Bout the go from a 255/70/16 to a 285/75/10 size tire and was just wondering how much my speedo will be off? How much does it cost to get it reprogrammed ?

 

Not much. Not enough to worry about you'll be fine

 

I assume you mean going to a 285/75/16 not 285/75/10.

If thats the case, your speedo will be off by about 10%, which in my opinion is pretty sizeable, if you're going down the highway and your speedo is reading 72, you'll be going about 80. Just make sure you keep that in mind to avoid tickets.

A programmer will be able to adjust your speedo to make it accurate, or the ford dealer can do it for you, and probably charge between $50-$75

 

Yea that's what I meant ... I will prolly have it done then. I don't want a ticket and ill forget about the difference lol

 

I went to 305/70/16. Yea at 70 your goin around 75 but at 55 it's more like 57.

 

How much you're off by depends on what your orginal size was. But given that its a percentage of the original it does increase as you speed up.

Heres how to do it out in terms of math. My truck came with 235/70R16s and I now run 235/85R16. In order to get a sense of what that means you first have to calculate your outside diameter.

Starting with stock for my truck:
We take the 235 number, this is the width of the tread in mm. In order to get the profile hieght (the amount of rubber between the rim and the ground) we multiply it by the aspect ratio, in this case 70%. This gives us 164.5mm (about 6.5") which we then have to multiply by two to get the total height from the tire itself, 329mm. Now we have to add the diameter of the rim, since I've been working in metric thus far, I will convert the 16" rim to mm rather than the other way around. 1 inch = 25.4mm, so a 16" rim is 406.4mm. To get the total outside diameter of the tire, we add this to the hieght of the tire itself, so 329mm+406.4mm=735.4mm

Now we do it again for the new tire, I'm not going to explain all the math again, but for a 235/85R16 tire we end up with a total of 805.9mm

Now, divide the new tire size by the stock one, so 805.9mm/735.4mm and we get 1.0959. This means that the new tire size is 109.59% the size of the old one, and your speedo will read 9.59% too slow. (I just call it 10%)

 

Thanks for all the help guys

 

My truck is built to run 235/blah/16, i'm running 255/blah/16, and my speedo reads 3 mpg over my actual speed, from other peoples post and based off my own results it seems that with every tire size increase you will see roughly 1.5mpg reading difference

 

its 9,2 %
Know when you drive 50mph on your Speedo its real 54,6

Here you can see the different...sorry its in German

http://www.buschtaxi.de/923.0.html

 

I'm in the habit of driving off my GPS