I spoke to the dealer today to get a status report and they said they can't get it to program. He said they tried for about an hour but it just wouldn't take the setting. guess I'll try another dealer after I get it back.

 

One thing to be aware of is that the calculated tire height does not match the actual rolling diameter.

That is, if you take ((2 x 265 x .75) / 25.4) + 16 you will get 31.65"

From the BFG site, they give the diameter as 31.8" on a 7.5" rim and also give the revs per mile as 654 @ 45 MPH.

If you take 20168 / 31.8 you'll get 634.2 which is 19.8 revs less than BFG says. What gives?

Well because the tire won't be perfectly round when the weight of the truck is sitting on it, the effective diameter reduced because of the flat spot where the tire contacts the ground. Hence the circumference will be reduced too.

One mile is 63360 inches. 63360 in/mi * 1 mi/654 revs = 96.881 in/rev. This is the circumference of the tire. Diameter is circumference divided by Pi, so 96.881 / 3.14 = 30.838 in.

So the rolling diameter given by BFG is about an inch less than than the overall diameter given by BFG.

Where'd that 20168 number given earlier by R1ZOOM come from?

That's 63360 in/mile divided by Pi (3.14159.....) so it's really 20168.114...

And now you know the rest of the story.

Bottom line. Best to check the tire mfrs web site to see what number they've published for revs/mile for your tire. Or alternatively, measure your tires circumference directly by chalking the side of the tire and rolling forward a few turns and then measuring the distance traveled and dividing by the number of turns you rolled forward.

Divide 63360 by that number to come up with the revs/mile for your tire at ~zero mph. The actual revs per mile will be fractionally less because the tire will grow a little at speed.

Whew!

Daryl

 

Now I know why the dealer himmed and hawed and it took an hour! When I took the wife's Tahoe to chevy for this all they had to do was select the tire size from a menu which explains why her spedo is off about 3 mph now. So I think the moral to this story is to have your dementions already in hand when you go so all they have to do is input them.

 

exactly, and measuring the tire while cold on level ground should factor in your difference in true height versus "installed height", for instance my Buckshot 315's are 34.5 inches tall per the manufacturer, but at proper PSI cold on the truck they are a little over 33" tall

 

I took my truck to a speedometer place here in portland. He put it on rollers and set it. I told him that Ford told me that they could only set it to 35" tires. He told me he has setup 44" tires on superdutys. My tires are 38" Pj's. He said it should be within 1 mph. I have to find one of those speed boxes that are along side the road to check it. It seems to be better and shifts alot better.

 

Those spedo shops are generally the places that certify the police spedos and have never been unable to do something to fix a spedo. Thanks for reminding me, think I will go find one when I change my tires.

 

I just got off the phone with my dealer and they said mine is now re-programmed. Thanks a lot for the informative posts. They told me it couldn't be programmed for anything higher than 265's so I printed out the instructions posted here and the link explaining how to enter custom numbers and they said it worked that time. Thanks again, without this site I would have been out of luck. The info here is invaluable. I also got my truck re flashed today, so I'm anxious to go drive it!