Reprogram Tire Size
Oct 19, 2024 | 01:25 PM
#1
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From: cold & windy
Reprogram Tire Size
Looking around different forums, I ain't found nothing on folks reprogramming different tire size in Hummer H3s so that the speedometer & odometer read correctly. Are folks not fixing this when changing tire sizes, or are they just not talking about it? This correction doesn't just ensure the speed & mileage shown on the dash are right. It also affects ABS behavior (and I imagine traction control + stability control + the simulated limited-slip action) to ensure they all function correctly.
I simply want to confirm what methods are available aside from mailing the computer back to a shop (or buying a crappy handheld generic tune module). We're happy with our PCM of NC custom tune, which corrected for our current 285/75-17s (33.8"), but I'm eyeballing some 255/80-17s for our next set (32.8 to 33.3", depending on the model). Of the stealerships here in town: one quoted $110 for 1hr of computer flashing, and the other said it'd be ~$350 for GM to write a custom flash & them to download it to the truck... which is absolute horses#!t because I did my previous 99 GMC with a friend's Tech2 in about 10min after changing to 33s.
I obviously don't still live near that friend and ain't sure if the E38 PCM is as easy as a 0411 to update the revolutions per mile for the VSS. Thoughts?
I simply want to confirm what methods are available aside from mailing the computer back to a shop (or buying a crappy handheld generic tune module). We're happy with our PCM of NC custom tune, which corrected for our current 285/75-17s (33.8"), but I'm eyeballing some 255/80-17s for our next set (32.8 to 33.3", depending on the model). Of the stealerships here in town: one quoted $110 for 1hr of computer flashing, and the other said it'd be ~$350 for GM to write a custom flash & them to download it to the truck... which is absolute horses#!t because I did my previous 99 GMC with a friend's Tech2 in about 10min after changing to 33s.
I obviously don't still live near that friend and ain't sure if the E38 PCM is as easy as a 0411 to update the revolutions per mile for the VSS. Thoughts?
Oct 19, 2024 | 03:23 PM
#3
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Ask PCM of NC. Since you have their tune they might just do it for free.
And yes, A F A I K all the ECM cares about is revs per mile; doesn't give a damn about tire size, gear ratio, or any other things that affect it. Only their cumulative outcome.
And yes, A F A I K all the ECM cares about is revs per mile; doesn't give a damn about tire size, gear ratio, or any other things that affect it. Only their cumulative outcome.
Oct 20, 2024 | 08:03 PM
#6
When I was at the dealership, the Tech 2 could only reprogram a stock size tire, with the exception of a 285/70/16. A 255/80-17 doesn't sound like a stock size, is it? When a customer installed some Silverado SS wheels to his Tahoe, you could not just change the tire size in the EBCM. I had to call GM and get a reflash from them for that stock tire size. It was 5mph off with just changing the size in the EBCM.
Oct 20, 2024 | 08:44 PM
#7
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Right: the GM setups only account, DIRECTLY, for the sizes they put on NEW vehicles.
OTOH, it may well be, that SOME aftermarket wheel/tire combos, would come close enough to some GM size or other, to use those calibrations. Keeping in mind that the resolution of these things isn't INFINTELY fine; error of 1 or 2% might well be acceptable, or even unavoidable, in what's attainable. (8 bit? 12 bit? Iunno) And, that as tires wear, their radius changes, and the speedo calibration changes accordingly, ANYWAY. Meaning, you can get it perfect TODAY, maybe, butt a year or 2 on down the road, or a new set of tires or a different brand even of the same "trade" size, maybe not so much. Work around a tire store for a bit and you'll see how much variation there can be. 5% is kinda YYYYYUUUUUUUUJJJJJJE in 2024 though.
OTOH, it may well be, that SOME aftermarket wheel/tire combos, would come close enough to some GM size or other, to use those calibrations. Keeping in mind that the resolution of these things isn't INFINTELY fine; error of 1 or 2% might well be acceptable, or even unavoidable, in what's attainable. (8 bit? 12 bit? Iunno) And, that as tires wear, their radius changes, and the speedo calibration changes accordingly, ANYWAY. Meaning, you can get it perfect TODAY, maybe, butt a year or 2 on down the road, or a new set of tires or a different brand even of the same "trade" size, maybe not so much. Work around a tire store for a bit and you'll see how much variation there can be. 5% is kinda YYYYYUUUUUUUUJJJJJJE in 2024 though.
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Oct 21, 2024 | 10:28 PM
#8
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From: cold & windy
yeah, i get the variance (which also depends on your wheel width & inflation pressure). once installed & inflated properly, i'll measure circumference. then i'll split the difference between new tread depth & 3/32" to determine the programmed height and/or revs per mile.
i wasn't aware of the "stock sizes only" limitation of the Tech 2... but depending on which tire brand we go with, a 255/80-17 would be pretty close to an Adventure model that had stock 285/75-16s (~32.8")... only an inch & change skinnier.
i also found out a shop a couple friends & i have used here said he'd hook me up so long as his tool let him do it. otherwise, PCM of NC is reasonably priced for the quick change so long as we're able to deal with the down time. at that point, i'd also have him pull a little timing to run on 89 instead of 91.
i wasn't aware of the "stock sizes only" limitation of the Tech 2... but depending on which tire brand we go with, a 255/80-17 would be pretty close to an Adventure model that had stock 285/75-16s (~32.8")... only an inch & change skinnier.
i also found out a shop a couple friends & i have used here said he'd hook me up so long as his tool let him do it. otherwise, PCM of NC is reasonably priced for the quick change so long as we're able to deal with the down time. at that point, i'd also have him pull a little timing to run on 89 instead of 91.
Oct 24, 2024 | 07:50 PM
#9
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i'll measure circumference
Measure the ROLLING RADIUS; i.e. from the center of a rear wheel to the ground. The wheel RPMs don't care about what the part of the tire that isn't holding up your car is doing.
Then add your fudge factor adjustment based on wear from there. Multiply by 2 x pi to get the rollout per rev. Should come out somewhere in the 80s - 90s of inches kind of range.
The Tech 2 deal is, GM calculated all the #s needed for their combos, and it shows you the list, and you select one. You can't just enter random #s.
Oct 24, 2024 | 11:02 PM
#10
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From: cold & windy
Wrong way to do it.
Measure the ROLLING RADIUS; i.e. from the center of a rear wheel to the ground. The wheel RPMs don't care about what the part of the tire that isn't holding up your car is doing.
Then add your fudge factor adjustment based on wear from there. Multiply by 2 x pi to get the rollout per rev. Should come out somewhere in the 80s - 90s of inches kind of range.
The Tech 2 deal is, GM calculated all the #s needed for their combos, and it shows you the list, and you select one. You can't just enter random #s.
Measure the ROLLING RADIUS; i.e. from the center of a rear wheel to the ground. The wheel RPMs don't care about what the part of the tire that isn't holding up your car is doing.
Then add your fudge factor adjustment based on wear from there. Multiply by 2 x pi to get the rollout per rev. Should come out somewhere in the 80s - 90s of inches kind of range.
The Tech 2 deal is, GM calculated all the #s needed for their combos, and it shows you the list, and you select one. You can't just enter random #s.
1. check your air pressure
2. mark the center of where the contact patch meets the ground
3. roll straight forward one revolution
4. measure the distance between where the mark hit the ground the 1st and 2nd times
that will provide your rolling/loaded circumference to use in a revolutions per mile calculation.
as for entering non-stock sizes with the Tech 2:
1. With the Tech 2®, select Service Programming.
2. Identify vehicle information as requested by the Tech 2®.
3. Select the type of module you are programming.
4. Select the type of programming to be performed.
5. Verify the displayed VIN with the vehicle VIN. If the displayed VIN does not match the actual VIN, write down the actual VIN and correct the VIN at the Techline® terminal.
6. When complete, Exit Service Programming.
7. Turn OFF the Tech 2® and disconnect the Tech 2® from the vehicle.
8. Turn OFF the ignition.
9. Connect the Tech 2® to the Techline® terminal.
10. Select Service Programming.
11. Select Tech 2® as the tool you are using.
12. Select the type of programming to be performed.
13. Verify the displayed VIN with the vehicle VIN. Correct the VIN as necessary.
14. Select the type of module you are programming.
15. Select Speed Calibration. You need to know how to get to this place
16. Select Normal.
17. After the download is complete, turn OFF the Tech 2®.
18. Disconnect the Tech 2® from the Techline® terminal.
19. Install the Tech 2® to the data link connector (DLC).
20. Turn ON the Tech 2®.
21. Turn ON the ignition, with the engine OFF.
22. Select Diagnostics.
23. Select the correct model year of the vehicle.
24. Select XXXXXXXXXXXXXXX
25. Select Powertrain.
26. Select the correct engine size.
27. Select the correct transmission for the vehicle.
Important: Be sure to select the correct transmission for the vehicle. Even if the correct axle ratio and tire revolutions per mile are entered if the transmission selected is not correct for the vehicle, inaccurate speedometer calibration will result.
28. Select Special Functions.
29. Select Engine Output Controls.
30. Select Speed Calibration Reprogramming.
31. Select Reprogram Speed Calibration.
Important: This procedure will allow the user to change the speed calibration for the vehicle. This procedure should only be done when a new ECM/PCM has been installed or when the axle ratio or tire size has been changed.
32. Select Done to continue.
33. Enter the axle ratio using the F0 through F9 keys on the Tech 2® key pad.
34. Select Done.
35. Enter the tire revolutions per mile using the F0 through F9 keys on the Tech 2® key pad.
36. Select Done.
37. At this point all entered information can be verified if anything needs to be changed. Select previous screen and the input information can be changed.
38. If the information entered is correct, select reprogram and the Tech 2® will indicate Reprogram Successful.
39. At this point if for any reason the Speed Calibration still needs to be changed select restart. The axle ratio and tire revolutions per mile can be re-entered again without having to go back to the Techline® terminal.
2. Identify vehicle information as requested by the Tech 2®.
3. Select the type of module you are programming.
4. Select the type of programming to be performed.
5. Verify the displayed VIN with the vehicle VIN. If the displayed VIN does not match the actual VIN, write down the actual VIN and correct the VIN at the Techline® terminal.
6. When complete, Exit Service Programming.
7. Turn OFF the Tech 2® and disconnect the Tech 2® from the vehicle.
8. Turn OFF the ignition.
9. Connect the Tech 2® to the Techline® terminal.
10. Select Service Programming.
11. Select Tech 2® as the tool you are using.
12. Select the type of programming to be performed.
13. Verify the displayed VIN with the vehicle VIN. Correct the VIN as necessary.
14. Select the type of module you are programming.
15. Select Speed Calibration. You need to know how to get to this place
16. Select Normal.
17. After the download is complete, turn OFF the Tech 2®.
18. Disconnect the Tech 2® from the Techline® terminal.
19. Install the Tech 2® to the data link connector (DLC).
20. Turn ON the Tech 2®.
21. Turn ON the ignition, with the engine OFF.
22. Select Diagnostics.
23. Select the correct model year of the vehicle.
24. Select XXXXXXXXXXXXXXX
25. Select Powertrain.
26. Select the correct engine size.
27. Select the correct transmission for the vehicle.
Important: Be sure to select the correct transmission for the vehicle. Even if the correct axle ratio and tire revolutions per mile are entered if the transmission selected is not correct for the vehicle, inaccurate speedometer calibration will result.
28. Select Special Functions.
29. Select Engine Output Controls.
30. Select Speed Calibration Reprogramming.
31. Select Reprogram Speed Calibration.
Important: This procedure will allow the user to change the speed calibration for the vehicle. This procedure should only be done when a new ECM/PCM has been installed or when the axle ratio or tire size has been changed.
32. Select Done to continue.
33. Enter the axle ratio using the F0 through F9 keys on the Tech 2® key pad.
34. Select Done.
35. Enter the tire revolutions per mile using the F0 through F9 keys on the Tech 2® key pad.
36. Select Done.
37. At this point all entered information can be verified if anything needs to be changed. Select previous screen and the input information can be changed.
38. If the information entered is correct, select reprogram and the Tech 2® will indicate Reprogram Successful.
39. At this point if for any reason the Speed Calibration still needs to be changed select restart. The axle ratio and tire revolutions per mile can be re-entered again without having to go back to the Techline® terminal.