hotrozz

I just picked up a 97 f150 2x4 with 5.4L and automatic. The previous owner says the speedo has always shown faster than actual MPH.
I calculated it and the speedo is reading about 10% fast. The tires are the right size, what is causing this and how can I fix it?

BigRedXlt

what size are the tires?

98blue54

im guessing a 245-65-17 or 16? typically dealer has to reprogram speedo recalculation... my speedo is off too but im also running 35's

hotrozz

Tires are 235/70-16, same size as whats printed on the door sticker.

TheJMC

Sounds like some one changed the speedo gear for larger tires but didn't change it back. If you can tell me the rear axle gear ratio I can get you the part number for the correct speedo gear. Look for the axle code on the Vehicle Id sticker on the driver side B pillar.

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hotrozz

Hey thanks! I'm thinking that's what happened too. The rear axle ratio is 3.55.

If you can tell me what tooth count it is, this place has driven gears for only $13.

http://www.transmissioncenter.org/Fo...eter_Gears.htm

TheJMC

The gear is a 21 tooth purple. If you are interested here is the old Ford part number [IMG]file:///C:/Users/JMC/AppData/Local/Temp/moz-screenshot-2.png[/IMG]# 17271-C9OP-A. This is for an automatic trans 4x2 with 3.55:1 gears and P235/70R16 A/S tires.

Regards

Jean Marc Chartier

BigRedXlt

JMC, if I wanted to make my spedometer accurate with my larger tires is it possible for me to recalibrate it using the method you just mentioned or will it only work with stock tire sizes? Also how hard is it to replace the spedometer gear?

TheJMC

The gears I know of for these model trucks start at a 16 tooth and go to a 21 tooth. That doesn't allow for much of a change. Depending on your gear ratio and what you started with compared to what you have now you could possibly make it work.

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