Thread: custom tunes and tire size and gear ratio
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Old Thu, May 14th, 2009, 09:47 PM
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Jackpine Jackpine is offline
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Originally Posted by Down N Dirty View Post
Hey Jack, I whipped out my calculator, and did a little research and it appears the default value of 2356 that appears in the TS section of Custom Options would actually be the correct number, including a 5% squish compensation for a popular tire size used on many F150's, 255/70/17. Now this may be a weird coincidence, but I somehow bet that number was picked for a reason, and isn't just random. I could be wrong, lord knows I've been that many times in my life.
No, I suspect you're dead right and I congratulate you on figuring the reason for this "funny" little figure out. There seem to be very few of us in the world who ask "why" anymore - I'm glad you're one of them, David! (I like the pup in your avatar too).

Everything I'm about to say comes from internet conversations with Bill - a great deal of what I talk about comes from those interchanges, so give him the credit, not me.

Bill says Ford applies a 5% total "squish" factor into their calculations of tire revs per mile. This means you can simply calculate or measure the number of revs that an unloaded tire would make in a mile, and then simply subtract 5% from that figure.

Bill further states that when you open the Custom Options menu, "placeholder" values for TS and GR are retrieved from the "Calibration Section" of the PCM. Clearly, Ford could have put ANY values here, but why not something that is used in lots of trucks? So, we could guess that Ford put a TS value there that was proper for a popular tire. Additionally, they MIGHT just put 3.73 for GR into the Calibration Section for any truck with the Tow option, like mine. I think you hit the reason for the TS value exactly right! Good on you!

If the truck did not have the Tow Option, possibly a different GR would be found there.

But, Bill has said that if you bypass the Custom Options menu, values for TS and GR come from the Vehicle ID (VID) portion of the PCM. This area is "fixed" at the factory when the truck is manufactured and cannot be altered - it even contains your VIN (I think). So, if you don't have new tires or gears and simply bypass Custom Options, everything will be fine. The GR and TS that were correct when the vehicle left the factory will be the values used.

Bill's custom tunes will set a "switch" that says "bypass the VID" if he knows the TS or GR is non-stock.

But now I'm going to confuse the issue slightly. I keep my tires inflated to the recommended 40 PSI religiously! I find my speed and odometer are correct if I use a 3% squish correction. I'm going to guess here (my opinion) that Ford assumes most of us will drive around on underinflated tires - hence the 5% figure that they use.

And, here's where it gets interesting: If I use Bill's "Gear Ratio Calculator" in Pegasus, I find my TS corresponds to a 7% squish there! "What the heck?" So, I asked Bill about this. His Calculator assumes that the metal wheel does not "squish", only the tire does. So, a 7% "rubber squish" in tire sidewall height is pretty close to a 3% total reduction in tire circumference, and probably more accurate!

We could spend hours figuring out these fun facts, couldn't we? Glad you're a kindred spirit!

- Jack