One thing to keep in mind when calculating your MPG too is tire size. If you have larger tires, you will show MPG that is higher, as you're recording less miles than you actually drive. Also, if the truck has smaller than factory tires, you would likewise show lower than actual MPG.
I think that's backwards. Bigger tires travel farther across the road for the same RPMs, and hence log less relative miles on the odo.
(At least I hope that's the case. If not, then I'm getting 12MPG with 33s! <img src="/forums/images/graemlins/zombie.gif" alt="" />)
No, it should be correct. If you have an oversized tire, and your odometer says you traveled 1 mile, you actually only traveled something less, let's say .75 mile. If you say you used 10 gallons to go 100 miles, based on the mileage your odometer spits out, then you would think you were getting 10MPG. In reality, you would have only traveled 75 miles, and thus your actual MPG would be less, in this case 7.5MPG.
Likewise, if you have an undersized tire, it may cause the odometer to show you traveled 1 mile, when you may have actually traveled 1.5 miles. If you calculate your MPG based on using 10 gallons in 100 miles from your odometer, you think you're getting 10 miles to the gallon, when in actuality, you would have gotten 15MPG.
The odometer spins slower with oversized tires, and faster with undersized tires.
Of course I realize these #s do not apply directly to this case, and I;m not saying anyone gets 10MPG or 7.5MPG, or for that matter 15MPG. I was only using those numbers to make it easy to calculate for demonstration purposes.
