Hello all, I recently found what I consider a gem of a truck in the body style that I love. A 1997 F150 extended cab 4X4 with the 4.6 V8 and a 5 speed manual and the truck is close to mint. With that, the only real upgrade I wanted to do is change the the gauge cluster out because for whatever reason it did not have a tach. Bought one from the salvage yard after researching and plugged it all in (easy job) and everything works just like it's supposed to except my speedometer is now 6-7 mph off at 55 mph. I am running a 265x70x17 tire where the factory tire is 235x70x16 but I don't think that would make a 7mph difference? I know this year truck the speedometer is gear driven in the transfer case and maybe other places but I wouldn't be sure where to start. Nothing mechanical in the gauge cluster so there are some electronics controlling some of this. I have used Forscan for other things but I wanted to reach out to some of the experts here and get some suggestions. On another note this truck has a build date of 02-96, I had no idea the produced them that early before 97 and sold the square body along side the new body style.

 

So I read a little deeper and then I went and pulled the speedo gear to see what I have, It's red and has 21 teeth. I also googled the difference in the tire sizes and it's a bit more than I thought at 2.5 inches. I'm guessing I need to drop 3 to 18 teeth to support the 2.5in tire size and the 7mph difference. Any words of wisdom before I order a gear? I didn't think this was going to be as easy as a fox body Mustang change after putting 4.10 gears in the rearend.

 

If you can get your hands on a very good scanner/reader that lets you change stuff inside the PCM, you can change the rim/wheel size in it.
I know a snap-on can do that and the one I have can, but most people do not have a scanner/coder like these.

 

I don’t believe that’s how the math works with those speedometer gears.

Once upon a time, I swapped some Exploder 15” wheels on my ‘94 Ranger and went with a bigger tire size. There was math involved and it wasn’t inches to teeth.

This post says 1 tooth = 3 MPH

https://www.ford-trucks.com/forums/4...-issues-2.html

A 19 tooth gear will have to do according to your findings.

 

I found this formula and used it. 8 x Diff Ratio x 638) / 1000 = the number of teeth you need on the speedo gear. 638 being the revs per mile 8x3.55x638=18,119.2/1000=18.1 I went with the grey 18 tooth gear. I will see how close that one is when it comes in. Thanks for the replies

 

Reprogram the psom?
That will fix it
You need to get in there with a NGS and tell the programmable speedometer odometer module (PSOM) what size the tires are
Pretty simple at a ford dealer (well maybe not so much 26 years later)
Pretty tough everywhere else

 

If I read everything right the 97 cannot be done with electronics, only a gear? I have reprogrammed my 13 Eco several times but this old gem is old fashion I think

 

PSOM started as early as 1992
Your 97 is definitely programmable with a NGS
As far as aftermarket can tools scratching the psom
I doubt you will find anything aftermarket that will let you change tire sizes in the module