Tire calculation question
 

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.

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.

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

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.

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!!!! :woohoo: 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.