I'm sure others have come across this, but still worth considering.
Overinflating tyres (within safety limits... usually!) is known to cause an increase in MPG. However, it also has an effect on the recorded mileage that will actually 'hide' some of these gains. This is simply because the effective diameter of the tyre increases, if it inflated at a higher pressure (since there is less deformation at the contact patch).
I worked out a few weeks ago that my replacement speedometer was 4% optimistic in the distance travelled, but now I have my tyres at a higher (but within the safety limits) pressure, it actually underestimates the mileage by about 1%
(worked out by digital map data + an actual journey I did yesterday). I won't be correcting my MPGs at the moment as that would make
things more complex, but for those who have changed their tyre pressures, it might be worth rechecking the accuracy of their mileage meter. This is because I have noticed a nice increase in MPG, even when using the speedometer mileage readings that appeared to 'lower' my MPG by 5%.
landspeed 
Threaded Mode
