I've had a feeling my speedo was off for a while now, everytime I pass those roadside speed things I'm going slower than the sign says. Now that I have GPS I'm sure of it. While tuned my GPS said I was going anywhere from 5-10km/h slower than I was, depending on my speed. I returned it back to stock and I was only 3km/h off which is acceptable. I'm assuming I have the wrong tire size in there, so what is the proper size?

The overal diameter is 774.7mm and circumference is 2433.5mm but I think I read somewhere that you're supposed to include 5% squish? What is everyone else using with this size tire (stock 2005 F150)?

 

If you're "indicating" a faster speed than you are actually doing, your Tire Size is too large. The 2433mm figure doesn't account for any squish on the tire (the flattening effect of vehicle weight which reduces the wheel's effective radius. I'd try a value of 2357mm for your tires. You MAY need to adjust it further from there. When you have it set correctly, an Edge speed readout should match your GPS and the dash speedo will be about 2 mph faster at 65-70 mph.

- Jack