speedometer

[raceking1324]

ok now on my 91 explorer my speedometer is off by like 5,10 mph how can i fix it or what can i do to fix it

Topic

[Matt]

Odd question, but are yer tires set at the correct pressure? If they are both low and severly worn, that alone will cause your speedo to read a bit faster than yer actually going. Just a thought.

[jbone]

And how do u know this? what are u comparing it to?..Like beefy
said tires will do this, if size is wrong or pressure is low,or maybe your instrument panel is on the fritz…Post back on your findings….thanx

[dreamer2355]

Does your speedo needle bounch around at all? You could try using some graphite lube at the back of the speedo where your cable plugs in.

[Matt]

The reason I have noticed the difference between old low pressure tires is because I deliver pizzas, and in the area I deliver in, the cops set up alot of those radar guns attached to a giant LED sign telling you how fast you’re going. I have been past the same one and my speedo shows 4 or 5 mph higher than the sign, and then literally the next day when I replaced the front tires on my geo prizm, it was a couple mph closer. I also had an astro van that had a speedo that was WAY off, and I pulled the whole cable, and pulled the spring out of the sheath and greased the whole thing up very liberally. That fixed my problem, because there was corrosion in the cable. Another thought–make sure you have the correct size tires on yer truck.

[EricTheCarGuy]

Beefy is on the right track with tires but the real issue would be if you have different sized tires than stock. If you changed the wheels and tires to a different size that could cause that issue. Other than that it could be a problem in the cluster or the gear that goes into the transmission, to make that determination some testing would be in order. I would start by pulling the gear out of the trans to have a look at the teeth on the gears to look for any wear.

[raceking1324]

ok the rims are 15 and i think they are stock and do u think if i buy a new cluster it mite fix it??and i just had a shop rebuild my trans so i know that’s all good

[EricTheCarGuy]

Now THAT is an important clue, if they broke the speed sensor during the rebuild they may have put a new one in that is not correct or perhaps the old one is not installed correctly. Anytime you have an issue after doing work it’s always a good idea to go back and check the work.

[BigC]

Hi raceking1324. I checked tirerack.com for the tire size of your ’91 Ford Explorer. There are two equipment packages listed.

Package #1: 225/70R15
Package #2: 235/75R15

DId the concern with the speedometer occur before or after the transmission rebuild? If it occured before the rebuild, then double check your tires to make sure they conform to the specs listed above. I deviation in either tire size would result in a different revolutions per mile for that tire. That could be enough to affect the speed sensor in the transmission and produce an inaccurate speed.

[Rich]

The first gen explorers had two different size tires that you could mount on the rims(BigC is correct). P225/70R15(stock) and P235/75R15(optional) depending on which size you have on your car, you needed to change the gear on the speedo. Do exactly what Eric said and count the number of teeth that the gear has. I cant remember though if you had to lose or gain a tooth on the gear W-| The speedometer cable goes into the transmission tail housing, it should be a black cable. Hope this helps!

EDIT: To find out how many teeth you need use the following formula
New Teeth = (Current Teeth x Speedo Reading)/Actual Speed

There are different colors for each gear here are the gears and the Ford part numbers

16-teeth-blue (D0AZ-A)

17-teeth-green (C7SZ-A)

18-teeth-gray (C7SZ-B)

19-teeth-tan (C7VY-A)

20-teeth-orange (C8SZ-B)

21-teeth-purple (D0OZ-B)

[Author]

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