My FJ is actually closer to actual after upgrading to 33" tires than it was beforehand. I'm reading about 1.5 MPH lower than actual speed now, versus ~3 MPH high with 31" tires.
I just finished my third long trip and have been wondering about the discrepancy between FJ speedometer and GPS reported speeds. I still have the stock tires on mine and the GPS was showing somewhere between 3-5 MPH less than the speedometer. The odometer was spot on according to the GPS and the mile-markers used for aerial speed enforcement (big white lines painted across the highway). The only thing I could figure was that Toyota, knowing I might speed a little above the limit, intentionally designed the speedometer so it would be off. So I set the cruise for 73 (according to the GPS) in a 70 MPH zone and waited for the highway patrol. I found one and noted that the Toyota was registering damn-near 80 MPH and he didn't come after me. I have to consider a few possibilities:
1) GPS is wrong, FJ is correct, and highway patrolman owns a FJ Cruiser
2) GPS is wrong, FJ is wrong, and true speed is somewhere in between
3) GPS is correct, FJ adds 3-4 MPH to my actual speed so I wont get a ticket, and my 3 MPH over the limit wasn't worth the patrolman's time
I even went so far as to attempt some math. (1/60)*75 = 1.25 miles in one minute. So I set the cruise at 75 on the GPS and started the clock. Since the FJ odometer only does tenths of a mile, I had to guess where the half way point was between 1.2 and 1.3 miles and it was pretty close if not exactly 1.25 miles. If I was really traveling 78 MPH or more (as the FJ reports), then It would have exceeded 1.3 miles; (1/60)*78 = 1.3 and (1/60)*79 = 1.3166. Next time, I'm going to try it over 10 minutes to see if I go 12.5 miles, 13.1 miles or whatever. I just need 10 minutes without interference from the other drivers, hills, or anything else that will affect speed.
Has anybody else had a chance to experience this anomaly?
BTW, the GPS also shows my Jeep as being off, but it has bigger tires and no mods to the odometer to compensate. Mileage and speed are both consistently 10% off, but I still love her cuz she likes to do it topless.