Just curious.. Just bought 03 F150 XLT and coming home from work noticed I was passing everybody. Cruise set on 74 as usual. ( Interstate travel w/ speed limit 70) then it dawned on me... Truck set up for 16" tires and I'm running 20" now. Figured speedo was off. Opened my Waze app on phone and speedo was 7 mph off. I was actually running 81. I was told some GM's you can reprogram computer for different size tires , can it be done on Fords too?

 

So many things wrong here. 1- you didn't have 16" tires. They are called rims. 2- rim size won't change speedometer calibration. Only tire size. So for instance if you had a 33" on a 16" rim, then ran a 33" on a 20" rim it's still a 33" tire. Yes you can reprogram the speedometer with a programmer.

 

My kids bike has 16 inch tires

 

Easy now guys! Is it possible that when you started running 20" rims that your new tire diameter increased? If so then that would cause the speedo to be off.. Say If you went from stock 16" rims with 265/75 tires to a 20" rim with 295/55 tires.

Yes you can reprogram the computer for tire size. You will need a Programmer like SCT or Edge.

 

Also your speedo is not off by MPH. When you're not moving it's not saying you're going 7 MPH. It's off by a percentage. For example, when I'm going 50, my speedo says 43 so I know my speedo is 14% off.

 

The Dealership scantool have the capability of adjusting the Tire & Rim size.

 

Should've mentioned stock size is 230/70/16 according to door tag. I'm running 275/55/20. Thanks for the info

 

Tyre size calculator shows an 11.1% difference between stock and your current size

also shows 70=77.8 so yup you are roughly 7mph out as expected

 

I don't think so.

 

Better than the '86 Elco I'm driving, lol. This one reads 15-22 MPH too slow.