You're "overthinking" this a bit (I do it all the time, so you're not alone).
Don't worry about entering your stock tire size at all. You just want to know the circumference of the tires on your truck. Enter 285/75-18 in the "Old Tire Size" window and leave the "New Tire Size" blank. This gives you 2711 mm as the circumference with 5% squish.
What you did by entering your REAL old tire and then the new tire sizes and the gear ratio was to find out what your new, "effective" gear ratio is (which is 3.66). So, you don't have quite the "pulling power" with the new, larger tires as you had with the old ones, but you may get better mileage at cruise, since your RPMs should be lower.
If you wanted to keep an effective gear ratio of 3.73 with the new tires, you'd actually have to change your gearing to 3.79, and, if you could do this, you'd effectively be back to stock.
So. use the 3.73 gear figure if that's what's in the truck and a starting circumference of 2711.
As far as verifying your speed, if you can't borrow a GPS that shows speed, you can do things the hard way, which is what I did at first. The next time you drive a long stretch of highway with mile markers (interstates are best) note your odometer reading as you pass a mile marker. Then, after a long drive (I used an actual distance of 100 miles, according to mile markers) note what your odometer reads again. If it says 100.0 miles after 100 actual miles, you are dead on for speed AND distance. If it shows, say 101 miles, it's because the tire size you entered is too large and you are driving 1% slow. Decrease the circumference by 1% or 27 mm.
You can do this over shorter distances, but it won't be as accurate.
By the way, you'll see some people recommending measuring the distance your tire rolls on your driveway in one revolution and using that figure. It's just a starting value too and doesn't take into account the "expansion" of a tire as it rotates at high speed. (Think of a dragster's tires during a burnout).
- Jack
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