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My stock pcm says it is set fot a stock 245 50 16 tire as 24.95xx inches (even thought I have 315s on there now). The calculator in HP Tuners says stock tire height is 25.6".
Now, my 315 35 17 tires in the HP Tuners calculator comes to 25.7" which is only .1" difference. Do I plug in 25.7" for the new speedometer calibration, or the stock pcm reading (24.95") plus the .1" difference (25.05")??
Thanks.
Use the 25.7" measurement since it is actually what your tire size is.
OK. I was just wondering if it would matter that when mounted on the car it would be a bit shorter than calculated because of the flat section on the road.
Wrong.
I have found the tyre height calculator completely useless for determining tyre size. If I were to go off that my actual shifts are be out by 10% - not to mention every other car I have fingered.
Measure the tyre height on the car from the centre to the ground and on the car, not to either side or off the car. HPT does not take into account sidewall flex.
Best results are to use a multiplier (*hint HPTuners*) to simulate the sidewall flex. The base modifier that works is:
0.976377952755906 x [calc. tyre height]
In other words, it accounts for 2.4% flex and ballooning.
The way to test is is on the WOT downshifts from 3rd to 2nd. Whatever your WOT downshifts is set at the car should downshift up till 2mph less. This takes into account shift lag and speed increases during the WOT transition.
I agree, you have to take into account the side wall flex, measuring the true loaded radius is the only way to do it. Then i use a gps to verify speedo accuracy which automatically looks after the trans shift points because they are based on speed.
Hardest part it telling the guy that just bought brand new 34 inch tires they are only 32.7 inches.
Using that calculation I get 25.09. I guess that is where I start.Quote:
Originally Posted by MNR-0
Thanks for the info. I knew sidewall flex and balloon would make the number different than the HPT value especially since my stock pcm said the stock tire was 24.95" high.
Oh, I am sorry I didn't know that we need it to be precisely correct down to the .001" degree. Well, since you guys need it very precise, we need to have HPT also add in to the VCM a correction factor for rolling diameter when cruising and WOT. Tire expansion varies greatly (if seen in .x" values) from the static size during various driving conditions. We would also need to have Tire Size vs. %TPS vs. Ambient Temp (or Road Temp) table to be able to accurately correct for the exact tire size. Don't forget different brands/types of tires expand at different rates (all-climate vs. performance vs. drag radials vs. slicks), so there is another factor.
You do not need it that precise. If the VCM did, GM would have added those corrections into the program. But they didn't, they just need a static tire size for the speedometer calculations. You guys do realize that the only true way of seeing the actual speed is by measuring distance over time, and not by computer derived revolutions? And tire growth/reduction is constantly happening, so you can not lock down an exact figure for the VCM calculations really. For the VCM, the static tire size is adequate, anything further is just "splitting hairs".
:cheers:
OK. I have loaded 25.7 for the time being since that is what the conversion calculator in HPT told me the tire was. I imagine it is shorter than that since it is a Drag Radial with only 21psi. But I wonder if that .7" will make that much difference.
I did once get a ticket for 65 in a 55 when I thought I was going 62 (245 tires vs. 255 tires). I just don't want that happening again.
Well I have noticed that my speedo doesn't jive with my GPS and have been wondering if my GPS is off or if I somehow screwed up the calculations even though I triple checked them.
It's off by ~1MPH usually @ 65 (no big deal) but on other days I've noticed my GPS and my speedo to disagree by up to 3 or 4MPH, so If I'm going what I think is "a mere 5 over" and the speedo is really off by 4, I can rapidly hit that magic 10MPH limit where they pop you. I've had 4 tickets in the last 2 months. :(
Obvously I've learned the hard way (fines, court costs and lost time) that I need to trust my GPS, not my speedo even though I supposedly can tune it, and don't drive 5 over. :banghead:
Damn man, I may just load up 25.00 to be on the safe side :o.
so for comparison's sake, i loaded up few different logs and started doing calculations of speed reported, vs speed calculated from gear/final gear/tire height/rpm.
the reported tire height (in HPT) on this particular car is 24.97, but for my calculations i had to set the tire height to 27.4 for the speeds to match closely!
that's a huge difference, and i'm just wondering, what could be a culprit? is it wrong tire size, expansion under heat (2inches though??) or are RPM's just not reported correctly.
HPT people, please let me know if RPM's are always dead on, as it's very imporant to few projects of mine.
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.