Has anyone here been able to take their trucks to a dealer to have their speedometer recalibrated because of taller tires? I went from 275/65/18 to 275/70/18, and because Ford does not have that size tire in their software they say it can't be done.

 

They're probably right. Ford's calibration software doesn't allow inputs of something more than about a 32.5" tire. You're going to need a tuner. If you tell them to go the maximum revs per mile (it's 644 revs per mile on my 2008), you should be within 2%.

 

According to this web site http://www.rimsntires.com/rt_specs.jsp
and with the tire criteria you have given you will be going 2.1 mph faster then your speedometer reads.
For such a minute difference I would just live with it.

 

Ford sets up the speedometers to read 2-3 MPH high (from the factory). If I thought about this correctly, he'll be pretty close to dead nuts on.

However, his odometer will be off (low).

 

I did a combination GPS and stopwatch and with a speedometer reading of 60mph it took 57 seconds between mile markers. 63mph, which the GPS agreed with. At speedometer reading 70mph it took 50 seconds between mile markers. 72mph, which the Garmin also agreed. I was just suprised it was that much.

 

Looks like I have no choice but to live with it. Unless I want to shell out more $$$ for a custom tune.
When I saw an F 350 in the dealer lot and noticed the tires were 275/70/18 I was hoping Ford would have the software for that size tire. But since the software goes off the vehicle VIN I'm SOL.