can new tires really reduce mileage?

disconnecting the battery doesn't erase the codes. You need a code reader for that.. check the gas cap gasket for cracks. If it has any replace it.

Well, were the new tire exactly the same size as the old ones? Even saying they were the same size and model of tire, the new tread height means the new tire travels farther per revolution that the old worn out one did which makes the subjective gas mileage 'seem' lower. You are actually driving farther now than the odometer thinks, compared to the worn out tires.

The way to tell for sure is to use a GPS or a measured mile to check your odometer for accuracy.

Mike

The rolling resistance of tires does vary, though that seems like a very big change. I would not expect that much of a change unless you changed to very wide, or very agressive tread tires. More and more mfgs are offering high milage (low resistance) tires, but it will be awhile before they are mainstream.

I see tons of vehicles equipped with Michelin "Green X" tires, although how "low" their rolling resistance really is compared to a "normal" tire I don't know.

nate

I should also say that low friction tires would affect highway milage a lot more than city driving. In highway milage the energy losses are primarily dissipative, like rolling and wind resistance, while a large percent of city driving goes into increasing kinetic energy and then destroying it with brakes.

yes, a big change. I guess another trip might some week be informative.

Btw, does it follow that high mileage tires would have less rain/snow traction? Just curious.