so i can correctly program it into my gryphon. the reason i ask is b.c i had done it and programed it and if my truck spedometer says 30mph, the gryphon will say 28mph. its like the gryphon is off by 2-3mph.
This is my tire size> 275/65r18,
rim size> 18" .
the programer goes by millimeters, so i have used the equation in the booklet provided with the gryphon to figure out the millimeters.
this is the equation
[(width x ratio x 0.002) + (rim x 25.4)] x 3.1416 x 0.97 = tire circumference

i got a tire circumference of 2482. sound right??

 

The Gryphon displays the true speed. Ford, in it's infinite wisdom, programmed the speedometer to display 1-2, sometimes 3 MPH slower than your actual speed. So, when you're adjusting tire size, you want the Gryphon to display the actual speed (i.e., match it to a GPS) and let the trucks speedo do whatever it wants. There are guys on here that don't even use the truck's speedo anymore; instead, they just rely on the Gryphon's digital readout to get a more accurate speed.

 

well in this case, ford speedometer is 2-3mph faster. i dont recall it being like this before i changed my tire size. when i got the programer yesterday, i didnt change the tires size it came programed with. so i entered mine and i did noticed the shifts werent as violent but still better then stock. and thats when i also noticed the speedometer issue..

 

Actually, cstanek, the truck's speedo will probably read FASTER than true, not slower.

But the rest of your post was dead on.

- Jack

 

You either your math wrong, or entered the wrong equation in your first post.

The equation you posted would be [(width x ratio x 0.002) + (rim x 25.4)] x 3.1416 x 0.97 = 1502

You did was [(width x ratio x 0.02) + (rim x 25.4)] x 3.1416 x 0.97) = 2482.

Regardless, the correct answer was the one you had. A 275/65/18 tire is approx. 101" in circumference, which is approx. 2480mm after factoring in the "squish" of a loaded tire.

- NCSU

 

This is the most accurate way to correct the Gryphon speedo. Drive on level ground and adjust your tire size until the Gryphon matches the GPS mph.

I have been tinkering with mine. It came with the Gryphon displaying a few mph faster than it should have been. Increased the tire size and now it's showing even faster than before. Gotta go back and make them smaller but I've been too lazy.

 

Yup, you should have reduced the tire size. You can probably get it almost perfect by calculating the percentage error in your speed. Then, reduce the tire size you used by the same percentage, since it's a linear relationship.

- Jack

 

Infinite wisdom? Why act like Ford doesn't know what they're doing.

If you want to know the truth of the matter, federal regulations allow a speedometer to read up to 10% faster than actual but not one mph slower than actual. So, Ford is just being rational (like most auto makers) and erring slightly on the safe side. Factory speedos read a little faster than actual (even when the odometer is almost right on).

 

My intention was not to sound cynical. I'm just confused why the speedometer in my MDX (and every other car I've driven lately) exactly matches my Garmin, but the speedometer in the F150 doesn't.

Dang it!

 

It doesnt matter if the odometer is off because your Ford is a Ford. =)

 

Jack, Jack, Jack. Smart man, yes, but I try to stray from thinking too much like an engineer outside of school. My head hurts enough as it is.

Thanks for the tip, though. Think I'll do that after my exam tomorrow.


Matt

 

O.K., fair enough. The F-150 series comes with a lot of different tire and wheel options and Ford switches tire brands due to pricing, availability, etc. The slight error to the high side probably keeps them from having to calibrate every tire and wheel combination. It could get expensive to realise after the fact that 500 trucks had come off the line with a slightly larger tire and now read .5 mph slow (and run foul of speedometer laws).