Ok I have a 96 f150 5.8 when I bought it had 235/75/15s on it and I put 31s on it I looked around the web about the calabrations the truck have a 972 factory cal and I dropped it to 918 hiway driving is great now but with speedo says 70 I'm really doing 65 does anyone know how to change the calabration to a higher number I can only make it go down!! Someone also said it would change my shifting better

 

cant help you but im curious how you can change that number for bigger tires. i got 35s and my speedo is 5 mph off.

 

I did it on my 93 for 35s ill see if I can find the link.
It made it dead on and shifted better.

 

Here ya go.

 

Where do you get 906 I'm only finding 918 for 31s wouldn't a 906 make it worse?

 

Make what worse? Lol
I have used these numbers for every size tire listed and it works great!
After the adjustment the speedos were checked with GPS. and all within 1mph of the GPS.

 

http://www.fourdoorbronco.com/board/...edometer-(PSOM)

Heres a good write up. The sheet 7700 shows is correct.

If you go all the way down with the cal number it wraps back around to 10 or 11 and starts again.

 

Just thought I'd follow up with this thread. Put on some new tires 255/65/17, the outside dimension is almost identical to 31/10.50/15. Using the formulas from above I came up with a conversion constant of 925. Well before adjusting the PSOM my speedo read 3 mph slow(factory 972), after putting in this new conversion constant it was 3 mph fast. Went halfway to 948 and speedo is perfect.

So the moral of the story...... I don't know what the correct way to figure a new conversion constant is, but it sure isn't in this thread or that Bronco thread I referenced.

I just thought I'd throw this up for anyone fighting with a speedo calibration.

 

the rim size plays a role in the final number, my factory setting was 984 and it needed to be 898 for my 35s. after i set it to that number my speedo is dead on and my transmission shifts perfect.

this is the link i found that helped me figure it out.
http://www.ford-trucks.com/article/i...ND_BRONCO.html

 

Rim size doesn't mean ****. A 35" tire on a 15" rim and a 35" tire on a 17" rim both measure 35".