How to calibrate the speedometer?

ScottG wrote:
The dealer can only make adjustments to stock size tires that were avail.
For true custom adjusments you need a programmers like CnF suggested.

And you know this to be fact about the dealer? On my Ford they plugged some number in their computer that they had some forumula had to do with the circumference. Like when I added the 50 aux fuel tank, they had to reprogram the computer so it knew the total number of gallons avaialable

That was the case with my F250. The base tires at 245/75R17 were 31" tall and the speedometer over reported the speed. Once I went to the optional 275/70R18's at 33", It was much closer to being accurate. Later switching to 245/70R19.5, the tire height was the same as my 18's.

Yes, depends on what tires on the truck OE. I’ve had numerous work trucks that always come with the smaller tire options. Speedos consistently read a bit fast. Up the size to around the “optional” bigger OE sizes and speedo is right on. I wouldn’t even dink with 1-2” tire diameter adjustments and if the early shifting is an issue hit the t/h button.
JMO

I would definitely use a GPS to see what your actual speed is vs the speedo before buying a tuner just to adjust the speedo reading. As was said, you may find that it is right on or within a mph or so.

If you only go up 1 inch your speed o will probably be on the money , most factory are off by about 2 mph

Yes the Hypertech was at the top of my list. I just didn't want to accidentally overlook a good alternative. Thanks for the input!

The dealer can only make adjustments to stock size tires that were avail.
For true custom adjusments you need a programmers like CnF suggested.

I use a Hypertech speedo calibrator for my Ram 2500. Around $200 online but they are vehicle specific so price may vary a bit. Easy plug and play with a laptop. Only downside is it will only work with one vehicle at a time. You have to return a calibrated vehicle to stock before programming another.

That's not enough of a change to matter, and most speedometers aren't that accurate anyway. I would use a GPS to get an idea of your actual speed versus the speedo and just keep it in the back of your mind.

I've run oversize tires on most of my rigs in the past and this always worked for me. The only reason I'm not on my current truck is I'm already pushing being too tall with my fifth wheel and getting into my garage and parking garages.