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

[Longshot270]

Originally Posted by JackandJanet

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.

Thanks Jack, I thought something looked funny.

[MileHigh04SCREW]

Another way, personal favorite lazy-way, is to chalk a tire on the edge near the tire tread and on the ground, and then roll one complete tire rotation (making sure your tire chalk mark is exactly where it was before the rotation - a plum bob works well). Then straight-line measure the distance. Just make sure all of your tires are air'd up correctly and evenly.

milehigh

[Paw Paw]

Hey guys ! My actual calculation on tire size came up to 2559 - not subtracting anything for tire squish. So, I decided to set the tire size to 2550 as a starting point.
Did the Longshot270 suggestion an got a milage distance from Mapquest on a route that I was going to travel on Sunday evening . Zeroed the trip odometer on the truck .
Mapquest mildage 11.03
Truck 10.9
11.03/10.8=1.011926
2550x1.011926 = 2580
This is where I am right now . Should I leave it at 2550 or bump it up to 2580?
I grew up working on the ole FLATHEAD V8'S , and 8N Ford tractors (which I still have ) and this whole digital thing is just too much fun and facinating .
Gotta love it .
THANKS FOR ALL THE REPLIES
PAW PAW

[Jackpine]

Paw Paw, I don't trust the mapquest mileage in your case. If you DON'T factor in "squish", your PCM's going to record a LONGER distance than you actually traveled.

No matter what Tire Size you use, the tires rotate a "fixed amount" in a fixed distance. I think mine rotate 647 turns in a mile. So, in 11.3 miles, they turn 7311.1 times. With a TS of 2485, I think the PCM will "see" 11.3 miles for that many rotations. But, if the entered TS is bigger, at 2550, you get:

(2550 x 7311.1)/25.4 = 733988.39 inches, or 733988.39/12 = 61165.7 ft and that is 11.58 miles on the odometer.

By the way, you have the same size tires as I have (275/65-18). I am now using 2485mm for TS in the old Gryphon or a 31.1" diameter in the CTS, at 40 psi inflation.

Tire rack says my Goodyear Wrangler SilentArmor tires spin 651 revs per mile. They expect more "squish" than I experience, possibly because they assume a lower inflation. If they were a particular Michelin, they say 648 revs. A Yokohama tire is 645. If you tell me the exact make and model of your tires, I can point you to the page that will show you the manufacturer's "rev" number, and from that, it's an easy calculation to TS.

But, I am CERTAIN that 2550 is TOO LARGE!

- Jack