Tire size change calulation

Not sure why I'm such a rock on this; trying to change PCM for larger tires on my truck. Stock size was 31.7" new size is 33.2". I'm using the Speedometer Parameters table. I have a 4.10 axle and if I'm reading the calibration table correctly my '04 truck should have 40 pulses per revolution of the drive shaft. My base tune shows I have 128000 pulses/mile. But that number given the 4.10, and 40 pulses/rev comes out to a 25.8" tire. Like I said above stock tires were 31.7" Goodyears. Any advise on how to make this come out right? Thank you in advance.

Kevin

Are you sure the original cal "Vehicle Speed Sensor Pulses per Mile" was 128000 ppm? The "Secondary Pulses per Mile Output Rate" is possible 128000.

With a tyre size of 31.7" the wheel circumference is pi*31.7 = 99.588 inches.
1 mile = 5280 feet so the wheel will rotate 5280*12/99.588 = 636.22 times per mile.

If your differential ratio is 4.1, then the drive shaft will rotate 626.22*4.1 = 2608.51 times per mile.
If your VSS pickup has 40 pulses per revolution, then it will transmit 2608.51*40 = 104340 pulses per mile.

Same deal with your 33.2 inch tyres:

5280*12/(33.2*pi)*4.1*40 = 99626 pulses per mile.

Those are the values you should see from the EFILive speedo calculator.

To get those values, enter data in this order:
1. Set the correct pulses per revolution
2. Set the correct diff ratio - make sure Fitted to rear axle is not checked (unless your VSS pickup is on the rear axle).
3. Enter your tyre size.
Then, the value you see in the "Pulses per mile (approx)" box is the calculated ppm based on what you entered. You then click on the [Update] button to copy the ppm and the diff ratio values over the current calibrated values below.

Note: changing the "Diff ratio" or the "VSS pulses" or the "Fitted to rear axle" values, alters the tyre size, so you need to enter your tyre size after changing those settings.

Regards
Paul

Paul, The caluator seems to be working fine, and I got same values as yourself. Values didn't add up when checking what my base tune read. Right at 128000 Pulses/mile, not the secondary pulses per mile output rate. That number is only 996. I'll make change; flash it and see how it goes.

Thank you, Kevin

Paul, flashed to PCM. After short drive I have ABS warning light. Sounds like my base tune is wacked from factory, but somehow it was working. I've attached file if you have time can you look it over for me? Oh, can't attache .tun files. I'll try to get screen shot and post.

http://www.f-body.org/gears/

use this if you want to know your tire size

enter the tire size as written on the tire (ie 275-40-17)
It will convert it to inches
Enter the inches into tire diameter and hit update.

Might not solve your problem, but for those who might need it.

The abs light will come on if it senses that the front tires are moving at a vastly different speed than the rear tires, like on the dyno. I have no idae how to help but just wanted to share some info.

Quote:

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Originally Posted by Z16DNA

Paul, flashed to PCM. After short drive I have ABS warning light. Sounds like my base tune is wacked from factory, but somehow it was working. I've attached file if you have time can you look it over for me? Oh, can't attache .tun files. I'll try to get screen shot and post.

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Can you please email your original *.tun file and the one that you modified to support@efilive.com with the subject "Re: Tire size change calulation".

Regards
Paul

Paul, I'll send it ASAP. FYI, I went on line and obtained a stock tune for a my truck, both '03 and '04 years. Reviewed the data, it doesn't match what I read out of my stock tune. So I used the data from the '04 and adjusted for tires. Didn't work. Trans shifted hard and made some loud noise. I stopped within a few feet and put stock tune back in. No problem now.

Interesting problem. Thank you for your help Paul,

Kevin