I have downsized my tires to a 195/50R15 and I have achieved much better combined MPG. If off set my speedometer a bit and i have calculated that my odometer will be off by 10% but even still I have been achieving around 42 MPG Combined Driving (thats city and highway). My highway MPG has dropped form 47 to about 44ish (depending on how badly i decide to speed). But my complete city driving MPG has gone up from 34 to around 38. I also run my tires at 38PSI cold pressure and i also have eibach sportline springs with Koni shocks. Oh i currently run Bridgestone Potenza G009. The next thing i am going to try is going back to a 16" rim but a much lighter one with a 40 series tire on it, because if you lighten up the rim and have a low profile tire on it, it should decrease the overall weight of the rim and tire combination which should help lessen the power loss due to a heavier wheel and tire and up the mileage. Let me know how you make out, and if you get the same results.

 

I have a theory that if you run a larger diameter tire it will dramatically impact your mileage on the highway.

3500rpm seems to be the Vtec engaugement point and also the exact RPM you run at typical highway speeds. By using a tire that has a diameter of about 1-2" larger I think it would in affect create taller gearing and keep the RPM below that gas guzzling Vtec engaugement point.

Just a theory, but I noticed a 3-5mpg loss this year doing the same trip... I went from a 205/40 tire to a 195/40 tire (17' rim). Granted I have other factors, have not really had the opportunity to test this theory out (mainly because it is expensive to get a whole other set of tires!)

Tire & Gas Milage Relationships (theories)
1) Larger dia = Better Highway mileage
2) Tires with higher treadwear ratings = less rolling resistance
3) Ligher rims/tires = Better City Mileage
4) Continous center tread > broken pattern treads for gas mileage
5) Get tires balanced at expensive shops = less weights needed and more presise balancing
6) Use 2-4lbs more tire pressure over reccomended when loaded with weight to counteract affect.
7) Summer Tires for Summer! Not all seasons!!

 

Your speedometer and odometer are only off 3.2%, not 10%. What happens when you subtract 3% of the mileage distance before you divide by gallons used?

And if the low profile tire weighs more than your current tire going to a lighter wheel won't overcome the energy it takes to rotate the heavier tire.