On the Bullydog ( same thing as sct ) their is also a gear ratio setting.
I can change or adjust the speedo by a percentage not just revs per mile .

 

I recently went from 35s to 285/75/16 and had to reprogram on my 5Star tuner. I used the calculation from tiresize.com/comparison as a baseline to compare against the speed recorded from my GPS. It was close, but obviously, actual tire sizes vary from one manufacturer to another. I had to adjust only once from there and now I'm within 1 mph at freeway speeds. I can live with that.

 

You need the IDS to change the speedometer.

The odometer and trip meter will be accurate but the speedometer will not.

 

I was under the impression you could correct both with a tuner?

Both are still off, my GPS shows 310 miles traveled and my odometer is stating 287 miles. my speedometer is off by about 6 mph at 70.

The tough part of this is I'll be running the 34.2" tires only during summer months (April to October) and back to stock size snow tires 31.6" during the winter.

Right now I'm just leaving my GPS attached to the windshield so I know my true speed.

 

I spoke with someone at 5Star regarding the same thing (I have the SCT iTSX). I was told that you cannot correct the speedo with a standard tuner on the V10. He specifically said V10, so I don't know whether that means you can change the speedo on an oil burner or not.

 

I might just be up a creek during the summer months..hmm

 

 

I could not change my V10 either, tried the gear ratio and revs per mile options.

 

That's weird, they asked my tire size when I got my tunes, and on larger than stock tires, my speedo reads 100% right on, GPS verified.

 

What size? My 285s were right on without a tuner. Now I have 33.5" tires, and it is off ~5%.

 

This is indeed interesting. My 2001 V10 4wd had a lift and 35s on it when I bought it last summer. According to my GPS, the speedo was off by about 10% @ 70mph even with the 5Star programming for that size tire. I recently removed the lift and put on 285/75/16s and reprogrammed the 5Star for the smaller tire and now I'm within 1mph of indicated speed on the GPS. I guess I had incorrectly assumed that the programmer was actually doing something when it was the difference in tires this entire time. Misunderstood, I suppose,

 

I've found with my Fords that the factory tire size actually reads a little fast, so with one size up in tires everything is perfect. Same goes for my Explorer factory 235/75 R15s read slightly high but with 31x10.50 15's it reads dead on correct GPS verified.

 

32.6" tires, so an inch bigger than stock.