Tire Size

[Jackpine]

Originally Posted by Dfishrmn

Should the tire size be set at actual size according to the chart or allow for a "squish" factor. I currently have the stock 255/70/17 tires inflated to 37#.
I have the tuner set with a 3% squish factor that I have read about in some threads on FTE. Was wanting to make sure the setting is correct to get accurate benefit from the tunes. Thanks.

Mine is set with "squish". You DO need to include this correction if you want the speed and odometer readings to be correct, and, it will have a small effect on your shift points which can effect the benefits of your tunes. I've subtracted about 3% from my tire's "unloaded" circumference and find that to be about the right amount. My tires are 275/65R18s and they are inflated to 40 PSI.

Whatever value you try, it's a "starting point". The way I first calibrated mine was on a trip to California on the interstate. Over a distance of 100 miles (using highway markers - which are quite accurate) I found I had less than 0.1 miles error on the odometer. Later, using a hiking GPS with WAAS, I found the Gryphon's and GPS' speed matched (while the dashboard speedo read 2MPH faster - it always will read fast).

My guess is, you MAY have to increase your "squish" factor more for the tires and pressure you use. Let's say you tried the "distance" test like I did and found your odometer read 101 miles after 100 actual miles. That would be a 1% error, so, you could subtract an additional 1% from your tire's circumference.

Hope this makes at least a little bit of sense!

- Jack