......did I miss something????

24.12 was original w/ calculated tire reading 24.13

vss was 25080, then entering the new tire size made it go to 24690, tans revs went from 2750.4491 to 2707.6756

Does this look right to you guys?

[louvered97gtp]

Don't feel bad. I have been screwing with mine for 6 months and can't get my speedo correct either. I've used stock gear ratio, 3.29, and 3.69 values in the table but nothing seems to works. Hopefully russ can give us a lesson on this. I'm using 55 series tires so I'm assuming that's throwing me off as well.

[krunchss]

Well, according to an online tire calculator I found, your 225 50R 16 tire is actually 24.9" high, not the 24.12 that you entered. That's almost a 1/2" difference in tire height. Your 235 45R 17 is 25.3" high, not 24.51. That's a difference of over 3/4".

Could that have skewed your results?

Krunch

[MMGT1]

That's what has me totally perplexed!!! The car came with 225/50/16 out of the factory, yet that(24.12) is the measurement in the origional scan that I took of the car when I got the hp software...I am the origional owner as well so I know that the info is what it had out of the factory, wierd eh'

[krunchss]

What kind of a car are we talking about here that has 225/50/16 from the factory? Both my Monte and gf's Impala both had 225/60/16 from the factory.

Krunch

[MMGT1]

Attachment 12861

This is the stock file from the car 2000 Grand Am GT1

[GRRRR8]

The method I have always used with different software is: mount the tires on the car, face valve stems at 6 o'clock, make a markon the ground and roll vehicle back until they are at 6 again. Measure between both points and divide by 3.14. Alaways within 1 mph 2 max at high speeds.