Ok After talking to 4 different Ford dealers I'm beginning to think I'm losing it. My truck is a 97 with a 3.55 rear end and originally came with 255/70/16 goodyears on it. I have since changed to 285/75/16 BFG AT's. I was under the impression that my truck's speedo wasn't adjusted by the computer, but by the gear on the side of the tranny. I have had this gear in my hand and know its a 3 or 4 dollar part. Shouldn't it be really easy to figure out which gear I need to correct the speedo? Am I out to lunch? Any help would be appreciated this has been a thorn in my side for a while. Thanks Phil

 

Do a search for speedo correction. There are tons of topics on this as well as some sites that will show you how to calculate your tire height to know what vss gear to use.

 

Hi Phil,

Don't worry, this is pretty easy.

Yes, you are correct, you will need to change the speedo gear as up thru and including the 1998 model year, while these trucks do use a VSS, they also still have a traditional speedomter gear in the tailshaft of the transmission, so it's easy & cheap to correct your speedo for gear ratio & tire size changes.

The first 2 things you will need to know are:

1.) Exactly how far off your speedometer is after teh change in tire size and/or gear ratio change. Speedo errors are straight linear percentage errors, so if it's 2 mph off at 50 mph, then it will be 4 mph off at 100 mph, meaning it's 4% off. If it's 5 mph off at 65 mph, then that's a 7.69% error (5/65=7.69%).

2.) How many teeth are on your current speedometer gear.

Take your percentage of speedo error and apply that to the number of teeth on your present speedo gear to get the number of teeth needed for the new speedo gear. Don't make simple mistakes like not adjusting the math for whether the speedo is currently reading too fast or too slow!

If you need help with this, give us a call once you know your speedometer error and the number of teeth on your present speedo gear, & we can tell you the number of teeth you'll need on the new gear.

The speedo gear is located in the tailshaft of the transmission, look on top of the tailshft & you will see a little spring-loaded door. Open it and look inside to count the number of teeth, usually you have to pull it to count the teeth, though sometimes you can get lucky, it's not hard either way.

I'm not really good at explaining these kinds of things via the written word especially. Though I can easily do the math & get the correct number, I'm just not a good teacher (terrible, in fact), so I'm sure someone else can give you a much simpler & precise explanation, but if not, feel free to call us if you want help on this, ok?

Good luck!

 

Cheers guys... I think I have it figured out. I found an old post with the same tire swap on an f-150, and did the math, and came up with the same change. Now I just hope I can find a 17 or 18 tooth gear. Thanks for the help....Phil