....To improve milage:
1. If you tend to drive fast, drive slower.
2. Maintain proper air pressure in your tires.
3. Don't sit and idle too long, don't use remote start.
4. Make sure it is tuned up and running properly.
I'll add #5.
5. Don't drive up to traffic/intersections/etc and brake hard. I'm not saying to coast from a mile away, but the harder you're braking means you've wasted momentum and therefore fuel up to that point.
There is a whole thing on hyper-mileing. I'm not that guy and those people go to the extreme. At the same time there's no point in keeping rpm's up to maintain a speed that you're just going to slam on the brakes a few seconds later. Easy throttle, easy brake. That will give you the best efficiency.
He thinks he's getting 10-11 MPG because his computer is programmed for 32" tires. I don't think anyone here is so dumb that they think recalibrating the speedometer will actually give you better fuel economy.
Hand calculating won't do him any good, because his odometer isn't accurate. In order for hand calculations to work, he would need to factor in the reduced revolutions per mile.
I won't say anyone is "dumb" if they thought recalibrating the speedometer would improve mileage. I'd call them uninformed. If you don't know how it works and/or how it does calculations you're not dumb for not knowing. Which is why I pointed it out in the first place. I wasn't trying to imply anyone was unintelligent. Only that they simply may not have known, and therefore made a false assumption (OP or anyone else reading the thread).
There's a big difference between unintelligent and uneducated.
Washoe (a chimpanzee) was taught sign language in the '60s-'70s. She wasn't some genius level chimp, just a common primate. She knew something the other chimps didn't, which doesn't mean they were dumb/unintelligent. That just means the other chimps were uneducated.
The same holds true here. If you don't know the adjustment won't effect efficiency you could make an incorrect assumption. That doesn't make you "dumb" because you didn't know what someone else knew.
I would never say or imply that.
As for hand calculating. Yes you would have to account for the odometer error in your calculations too, as you noted. It can be done. It would be a pain, but not difficult. Just extra work. However, doing the extra math for a few weeks to figure out a true loss from the modification may make the OP happier about the mod.