Speedometer gear trouble or rear end?

[rustyzpr]

I just bought a '97 Explorer, Eddie Bauer Edition, Control-Trac, V6, 4wd. I noticed that a radar sign posted me at 36 mph while my speedometer read 40. I first thought that the previous owner had changed 15" wheels and tires with the 16"ers that are on it now but the door tag calls for 255/75/16. I do know that the left rear door has been replaced & repainted due to some type of accident. I'm wondering if the rearend was replaced with one from a 15" drive train would this cause the problem? Also will this cause a problem like using 2 different sizes of tires? Should I be worried or not?

 

[swetrid]

Normal variation, Ford allows 10% percent error. As tires wear the circumference decreases and therefore will affect speedometer accuracy. None are perfect except for certified police calibrations.

 

[BrooklynBay]

Welcome to this forum! I've moved your thread into the stock 95-01 section.

 

[gavin]

less than 10% margin of error, but if you want to be sure, check the axle tag on your rear axle.
Then check the axle code on the door jamb sticker (assuming it's the original front driver's side door).

it's certainly possible the rear axle got replaced with one with a different gear ratio

 

[RomeovilleIL]

As said, 10% below is in spec. Check your tire pressure as well. Low air in the rear tires can throw you off quite a bit. We have issues around here with cab drivers who like to air down their tires to increase their fares in the city. News crews like to investigate every so often - at $2 per 1/4 mile it is big $$$.

 

[pjtoledo]

less than 10% margin of error, but if you want to be sure, check the axle tag on your rear axle.
Then check the axle code on the door jamb sticker (assuming it's the original front driver's side door).

it's certainly possible the rear axle got replaced with one with a different gear ratio

and while you're under there, check the front axle too.



I would borrow a GPS and see what it reads, way more accurate.