Hey guys, I bought some BFG All Terrain 285 70 R17s on my 03 Supercrew FX4, and using my GPS I noticed of course the speedometer is now off by about 1 mph. Is that enough to have everything re-adjusted? One dealership told me $86 and it would take about an hour, and another Ford dealership told me $50. Is it really worth it? I used to have a Toyota that was 4 mph off, and I just learned to adjust to it. I'm not worried about the speeding factor because I drive 5 or 10 under the speed limit. Suggestions??? Thanks!

 

You didn't mention at what speeds it is off by 1mph. If it is off by 1 mph at an indicated 10mph, you have a 10% error and would probably want to get it adjusted. If you are off by 1 mph at an indicated 50 mph, you have a 2% error. Most folks would probably not worry at all about 2%, and some brand new vehicles might not have that kind of accuracy. My '03 Jeep Wrangler came from the factory with a 3-4% error. For now I just mentally compensate for the error when I check the speedo. I have determined that larger tires would actually make mine much more accurate. How's that for rationalizing a lift and bigger tires?

I'm guessing that you were referring to a 1 mph error in the 50 to 70 mph range, so I think hardly anybody would worry about that.

 

No, you don't have to get it adjusted. Something else, I don't think the dealer can adjust it. I've been told the dealer can only program the computer for an OEM tire size. Since a 33 inch tire was never offered on your truck, how can they reprogram it. I know for a fact you can reprogram it yourself with one of the aftermarket computer tuners (Superchips, Hypertech, etc.).

 

Up to 6-8 mph +/- at 60 mph is allowable according to Ford, and will not affect odometer operation.

 

Quintin,

Are you saying the tech can adjust up to 6-8 +- mph at 60. regardless of the tire size? Mine is off 3 mph after installing 275/70's.

 

No sir, we can't adjust for specific speeds, the max allowable error is 6-8 mph either way when compared to stock. As Wandell said, the PCM can only be adjusted for OE available tire sizes.