Tire calculation question

Slowpoke · #1 Thu, March 5th, 2009, 08:28 PM

Just got my Grypon an hour ago. Started reading the manual, then got confused about the tire size. I read on these forums that there is a tire calculator in the Pegasus softward. Downloaded that and tried to figure out my tire size in mm. I have 35x12.5/18 tires. I used a converter which converted the tires to a 318/68-18. Punched those numbers in the calculator with a 5% squish and my tire size is 2726 mm. Is there anyone that can double check that for me before I enter that figure into the Gryphon. Just making sure before I screw things up. Mahalo.

blackfx4ny · #2 Thu, March 5th, 2009, 08:44 PM

Who knows for sure, I get 35 inches tall equals 2792 mm around (diameter). 5% off of that and I come up with 2652. The best way is to try a number that's close and make adjustments until you are satisfied. The speedo should be 2mph higher than the Gryphon, this is from Ford, cuts down on speeding I guess. Hope this helps some. BTW, I do not like math.

***Jackpine*** · #3 Thu, March 5th, 2009, 09:36 PM

Dang, I hate these non-standardized ways of measuring things. Black, the conversion ratio is EXACTLY 25.4 mm/inch which gives an unloaded circumference of 2792.875869 mm or closer to 2793 mm (if we want to quibble over one millimeter).

Personally, I've found that a 3% reduction in that total circumference is about right, since the wheel itself does not "squish", only the rubber part does, and Bill's calculator keeps the wheel radius constant. So, 2793 - 3% = 2793 - 84 = 2709 mm.

In my case, I found a 3% squish in total radius was about equal to a 7% squish in the rubber part. So, if I use Bill's calculator, which wants "rubber squish", I enter a squish value of 7%. This of course changes depending on your tire construction, wheel diameter, and inflation pressure. My tires are inflated to 40 psi, and I have 275/65-18s.

So, Slowpoke, I'd start with 2709, and, Black's advice is dead on - this is just a starting point. You have to try a setting and compare it to a GPS or radar gun measurement to get it right.

Hope I'm not confusing things here.

- Jack

***Power Hungry*** · #4 Thu, March 5th, 2009, 09:44 PM

In almost all cases for 35" tires, I usually start at 2760mm and work from there. You may also check the manufacturer's website and see if they list a "Rev per Mile" rating for the tires. Take that value and calculate mm using this formula:

1609344 / Rev per Mile = Tire Size in mm

Don't forget the "3 roll" method. Mark the tire and ground, roll the tire forward 3 times, mark the ground again and measure. Divide by 3 and there you are. Don't forget to convert inches to mm by multiplying by 25.4.

Hope this helps.

Slowpoke · #5 Thu, March 5th, 2009, 10:58 PM

Mahalo to blackfx4ny, Jackand Janet, and Bill for all of the information. I'll use the the figures that you guys provided and go from there. I'll contact my sister-in-law and see if I can borrow her GPS to try and get an accurate tire size. By the way, after I posted the thread, I installed the Gryphon. The instructions very were easy to follow. Went out for a short drive using level 2. All I can say is WOW!!!!

What a difference. Makes me anxious to know what the custom tune will do to the truck. Corey and Bill, mahalo for the great product and service.