I did a test over the last tank of gas, mind you it's been raining so I expected to get at least 10% less fuel mileage because of tire slippage.

My 99 Cavalier got 25.6 MPG over the last 268 miles. Not quite as good as it has been, but as I said it's been raining and I've been driving it more for short trips lately because it gets better mileage than the Trailblazer.