Tire Size Change Question
 

2008 Screw 5.4 went from tire size 275/65R18 to LT315/70R17. I first used the formula in the owners manual to calculate the correct number and by doing this I got 2741 so i programed this in took a 25 mile drive to the parents and used two higher end gps to verify speed and basically i had to show 75 with the speedo needle to go 70 according to both gps. at my parents i did another calculation this time taking the overall diameter of 34.7 and multipling it by 25.4 to get 881x3.1415 and got 2769 and then programed that new number and the speedo was still off by 5 according to both gps

Am i not doing something right or is the programmer not making the correct modifications to the cpu. any help would be great

When you change your TS you must also check your GR. When you open up CO on the gryphon it puts in place holder values. Follow Jack's Q&A in the gryphon programmers section it's pretty good and straight forward.

Lars

Right now all i am running is the canned tune on tow so I can get better shifts and update my tire size until my custom tunes are done so I dont know if this makes a difference or not to your response but I will be sure to check that out and make sure its right. Thanks for the info I apperciate it

You're not accounting for "squish". Try subtracting 3% of 2741 to get a more reasonable "loaded" circumference. When I do this, I get a value of 2741 - 82 = 2659 mm.

Unless you compensate for squish, your PCM will think each revolution of your wheels moves further than you are actually traveling. That equates to to a higher speed than you are actually doing. Try it with 2659. If you're still indicating faster than you're actually moving, reduce it still more.

Oh, and Lars was correct too. Make sure your Gear Ratio value is correct when you do this.

- Jack

To add to that, don't use the speedometer to gauge if you have the right tire size or you will be chasing numbers all over ther place; you should be comparing odometer to your GPS' trip mileage counter. There are different ways you can make it work or you, but you will find it is much more accurate and easier to know if you need to go up or down on tire circumference. If my GPS is clicking off tenths faster than my truck odo, I need to raise my tire size...if my truck odo is ticking out faster than the GPS, I lower the number. So far on Toyo MT 35x12.50-20's at 43 psi I found that 2702 is about as close as I'm going to get.

Thanks for the info. i have never heard of the squish until you just brought it up but i guess it does make sense. Another thing I was wondering is what should I run in these for air pressure? I currently have 35psi in them they are procomp xtreme mt in lt315/70r17 i was debating putting 40psi in them but i dunno

Call Procomp and ask. Every tire manufacturer should be able to tell you what pressure to run for the aplication, even loaded or unloaded. For example, on my 06 Screw with 35" Toyo M/Ts, I was told by their customer service to start with 45 psi, and adjust accordingly by gauging tread wear. I now run them 43 psi front and rear. If I have a 6000lb trailer on the back (600 lb tongue weight), I have to pump up the rears to 65 psi. They should be able to tell you all that info at Procomp's customer service line.

Grabber gave you a good answer, I'd call Procomp too. Somewhere, on the tire's sidewall is a max pressure value. DO NOT exceed this! And, I'm sure you know to check your pressure when the tires are cold, not after being "rolled on" for a while.

Just as a "guess", I suspect 35 psi is way too low. The OEM tires I had on my truck were recommended to be at 40 psi. At the recommended pressure, you maximize load carrying ability and treadwear, and minimize rolling resistance, heating (due to sidewall flexing) and hydroplaning on wet roads. If you over inflate, you tend to wear out the center of the tire tread and you increase stress on the entire tire and wheel which can lead to blowout.

I think, at the correct pressure, the squish value I gave you will be just about right. I was actually coming up with a 6% correction given your speeds, and that tells me you are underinflated too.

- Jack

You bring up a good point, J&J...perhaps a 5% squish lets you know you're where you need to be for cold psi? A long stretch, but I wonder how accurate that theory could be? That would take a lot more math than I'm comfortable with...

I'm guessing here, but I think the 5% figure that Ford uses accounts for the tendency of the "average" driver to ride around on underinflated tires. I'm pretty sure most don't check their tires at all, and they depend on the dealership or whoever changes their oil to do it for them, every 3-4 months.

I was in Discount Tires a few months ago getting a tire rebalanced and overheard some poor old soul being told that he/she (I can't remember which now) was going to have to get 4 new tires because the ones that were on the vehicle were worn beyond safe limits due to poor alignment and no rotation.

- Jack