I recently changed gears in my car from 2.73 to 3.42. When I went in to edit the speedometer information I noticed the tire size is listed as 24.88. The tire is a 245-50-16 which calculates to 25.6 inches. When I programmed the PCM is stayed with the factory tire diameter but I'm curious where this 24.88 inch number comes from.
Hey man, how did you calculate that tire height?
2001 Camaro SS A4
Some manufacturers' tires are way off from what you
would calculate. Just measure the true circumference
and divide by pi, that will give you the actual tire height
to use.
I understand how to calculate the actual height. Draw a line the tire, mark the ground when the line is perpendicular to the ground then rotate the tire one revolution(on the car to include the squish of the tire from the weight of the car), , mark the gound again, measure the distance between the two lines and divide by pi . I haven't done this yet, too lazy.Originally Posted by jimmyblue
I'm just curious how they came up with the number.
jimmyblue your a MOD on here now too, damn you must live on the computer.
Well I work 10-12 hours a day in front of the tube and got
to do something to keep from going insane (or more so).
I had same question as yours that I posted in the bug section last month... never got a real answer though.
I still think it's a "display" bug or HPtuner uses some magical math equation to come up w/ that #...who knows.
My guess that's the size of the tire at minimum tread wear. I'll have to go out and measure the true tire diameter of my tires.
Go to my site for all the calculators you need. Here's the tire diameter link.
It's in Editor | Tools | Unit Conversion, too.
we work it out from the PPM and PPrev. VSS has so many pulses per rev (tailshaft rev) and a number of pulses per mile, work out how many revs you do in a mile, factor in the gear ratio to get tire revs/mile and you can work out the tire dia (i forget how many inches in a mile...)
for you A4 guys the trans revs/mile number is tailshaft revs per mile. Again factor in the gear and you should ge the same answer... for M6's this number is junk.
Chris...
I count sheep in hex...
Hi Chris, a question on this if I may.
I looked at 2 stock bins for Commodores with 235/45 R17's - and found the Tyre Size value down as 24.80. I have the same tyres - which were set by a Tech2 change. The tyre size is also down as 24.80.
If you calculate the tyre size (using a number of tools) it actually comes out as 25.33.
The Trans Revs/Mile sanity check for both stock bins is 25.37 - close enough to the correct tyre size.
Because my own Trans Rev/Mile setting was wrong (LS1Edit was used to update diff ratio change but did not update it) I sat down today to calculate it as per your comments here and on another thread.
Revs per mile = 112558 pulses per mile / 40 pulses per rev = 2813.95
The sanity check on this figure returned 24.8 - which matches the stored tyre size.
Since I knew the tyre size was actually bigger, and because the stock settings seemed to take that into account, I calculated the difference between recorded and calculated tyre size, and reduced Trans Revs/Mile by 2.09%.
The adjusted Revs per mile = 2755.0714, with a sanity check of 25.33 - the calculated tyre size.
So... do you know why the stock tyre size and sanity check tyre size differ? Should I set my Trans Revs/Mile value so that the sanity check equals the recorded tyre size - or the calculated tyre size?
Cheers, Doug
on those bins i see:
100196 pulses/mile
40 pulses/rev
3.08 gear
now,
100196 / 40 = 2504.9 tailshaft revs/mile
2504.9 / 3.08 = 813.27922 axle revs/mile (number of tyre revs per mile)
1 mile = 63360 inches
63360 / 813.27922 = 77.90 inches/rev (circumference of the tyre)
77.90 / pi = 24.80 inches
yeah?
No idea why the trans revs/mile differs, i see many that are the same some that are a little off... your mileage varies.. so to speak...
We do our best to calculate the tyre and gear, but honestly there are at least a dozen different ways GM use the PPM, gear and such. Also, at a guess i'd say the speedo and trans cal are done by different engineers.
Chris...
I count sheep in hex...
That makes sense - although it assumes the pulses/mile value is correct
Since there isn't a clear answer on why there is a difference - do you have a suggestion on which way I should go? (Maybe based on what the majority of the bin's you have seen do?)
I can set Trans Revs/Mile so that the sanity check matches the current stored tyre size (2813.95 for 24.8), or matches the calculated size of the tyre (2755.0714 for 25.33).
Oh, here's my current settings
Tire Size - 24.80
Gear Ration - 3.46
VSS Pulses/Mile - 112558
VSS Pulses/Rev - 40.0000
VSS Output Pulses/Mile - 10020
Trans Revs/Mile - 2448.5740 (sanity check 28.50)
So you can see Trans Revs/Mile wasn't updated by LS1Edit. I'm almost too scared to ask what VSS Output Pulses/Mile means.
Hi Doug,
If the VSS Pulses/Mile is not correct then the vehicle speed PID output will not be accurate, we can only assume that GM makes it such that the vehicle speed PID is accurate
The output pulses/mile is the output scale factor to the instrument cluster, the cluster has it's own calibration that converts the number of pulses it receives from the PCM to the actual speed shown on the cluster. Again, we can only assume that GM got this right, and a linear change on the PCM side will results in a linear result on the cluster (which it does). Again, it all assumes the PCM calculated vehicle speed is accurate.
For the trans speed, i would set the trans revs/mile such that the tire check value matches the tire calculated from the cluster. EIther way it needs to be close to the cluster tire calc, otherwise the cluster will display 60km/h but the trans will think you are doing some other speed.
The fact the trans revs/mile is out will mean your shifts are out, if you only did a small gear change or tire change then it's not really noticeable (shifts out by a few mph), but if you change from 3.08 to 3.91's you shifts will be out by over 20%.
Try it yourself, by doubling the trans rev/s mile, all your shift speeds will be half what they are set at - i think i got that the right way around
Chris...
I count sheep in hex...
Thanks for the help Chris. My new logged trans data mades a little more sense after making that change. Wasn't the quick fix for my TCC issues - but now it looks like it will be easier to associate some of the funny behavour to specific calibration fields (where as before it was never close enough).
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