Thread: Which custom tunes for me
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Old Mon, January 31st, 2011, 04:59 PM
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Originally Posted by apdanos View Post
So this is what I did. I reinstalled the custom tune. The gear ratio was correct. The tire size that was in the custom tune was different than the two numbers that I determined. I determined the tire size using two different methods: 1)by measuring one tire revolution and 2)by calculating using the tire diameter given by the manufacture website. So which one should I use? The actual measurment or the calculated?
First, please read through the discussion of using the Custom Options menu here: http://forum.gopowerhungry.com/faqs-...ions-menu.html

The way Tire Size (TS) and Gear Ratio (GR) are stored is VERY confusing. What you saw for TS may not have been the value that was used.

But, as far as determining a TS, I'm not a fan of measuring a tire revolution. There's too much potential error and, it does not account for the expansion of the tire at highway speeds (somewhere in the neighborhood of 700+RPM).

If you have P-metric tires, Bill has included a VERY accurate calculator with Pegasus. It is the Gear Ratio Calculator. Enter your "Old Tire Size" like this, using my 275/65r18 tires as an example: "275/65-18". If you don't have "low profile" tires (less than 60-series) use a 7% "squish factor" in the calculator. (The squish in the calculator is "rubber squish" - no squish in the rim). With a low profile tire I'd reduce the squish to 5%. You can then read the circumference in millimeters.

Another VERY GOOD way is to find the number of revolutions per mile for your tires from the tire manufacturer's website. Most post this figure and it accounts for squish. Then divide this number into 1609344 (the number of millimeters per mile).

If your tires are "inch" sized, multiply the diameter by pi and then 25.4 to get the inflated circumference. Then reduce that by 3% for "normal" tires or 2% for low-profile tires.

All these should give a very close starting point. Match a Gryphon speed readout with a stable speed reading on a GPS or match your dashboard odometer with a measured distance of at least 10 miles (more is better). The dash speedo will read about 2 mph fast at 70mph if you have everything right.

Hope this helps.

- Jack