im going to take a sub par educated guess here and say that fixing the geo will reduce your mpg.
more of the tyre on the road means it has more grip, which i guess in turn means more effort to roll it.
i think hot weather has something to do with economy...
ive had mine for 4months now, and i was used to getting 400miles out of 60ltrs [light comes on for last 10ltrs], but the last 2 tank fulls, im getting only 320ish... ive lost nearly 20% in mpg.
odd really!
Id say hotter temperature would lower fuel usage as the air is warmer and less dense in turn cousing you to use more fuel in winter