GMs Stock Tire Size?

[2xLS1]

Why is the tire size GM uses in all their stock calibrations always smaller that what tire size calculators show for stock tire sizes? Seems to be that way no matter what the vehicle type.

[12secSS]

I would venture to guess it is because they do not want you going too fast, so if the speedo is slightly faster, then you would be OK. I always change it anyway to the measured tire height.

[EC_Tune]

Most of the tire size calculators don't take into account that the tire "squishes" when you put 1000 lbs on it.

The attached file is actually a ZIP file renamed PDF since we can't upload ZIP files yet. Inside you will find a pretty good tire calculator. (At least for truck tires) Car tires don't seem to work as well as there is less "squish" on them and the profiles are lower (20-45 series instead of 55-85 series) However the best way to get tire height is to do a roll-out.

Enjoy!

[johnh]

Yeah just noticed that too.. Stock Calibration shows 24.91 in, all the tire calcs, show 25.66 for my WS6 wheel/tire (275/40/17) combo. In fact none of the stock combos come close (25.61 245/50/16, 26.1 235/55/16). However, using the old fashioned way--measuring tape..shows just a bit under 25"...

[jknox]

On a similar vein, I've noticed that there's always a discrepancy between "vss pulses per mile" and "trans revs per mile". I would think that vss pulses should be equal to trans revs/mile times the vss pulses per trans rev multiplier.

But it ain't... And I'm guessing it's for the same reason - that nice federal reg that says that the average speed for a vehicle's production run should not be faster than what the speedo shows.

[EC_Tune]

Trans revs per mile & VSS are two separate animals. Trans Revs/Mile help to calculate shift points etc. VSS is strictly for the PCM & speedo.

[jknox]

Even so, shouldn't they be linked together normally?

[EC_Tune]

Yes, Usually to get VSS PPM you usually take trans revs/mile & mult by 40.

[bigmur]

I just looked at my tire size that came stock. It is at 30.84 and gears at 4.10. I used unit conversion and It came back as 31.6. Now the Vss Pulses per Mile, Trans Revs/mile, Shift Speed, Shift speed WOT, TCC Apply release to name a few are highlighted in red? Obviously I need to change something to adjust this correct?
New to HPT. Any help?
Attached bigmur3 is before and tires is after

[EC_Tune]

If you made a change in the speedo file (tire or gear changes) and clicked "Commit" it will automatically calculate the new shift speed/tcc lockup, VSS, trans revs etc.

[bigmur]

Thanks. I thought so. but just to make sure. I took my tire size. 265/75/16 and plugged it into the convertor an came up w/ 31.6 before it was like 30.84. Sound good? I saw another way to do it you had a post but the file was corrupt. wouldn't let me open.

[EC_Tune]

If they are the same size as stock, don't change anything unless your GPS says you are way off (+/- 2 MPH is way off in my book though )

[bigmur]

thanks for the help.
i'll check it out and see what it does

[BBA]

Ummm...tire squish? Well, if the tire was not coupled to the rim and was allowed to spin I could see that, but, the tire tread length around the circumference is what matters because the tires move exactly that far for every single revolution, squish or no squish.

But...squish sounded like a good answer if you didn't think things over very well.

I take the easy route...my GPS shows me my speedo is perfectly accurate so I don;t have to mess with it.

I would bet GM does the smaller size calculation as an average to make up for mid life tire wear that does reduce the circumference.

[Scooter70]

I read an article in one of the car magazines a while back that compared speedo accuracy across a bunch of makes and models. The end result shows that GM speedometers were more accurate than any other carmaker. I guess for whatever reason they do it, it's working.

[EC_Tune]

BBA - What you aren't taking into account is the tire radius reduction caused by placing weight on the tire and the contact patch that it creates. So tire "Squish" is an appropriate term. Thats what it does when you put 1000 pounds or more on it. And GM DOES take that into account per Scooter70's response.

If you measure the loaded diameter you will find that your "BFG LT315/70/17 35" tire" is now 33.69" on your Hummer H2. The radius of the tire will be shorter from the axle to the ground (always) than the radius measured from any other point. Guess what, tires don't remain *round*, even and especially while driving.

[2xLS1]

Ummm...tire squish? Well, if the tire was not coupled to the rim and was allowed to spin I could see that, but, the tire tread length around the circumference is what matters because the tires move exactly that far for every single revolution, squish or no squish.
But...squish sounded like a good answer if you didn't think things over very well.

I take the easy route...my GPS shows me my speedo is perfectly accurate so I don;t have to mess with it.

I would bet GM does the smaller size calculation as an average to make up for mid life tire wear that does reduce the circumference.

I agree with this and am surprised a thread I started back in 12/06 has lasted this long.

[WS6FirebirdTA00]

So you are telling me that if that "squish" didn't matter then if you have a 44" tire on a 15" rim go completely flat and you are riding on the rim that you still have the same number of revolutions?

If that is the case, then why do people blow up posi rears when you have a large difference in tire pressure?

My roll out is TOTALLY different from actualy size. Read Doug's post.

Also, a bias ply tire will grow at speed, so it loses the compression and gets "taller". So in that case, you guys are saying that is impossible too?

Just a few points I am trying to make. You need to measure the radius of the tire from the ground to the axle centerline. I see what you are saying with the circumference, but the bottom line is my speeds will not match up with that calculation.

[2xLS1]

So you are telling me that if that "squish" didn't matter then if you have a 44" tire on a 15" rim go completely flat and you are riding on the rim that you still have the same number of revolutions?

Think about that for a moment. Do you really think you are riding on the tread circumference on a completely flat tire? Poor analogy. Unless the tread length on the contact patch shrinks with squish, then the circumference of an inlfated tire does not change nor it's roll out. Amd a tire's circumference should grow if a tire's diameter grows at speed.

[WS6FirebirdTA00]

Radials do not grow at speed.

Flat tire or tire low on air for drag racing, same thing. You are not driving around on a perfect circle all the time, thus the reason for the difference. You don't have to believe it but if you guys were right my tires I use at the track would give me that extra 3 mph I need