The calculation I came up with for my 285 70 17 bgf at's was 2610. The book for my edge evo had a calc for 285 75 17 so I did not use that.

Gps'd my truck, and my speedo and edge are very close. the edge and my gps are 2mph off at 65 and 1.5mph around 40-45mph. My truck and edge speedo are faster than gps. Was going to start playing with the size some more, but was trying here to see if anyone saw the same results and found a size calculation that worked good for the bfg ats.

Thanks. tried to search this out and only found stuff for 285 75.

 

Bill Cohron at PHP recommends reducing the tire circumference you calculate by 5% to account for "squish". When I did that, it made my Gryphon's speed "dead on". And, as Bill says, Ford designed the truck's speedometer to read slightly faster (about 2 mph) than actual (what you'll see on the Edge) for some reason known only to them. I have essentially the same differences as you: about 1 mph at 40 mph and 2 mph at 75.

- Jack

 

thanks for the response!! Will try it out tonight!

 

I've found that a 3% squish is more accurate. For my 285/70/17's I have mine set at 2530 I think. I'll check later and confirm that. This is a good calculator I think. http://www.miata.net/garage/tirecalcold.html

Put in your tire size and hit metric. Then multiply the circumfrence by .03 and subtract that number from the circumfrence. My Edge and Gps are within .5mph!

 

You're right, Ftruck! I think 3% works better too, and in another current thread I said that. For some reason (maybe my advanced age) I was remembering 5%. I should have come back here and corrected myself! :o

But, it all comes down to a bunch of factors. One of which is the inflation pressure you run with. I keep my tires at Ford's recommended 40 psi. A lower pressure, like 32, will probably produce a 5% squish.

- Jack

 

What % squish factor would you use for 305/40/22's?

 

Why use a squish factor when you can use the actual radius of the loaded tire?

 

And how would you measure that smart guy????

 

I run 50psi. My bfgs are 65psi rated and I have been running them this hard to try and see if I get any mpg gains. So far I have not.

 

He must run solid rubber tires.

 

you do a roll out. Mark the driveway and mark the tire at the bottom, roll one full revolution of the tire till the mark is back to the ground and measure the didtance betwen the two marks.

 

The whole point really is to "get in the ballpark". Any measuring is going to have an error. Bluejay's measurement will be better than mesuring the "loaded" radius since you compound any measurement error by 2 x pi when you calculate circumference. I doubt you can mesure the radius with anything better than a 2mm precision, so if you have a 2mm error in the radius, it results in a 12.6mm error in circumference.

But, there's another factor. When the wheels spin at say 75mph, there is quite a bit of centrifugal force on the tire body which should increase the effective tire radius at that speed. Notice how a drag racer's tires expand in a "burnout" - same effect.

So, once you're in the ballpark, you need to play with the final setting more until you can confirm your speed and distance are correct. One way I've confirmed my speed is with those traffic radar signs that they put up sometimes at work zones or at Border Patrol checkpoints. I verify my odometer by driving a long distance on an interstate and comparing it to the mileage markers. I used a 100 mile stretch.

All of this gave me a figure that was about 3% smaller than the "calculated" circumferance using any of the tire calculating aids that are around. This is why I recommend starting with 3%. It's easy to multiply circumference by 0.03, subtracting the result.

I think this is a good starting point for ANY tire, but tire pressure and tire design will have an effect.

- Jack

 

Very well put. I could'nt agree more!

 

Take measurement device. Measure radius of tire from the ground to center of hub. It really isn't that hard.

I would agree with the error derived from the loaded radius. But the error from a squish factor is greater. And to be fair I would honestly say 3mm would be the closest you would get. 3*2*Pi = +18.85/-18.85 (37.7 Total error) Where 3% * 2500 = 75. So the loaded radius, mathematically, should get you closer than a squish factor.

 

I tried the Bill squish method once and it threw my speedo off by a good bit. I went back and tried the calculations per the Edge book and my speedo was dead on with my GPS. I guess I'm different than everyone else here.