Tire setup

onelildude · #1 Sun, September 6th, 2009, 07:20 PM

I have some 35x12.5r20's on my truck and I can't get the tire setup right. I have followed the directions to the T, but my speedo is off by 5 at 30mph and around 7/8 at 65mph. Can anyone help me with this???

JWBFX4 · #2 Sun, September 6th, 2009, 07:51 PM

Well it would have helped if you said what number you are inputing....plus if you are 7/8 slow or fast.

But I would say around 2760 would be a good number to start with. That could be a little low though...im just trying to do it from memory

onelildude · #3 Mon, September 7th, 2009, 12:53 AM

Ohh yeah my speedo says 70 but I'm doing 77

JWBFX4 · #4 Mon, September 7th, 2009, 09:30 AM

I guess you are checking your speed off of a gps.

If your speedo says 70 and your are doing 77...then you are actually about 9 mph off... Ford programs the speedo where its about 2 mph fast at 70 ( so if your speedo says 70 your going 68).

I just noticed you had 20's you might want to go a little higher than 2760mm because I believe they have a little less squish.

And what I forgot to mention earlier, when you are using the formula to figure out what number to put in for your tires...always remember to deduct 3%-5% for "Squish". Chances are you did the formula right, you just didn't subtract the squish.

Hope this helps

onelildude · #5 Mon, September 7th, 2009, 02:57 PM

I'll try it and see what happens

***Jackpine*** · #6 Wed, September 9th, 2009, 10:12 AM

Quote:

Originally Posted by onelildude

I'll try it and see what happens

If you are going faster than what the dashboard (and Gryphon) speedometers indicate, the number you have entered for Tire Size (TS) is too small. Also, since you are using the Custom Options menu to set the TS, you need to verify that your Gear Ratio (GR) is accurate too - it may well be incorrect.

Based on 35 inch tires, I get a TS value of 2709mm. That includes a 3% squish on the unloaded circumference of 2793mm. Try something in that range and remember to push [Enter] after you set both the TS and GR. Then, scroll to "Done" and press [Enter] to start the programming process. If you accidentally press [Menu] at any of these steps, you WILL NOT apply your settings.

And, JWBFX4 was correct when he said there will be a 2 mph difference between the dash speedo and the Gryphon speedo. If you can get the Gryphon speedo to agree with your GPS, (both indicating your actual speed), the dash speedo will indicate 2 mph faster at 70 mph (it has about a 3% error built in).

- Jack