ok i have a 2000 f250 4x4 v10. from the factory it had 3.73 gears and 285 tires.. i have lifted the truck and it now rides on 38" tires and 4.88 gears front and rear.

now i have done numerous gear changes , lifts and tire sizes on many vehicles. i have always been able to get the speedometer within a few miles of correct by installing the right gears to match the tire size.

according to 4 wheel parts. the 4.88 gears are the correct gears for 38" tires on my truck.so lets start from the beginning.. i placed the 38" tires on my truck first with the 3,73 factory gears. my speedo became 5 mph off. it showed 60 on the dash and was clocked ( 3 gps units) at 65mph actual. when i installed the 4.88 gears which should have corrected the gap.. it actually went the opposite direction. the truck is now 14mph off.. when i show 60 on the dash im doing 74mph actual.

i contacted 4wheel parts and they said to take it to ford to have them reprogram the speedo. so i spent $115 at ford. they programed 4.88 gears and 38" tires. the speedo didnt change at all. still exactly 14mph off. i thought ok well in my experience when the speedo is below actual the gear size needs to be above the tire size. so i had them reprogram it for 4.88 gears and 33" tires. nothing. speedo is still the same.

it seems every program variation i try , the speedo stays the exact same ..ford cannot figure it out . i also replaced the abs speed sensor on the rear axle with no change. i bought the speed sensor for the transmission but have not installed it yet.... but the way things are going it doesnt seem like that would even work. i have never had a speedo go in the opposite direction from which it should

what i mean by that is , when you have factory gears and install large tires the speedo goes below actual speed on dash... when you install large gears with stock tires the speedo goes above actual speed on the dash. but in my case it has done just the opposite. seems the more i try to correct it , the farther off it gets

im at a loss here. any ideas would be appreciated

 

oh and the pcm has not thrown any codes. other than the speedo off truck runs fine

 

It sounds like whoever is doing the programming at Ford doesn't know how to do it. The gears don't matter, because the tone ring is on the gear, and goes around 1 time per tire revolution, regardless of what gear you have. The only thing you do is put the correct tire size into the ABS module.

If you know someone with AutoEnginuity they can do it as well as the dealer.

 

Well the only problem there is there is a speed sensor on the tranny.. when you change gear ratios, yes the ring gear goes around 1 time per tire revolution but the drive shaft changes how many times it rotates. Exp. 3.73 gears the driveshaft rotates 3.73 times to 1 tire revolution so the gear ratio makes a big difference in your speedometer. Because the tranny sensor is supposed to pick up on this.

 

The tranny speed sensor isn't used for ABS input. Only the Vehicle Speed Sensor is. The VSS signal is repeated from the ABS module to the PCM and the instrument cluster.

Regardless, if the guy is changing parameters and the speedometer isn't changing, he's obviously not doing something right.

 

The Vehicle Speed Sensor (VSS) is on the rear differential. It reads a signal off of the ring gear. One tire revolution, one gear revolution. This is why the gearing doesn't matter. The tire "revolutions per mile" figure has to be programmed into the truck.

The gear parameter is important also, but it has influence in shifting points, not speedometer reading.

 

The VSS has been replaced and the PCM reflashed to 2 different tire sizes and the speedometer stays the same... doesnt change at all. Still 14mph off

 

Flashing the tire size into the PCM won't change the speedometer. You have to flash it into the ABS module.

 

Ok,,,then i will be going back to ford tomorrow then

 

My question now though would be if gear size is not a factor then why was my speedo 5mph off with 3.73 gears and 38" tires...and is now 14mph off with 4.88 gears and 38" tires? All i changed was the gears and accordi g to what yall are saying that shouldnt make a difference

 

I have a similar problem. I have 35" tires on my 99 F350and as the state trooper assured me i was doing 5-7 mph over the speed limit compared to what my speedometer read on cruise control. I checked my speed with two gps units and yes i was going 7mph over at 65mph compared to my speedo. I changed my gearing to 4.56s to get more power to the ground and fix the speedo, but nothing changed. I am not real sure on the math but 14mph seems to be about right with that size tire. My friend has a truck with 37s and he is way off when driving by his speedometer, but I am not sure about the actual difference.
I went to ford to get the speedo fixed and the guy told me that it was the ABS module that reads the rev of the tire and gearing had nothing to do with the speedometer reading. When he attempted to fix the speedo he could not get the tool to link with my ABS module and said he could diagnose the problem but that would be a minimum of a lot of money so I said I would deal with the inaccuracy. The ford tech told me it might be better to take it to an individual's shop to re-cal the speedo because the ford tool is very limited.

 

I don't really know why that would be. All I do know is how it's wired and what I did when I changed my tire size. You might post in your state's chapter forum and see if there is anyone around you with AE. It's extremely simple. I just have stock size tires right now, but I adjusted my tire size in 1/4" increments until my speedo was right on.

It's funny, BigBlue, that the Ford guy would say their software is limited. It might be, I've never seen it so I don't know. It's really simple with AE however.

 

What is AE? Automotive Experience?

 

AE is AutoEnginuity. It's a piece of hardware that connects to your OBD-II port with a USB connector that will connect to a laptop, and software that will do a whole lotta stuff.

Like read ALL Ford codes accurately, do all sorts of engine tests, monitor every available PID from all modules, and perform module specific functions like programming PATS keys and adjusting tire size in the ABS module.

It's a very nice piece of software, but pretty expensive. Riffraff Diesel Performance is a site sponser that sells the Ford Enhanced package for $359.89.

 

Ok so ford gave me a refund. Their program system will enter in whatever tire size they put and accept it, it ust wont program the truck. And it was the abs module they were trying to program. But it wont accept anything over a 33" tire with ford equipment. So today talked directly to hypertech and i bought a hypertech speedo calibration tool for $160. Supposed to go up to 43" tires and 5.13 gears. So we will see when it comes in.