I just had a tire dealer tell me that I can not change the tire diameter because it will screw up the computer in my 94 Explorer? How can the vehicle know what the diameter of a tire is? Has anyone heard this before. The tire dealer said they were advised of this by a bulletin from Ford.

 

Changing wheel size can alter the ratio of transmission RPM to vehicle speed. This is true only if the outer circumfrence of the new tire is different from the original. Your explorer is equipped with Vehicle Speed Sensor (VSS) that is mounted on the transmission output.

According to Haynes Manual, "The VSS is a pickup coil sensor mounted on the transmission. It produces AC voltage sine wave, the frequency of which is proportional to vehicle speed. The Powertrain Control Module (PCM) uses the sensor input signal for several different engine and transmission control functions. It also drives the speedometer. A defective VSS can cause various driveability and trasmission problems."



FordFool

 

And if you do change the overall circumferance/diameter from your stock size, you can change your speedometer gear for $7.82 by getting a new one from Ford and installing it in place of your original one. It will take you longer to drive to Ford and buy it than it will to put it in.

 

 

[updated:LAST EDITED ON 10-Feb-02 AT 00:14 AM (EST)]I put 265/75/16's on my 99 explorer xlt The only thing I noticed was with my GPS(global positioning system) Up to 50 mph My speedometer was right on for speed and 51 to 80 It was 2 mph faster than the speedometer read and 81 to 100 It was 4 mph faster than it read. The speedometer reads 100mph and my actual speed is 104 then the Governer kicks in. I talked To the service manager at Ford About adjusting the computer to fix the speedometer he said it was a waste of money most speedometers are off that much. and it wouldn't cause any problems. And 2 other Ford dealer service managers confirmed what he had said.

 

I put 275/70/15 s on my wifes 96, and like previous post, GPS indicated an error, but not enough to worry about.

I also had several tire dealers tell me they couldnt put those tires on because they wouldnt fit, per mfg "fit book" ie FORD.

I told them I wasnt interested in what FORD engineers advised about MY tires, and walked. ended up buying from JCW tires.com and paying Costco to mount and balance. a hassle yes, and I got what I wanted, not what FORD advised .....

 

[updated:LAST EDITED ON 14-Feb-02 AT 12:42 PM (EST)]If I remember correctly.. the Speed sensor is located on the speedo cable (like it is on the 1991-1992 Explorer), not in the transmision. When you change your tire size.. figure out how much bigger it is (in percentage).. Then take the driven speedo gear out of your transmision (or xfer case if 4x4) and count how many teeth it has. Now you need to figure out how many teeth you need to get it back to the right speed.

So if you change your tires size by 10% (and lets say you go bigger which means your speedo will now read low).. You will need driven gear that is smaller by 10%.. so if you have an 18 tooth gear now.. you want a 16 tooth gear.. Now your speedo will be close (not exact since you only wanted to change it by 1.8 teeth, but you can't do that). It is 1 bolt to pull the speedo out of the xfercase or tranny, 1 clip.. pull the gear off.. and then pull off the old gear and put it all back together.. it takes me <5 minutes (I don't have to lift the truck to get to it on our 1992)

Once you do this.. all will be normal again.. (your shift points may be off if you don't do this.. and mileage may suffer slightly)..

~Mark