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The Ontaribertan.
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Discussion Starter · #1 ·
Hey everyone,

For those of you with a mileage computer (or whatever they call them), are they programmed with the speedometer and therefore with tire size?

I ask because I am going to be throwing my 35's on the new truck and I love how handy the mileage computer is. I know you can reprogram your speedo with scanners and programs like AE, so if the mileage computer runs off the speedo, can they be reprogrammed for bigger tires as well?

Thanks, as always.
 

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I don't know all the exact workings but with bigger tires it did read lower than it did with stock tires and what actual mileage was so there is a correlation there. But I know it reads off of throttle position, I don't know if that is the only thing it reads off of or not. That is why with most chips/tuners on the truck you will see higher mileage than you are actually getting. If your chip makes the throttle very touchy you won't need to push very hard to go, so it thinks you are getting better mileage.

Thinking about it speed has to be in the calculating somewhere because it can tell you are getting worse mileage at a stop, so I would think recallibrating will have an effect on it. The change in size up to a 35 is about 10% give or take, meaning if you didn't recallibrate, your odometer says you went 100 miles you actually went about 110miles. So your mileage is also about 10% off so at 15mpg it might be 1.5mpg off, so it won't be a huge noticeable difference.

But like I mentioned, if you put a chip or tuner on the truck, you are almost guaranteed that it will not read correctly.
 

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The Ontaribertan.
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Discussion Starter · #3 ·
The change in size up to a 35 is about 10% give or take, meaning if you didn't recallibrate, your odometer says you went 100 miles you actually went about 110miles.
That's exactly what made me wonder whether or not these mileage computers could be reprogrammed, because you can adjust speedometers and therefore odometers to correct for tire size. If these computers run of throttle position does that mean there is a link to the PCM? If so, maybe tuners can adjust for these things??

Hopefully someone has tried to adjust these things before.
 

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Word
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Your PCM uses vehicle speed to calculate distance traveled....from the factory it's programmed with the exact wheel and tire size along with other things like transmission ratios, ring and pinion final drive ratios..etc...so by changing wheel and tire size or ring and pinion ratio you will throw off the vehicle odometer and speedometer, actually Ford built some tolerance into the programming features of it's PCM, the dealership can access these parameters and adjust for tire size increase and for ring and pinion changes, I know because I've done it ( 8 years as a Ford Master Tech working in dealerships ) now as long as you don't go crazy on tire size it can be adjusted, but the dealership will prob. charge you about $120 to do it...if they agree to do it at all, a lot of dealerships don't like to mess with factory parameters....Ford put alot of controlable parameters into there modules, you can turn off interior lights so they won't come on when the doors are opened, you can turn off the seat belt chime and warning....all kinds of stuff...
 

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The Ontaribertan.
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213 Posts
Discussion Starter · #5 ·
Alright, thanks for the help.

I aware that Ford could adjust your speedo for larger tires but does that automatically mean that it will adjust the mileage computer as well?

Maybe I'll give them a call.
 

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Word
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Yes, after reprogramming your PCM with the correct tire size your odometer will read correctly, along with your speedometer and your MPG ( miles per gallon or mileage computer ) feature, if you have this...because now your wheel speed input will be correct from your wheels to your PCM...
 

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Can only the ford ids do this or can other scan tools do it too?
 

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AE is coming out with a new version that is suppose to do this. BUT they have been saying its coming soon for at least a year.
 

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FNG
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AE is coming out with a new version that is suppose to do this. BUT they have been saying its coming soon for at least a year.
The new AE is out. But you have to have the newest cable to do it

Ford IDS can do it but only up to a 315
 
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