The best I've gotten is 35 MPG, the worst was around 23 MPG. Consistent, city driving, and traffic in Los Angeles I get about 27 MPG. This is with a JRSC, but it's very rare that I goose it.
This is as accurate as I can get, I use the trip odometer, reset each time I fill up. Every time I fill up, I go to the same gas station and use the same pump, and lett he tank fill up slowly until it first shuts off. That way I'm sure to get a consistent fill from tank to tank.
FYI, I believe it was Car & Driver, just a little while back that did a study on odometers and speedometers. Speedometers were, in all cases but one indicating a higher speed than the vehicle was traveling, however, the odometers were all just about dead nuts right on. So regardless of indicated speed, your vehicle should not receive any excess milage in stock trim. I have 205/50/15 on mine, but like was stated before, this is a very slight difference, certainly NOT .5 gallon per mile as was suggested by Red GSR.
In fact, let's do the math... we're looking at tire circumference...
195/55/15 = 73.65" circumf
205/50/15 = 72.48" circumf
if the stock tire were to travel one mile down the road, and the 205 were to turn an equal number of revolutions, the 205 would end up just under 84 feet short of a mile or 98.4% of one mile.
Over a 1000 mile stretch, the difference in odometers would be 16 miles. This may sound like a lot but there is much more error to be found in the actual amount of fuel consumed based on how full you fuel tank was to begin with. Tires that are 1.6% smaller than stock will not make a whole lot of difference in fuel milage.