Ever since I reprogrammed my truck after receiving an Evo warranty replacement, my speedo has shown a speed about 5 mph faster than my actual speed.

For example, at 1800 rpm in fourth gear I should be cruising at 65 mph, but the speedo shows 70.

The obvious issue is having the tire circumference or gear ratio entered incorrectly. However, I have checked both several times and reprogrammed each to time in hopes that the mistake will be corrected. I have both parameters entered properly, but still my speedo is off.

Ratio-- 3.55 Tires are 16 in 265/75R (IIRC). The circumference is definitely 2524mm, though after entering the number and programming, the Evo consistently reverts the number to 2526, though there is no way 2 mm would produce such a skewed result.

Any ideas?

 

Do you have a GPS to compare to? Try 2430 see where that gets you

 

Not anymore. I fried it attempting to hardwire it into the truck's electrical system.


I have a good idea of what speeds correspond to engine rpm when cruising. I can also have my fiancee drive alongside me in her car later this week to confirm.

I'll try the circumference of 2430.

 

Math isn't a guessing game...

(actual speed / displayed speed) = X

(X * old tire circumference) = new tire circumference

 

Or, here's a slightly different explanation. You are not accounting for "squish" - the flattening effect that weight puts on the bottom of the tire which effectively reduces the wheel's radius. The fact that your speedometer reeds high means your PCM thinks your wheels are bigger than they really are. Your 2524mm is exactly correct for an unloaded tire. I've found that subtracting about 3% from that corrects it for "squish", so, since 3% of 2524 is about 76, try 2524 - 76 = 2448mm.

And the difference you see when you reload the previous setting is just an artifact of the way the Edge truncates and rounds values in converting the circumference to revs/mile for the PCM and back again to circumference for the display. It's of no real consequence.

Remember too that your truck's speedo reads about 3% higher than you are actually traveling, if your tires are inflated properly. This translates to about 2 mph in the 70 mph range. Using i.ride.suzuki's formula, you'd be working with an actual error of +3 mph which is about 4.5%, so you may need to reduce your circumference by that much.

- Jack

 

I'll crunch the numbers and see that improves things.

The only problem I see is that my tires had more "squish" prior to using the current tuner. A faulty tire gauge consistantly gave me a reading of 35 lbs when in reality all four tires were down to 15. I know, I know, you would think that the rims would nearly be sitting on the ground at that pressure but they were not. The tires looked very normal.

When I was running the abysmally low pressure, the speedo was correct.

Just sayin'...

I'll post the outcome of your suggestions.

 

That's interesting! Here's one possible explanation: If your tires were OEM sized tires, and if you bypassed the Custom Options step in power programming, the Edge would take the tire size from the PCM's VID memory section. That tire size has been preloaded by Ford to match the OEM tires with a 5% squish (according to Bill Cohron at PHP). Why 5%? I suspect it's because Ford knows that many people drive around on underinflated tires. (And please, I'm not accusing you of being negligent here - the fact that you were checking your tires says you weren't). But we all see lots of folks riding on badly underinflated tires.

I believe you when you said you couldn't see the "squish". I had a slow leak in my car's rear tire recently and by the time I noticed it, the pressure was around 15 psi too.

- Jack

 

That Ford and their built in squish factor.

I can definetly tell when the fronts are low better than the backs.