Tire Size

[Paw Paw]

Just 2 quick questions. One on tire size . I have 275 x 65 x 18 tires on my truck at this time . I used the formula in the Gryphon manual to calculate the tire size and came up with the number 2482 with a 3% squish .
My truck came originally came with 235 x 75 x 17 tires . Tire size in the Gryphon was 2356 - If I remember correctly .
I have changed the size in the Gryphon a couple times and have got different results each time .
Can anyone fix me up with the correct tire size number for my tires and maybe how you did it .
I am waiting on my custom tunes . Should I go ahead and download the Pegasus software now while I am waiting .
Thanks in advance
Paw Paw

[Longshot270]

The best way to get your tire size correct is to use the odometers of the truck and a gps. First program your truck to the 2482 so you can be 100% sure of your tire size. If you dont have a gps you can use the mile markers between cities or get the distance between two intersections using google maps. If you have a gps give it time to properly locate and then clear the odometer for the gps and truck. Then drive about 10-20 miles and check the two odometers. To calculate the new tire size divide the truck's odometer by the gps odometer, then multiply that number by the tire size you programmed the truck to. This final value is what you enter into the programmer. Do not recalculate for squish or anything.

[Paw Paw]

Thanks Longshot270 , I don't have a GPS , so I will try the mile marker method.
Paw Paw

[Longshot270]

I would try doing two intersections or addresses on maps.google.com. That is accurate to more than 1/100th of a mile.

[Paw Paw]

Thanks Longshot . I will give it a try.
Paw Paw

[Jackpine]

Paw Paw - Your 2482 is going to be VERY close in my opinion. That 3% reduction works very well unless you have some extremely low profile tires, and you don't. Longshot's technique will "dial in" any small changes needed to correct for "slight" errors in the "squish" value.

I just returned from a trip to California and on the interstate, in a 70 mile stretch (using the roadmarkers), I found I had an approximate 0.1 mile error in my odometer (reading low). To put this in perspective, my "entered" TS was 2481mm.

Now, 70/69.9 x 2481 = 2484.55, or 2485mm. (Longshot, I think you need to divide the GPS (actual) miles by the truck's miles, not the other way around). If I INCREASE TS, I show a longer distance on the ODO.

So, my 3% squish value that seemed to give me correct speed, is actually a bit too much and each tire rotation actually moves me a whole 3.55mm more, and that works out to a whopping 7.6 ft more each mile! Wow!

So, in 70 miles I actually covered about 530.6 more feet and that is about a tenth of a mile.

I'm kind of having fun with all this, because if I were to let my tire pressure drop, I might easily get back to the 3% value. Everything is a bit of an approximation. Don't lose sleep trying to get things "exact".

- Jack