Changing tire size and odometer/speedometer stagger

abq02ranger · #1 03-31-2010, 11:29 AM

Ok so im pretty sure where this goes, but if im wrong feel free to change it and im sorry.

So i've got an 02 Fx4 currently sitting on stock t-bar crank and 31 AT's. say i crank t-bars a little bit so i can fit 33 10.5s, what would that do to the speedometer/odometer if its calibrated for 31s? obviously i mean with a larger diameter tire i would actually be traveling further for the same number of revolutions so the speedometer would read low, but by what factor?

i know a million of you have gone from 31s to 33s or even more drastic and how much of an effect does that have? how can that be accounted for?

thanks

Harleymedic53 · #2 03-31-2010, 11:51 AM

OK when i had my stock setup i meausred my tires and i was only able to measure 29" in dia. Im sure new they were 30's cuz they were pretty bald. I had a stock FX4 4x4. I went up to 33x12.5 and i'd say on average once you get over 40mph it adds about 4mph to your spedo. anything below 40mph your spedo pretty much reads right. so if i set my cruze at 68 i'm goin about 72. thats my setup.

Fx4wannabe01 · #3 03-31-2010, 02:17 PM

Whatever the percentage difference is in height, will be the difference in the speedo/odometer.

You can do things to counter this and read right. None are exactly 'cheap', but I use a ScangaugeII digital gauge/scan tool. It's one neat little gadget and I love it! It'll allow you to compensate for tire size, it'll do mpg's, trip cost, av. mpg's, miles driven, TONS of neat little things and it's even an OBDII scan tool. I actually gained about 2-3 mpg's by learning about fuel consumption by getting this tool. Check it out at Scangauge.com. *note: this will not change anything with your truck.....you look at this for speed, not your factory gauge.

Another option is heading over to the dealer to have them recal your PCM so that it'll read the single off the speed sensor correctly, and display on the dash correctly too.

But.....31's to 33's isn't that much of a difference. If anything, it'll be just remotely off. Ford's read too fast anyway so larger tires will counter that normal factory problem. I'd do the math for ya but I really don't feel like it.