Chart

Going from 265s to 285s on my 2000 PSD Excursion actually corrected the speedometer. (That's going from a 31" tire to a 33" tire). You might want to try it first to see if alterations are even needed to pull your boat. I'd be interested in you results.

LapseofReason

Brian you can go to the dealer and program the factory pcm to up to 34 inch tires and the only thing you will notice is a few hundred less rpm at cruise. All you need is the hand held odb II, Did my 96 and my 01 excursion took about 5 minutes in the parking lot, if you need a Ford tech that knows what he's doing call me. Most aftermarket tuners have an opp for it too since they overwrite that part also. I have and had 35's on both trucks and the speedo was dead on on my 96 because I used the Hypertech programmer, on the 01 I am about 2-2.5 off at 80 mph because you can only go to 34's in the factory pcm and the programmer in it doesnt have it right but it still drives and tows way better than stock.

Jassman

Brian, having the same problems with my 11.. I changed from 275 65 R 20's which are 34.2 to 325 60 R20 which are 35.5's.. tranny shifts early.. the dealer can mod different tire sizes..only going down in size that was recommended as the largest tire size offered for that vehicle.

E Dock DD

I feel this is a 4.04% change. I get this number by 33.0 - 31.72 divided by 31.72. You need to divide by 31.72 because this is where you are starting from. This gives you the change from where you are at.