I recently corrected my speedometer error and I wanted to post what I found out.

97 & 98 F150's require a speedo gear change in order to correct speedo errors. The gear is located on the driver side of the tranny or transfer case. To see the gear all you need to do is remove one nut (11 mm) and pull it out. The gear is held on by a clip. They cost approx $10 from your Ford dealer. Changing the gears out takes less than 5 minutes.

Tooth Count Color Ref. OEM #
16 Tan
19 Tan C7VY-17271-A
20 Orange C8SZ-17271-B
21 Red
*23 White J316-7393

*This is a retrofit gear to allow the speedometer to function with 3.73 and 4.10 rear end gears. Since the gear tooth pitch is slightly different than OE, the service life of this gear is limited to less than 25,000 miles.

If your speedo reads low you need less teeth. If it reads high you need more teeth.

My truck came stock with 235 70R16's. I put on 265 75R16's on the stock rims. This caused the speedo to read approx 10% low. My truck had a red 21 tooth gear. I replaced it with a tan 19 tooth gear and compared my speedo with a GPS. It is now within 1 mph at 60 mph.

I found this website when I changed tires and it calculated my speedo error very accurately.

http://www.dakota-truck.net/TIRECALC/tirecalc.html

I know that there are formulas to figure out what gear you need but I just took a guess and happened to get it right.

I hope this helps someone out.

 

Thanks. That really simplified things for me. I made the same change you did in tire size on my 98 and I think I'll stop by the parts counter on monday. Should make things easier with the numbers too. Thanks a lot!

 

Dpfolie,

I have a 98 2WD and this is a great tool you have posted, but I guess I am just slow on trying to figure out what speedo gear I should use if I go to a 33" tire. Currently, I have 31.6" tires as you have or 265/75/16 with 4:10 gears and I am considering going to a 275/60/20 or 33". I've plugged in the numbers and got a speedo difference of -2.4 on the speedo count at 60 MPH, but what speedo gear do I need to correct the difference between 33" and 31" tires with 4:10 gears at 60 MPH? I assume I need to go down to a 16 tooth gear based on your post, is this correct? If so, do you have the part number for the 16 tooth gear?

Thanks in advance.

 

The first thing you need to find out what speedo gear you have installed and then go from there.

 

My speedo is 10% slow and I still run stock tires. I think this thread gave me the info I need to get rid of this problem.
Pull out the old gear and go to one size smaller, correct?
Don

 

Reducing your speedo gear by 2 teeth is approximately a 10% percent change.

 

I need the 23 tooth "retro gear". Went to the Ford dealership and they were lost, and said that wasn't a valid part number. Anyone know where I can get one of these?

 

The 23 tooth gear is a MOPAR/Chrysler product. You have to hit one of their dealers.

It will work on Fords but it will usually not last over 25,000 miles because of the thin teeth.

 

Perfect, thanks. Called up the closest dodge dealer and they have 5 in stock.

Thanks gain!