Ok, I have a 2005 fx4. Tire cir.=2610mm (285/70/17), stock ratio=3.73.
If I'm trying to reset my speedometer to reflect the true speed using my EVO programmer, and the variances are: speedometer=45mph, Evo=43mph, GPS=42mph. Assuming the GPS is probably the most accurate at this point.
Question is? should I enter a larger gear ratio or a smaller gear ratio in the programmer?

 

Don't change the Gear Ratio! That would be much to big a change. You are actually VERY close. If the Evo says 43 and the GPS says 42, you need to reduce the Tire Size about 2%. So, try 2548 for the Tire Size. This actually sounds like you have not accounted for tire "squish", which reduces the effective tire radius due to the weight of the truck.

The goal is to get the Evo and the GPS to match. The dash speedo reads higher than actual due to mechanical errors, but the odometer will be correct.

- Jack

 

Thanks Jack. I will try changing the tire size. The book said you could reset the trucks speedometer if you programmed the tire size and the gear ratio. That doesn't work. Anyway, I just want to be close on the GPS and EVO. Thanks again Wes

 

The book is correct, changing the Tire Size and/or Gear Ratio WILL change the truck's speedometer (and odometer). But, if you get the odometer reading accurately, (Evo and GPS matching) the dashboard speedometer will read somewhat faster than you are actually driving due to it's mechanical translation of digital data from the PCM. Think of it as Ford's attempt to keep you from getting a speeding ticket.

If you get the dash speedometer reading accurately, the odometer will read low.

- Jack