Hi,

I have a 1996 Bronco. I put 285 75R 16 Goodyear DuraTracs on it. The tires are approx 32" tall.

According to the Goodyear website they are 634 revolutions per mile. One formula I found was to divide the revs per mile by .6666 so that comes to 951 to enter into the dash.

By using the other formula I come up with 32 x 3.14 = 100.48 / 12 = 8.37 5280/8.37 = 631/.6666 = 946. In theory the value should be around 946-951. I entered 951 and I am off 5 MPH at most speeds. If the speedometer shows 55mph I am actually traveling 60mph according to the GPS. I read the formula wrong the first time and thought I just put in the revs per mile so the first time I entered in 634 and it showed like 70mph on the speedo when I was actually going 40.

Does anybody know what number I should try below 951 to adjust the speedometer up 5mph? I only have 4 tries left. I plan to keep this tire size as long as I have the rig so I am not too worried about trying again. I just do not know if I should go down 10 numbers or 50 numbers. None of the dealers want to mess with it around here. Thanks for info.

 

Just some comments on all this.
Mark a tire side wall and the pavement.
Roll the truck one full revolution and measure the distance it rolled back to the tire mark.
Then try this calculation. 5280 feet times 12 = 63360 inches.
Divide this number by the distance your tire rolled on the pavement.
This should take into account most all the variables such as truck weight and air pressure except centrifugal tire growth as speed increases.
Here is an example:
32" tire rolls approx. 32 x 3.14 the number for pi = 100.48 inches.
63360 inched per mile divided by 100.48 rolling inches = 630.573 revs per mile or rounded off the 631 revs / mile.
Since we don't know how your trying to adjust or what system your working with on a 96 Bronco, it's just the basics of it.
Your speedo display may not be accurate for display.
Measure a known 1 mile marker distance with GPS and apply the known difference correction to what you see on the speedo.
You cold make a decision based on two known rev values and pick one number between them after some thought.
.
If you end up within 2 mph on the speedo, I would leave it at that.
My 02 electronic system reads 2 mph high from the factory with stock size tires at 60 mph. Nothing has been changed for tire size except brands and tread designs..
Hope this gives you a perspective.
Good luck.

 

Thanks for the reply. I just found out that the Jet 50109 should be able to dial it in and not count towards the adjustment count. I am keeping my fingers crossed.