I take my miles shown on the trip odo, multiply by 1.04, divide by gallons pumped.

 

no i am sorry is said that wrong how do you get the calculation on the percentage tire difference. i am running 315s

 

dont worry i've already factored in the difference in tire size for the mileage difference. Every bit helps.

To do the difference in mileage for hand calculations this is the way i did it:

I looked at the door tag to get the stock tire size. I think it was a 235/75. I went to a 315/75. It winds up being about 30.5" vs 34.5". So you have about a 4" differnce. so you take 4/30.5 = 13% roughly

So when I look at the mileage off my tripmeter, i multiply by 1.13, then divide by the gallons. It works the same when you are factoring speedo error. I'm going 13% faster than indicated.

But yes that is one of the reasons I think the smarty might be the first thing i buy, b/c of the speedo correction feature.

 

Plug your tire sizes (stock vs installed) in to the right boxes and it gives you the correction factor.

http://www.dakota-truck.net/TIRECALC/tirecalc.html

 

What is a 100 mph passing zone? Never heard of that.