I'm trying to decide whether it makes sense for me to get a Prius, and I think it comes down to the gas mileage I would get with it, vs. the mileage I would get with another small car, likely a Corolla.

I'll outline my assumptions first, the question comes last (sorry to be long-winded).

I own a 91 Corolla wagon with 210k miles. (Love it, wish they still made wagons...) Manual xmission, and I get ~33 mpg in town, slightly over 40 on the highway. In my commute now, I average 36-38 on mostly freeway, but with frequent congestion. (The road, not my nose...)

I suspect that's higher than most Corolla owners get. Partly that is probably due to the manual transmission, and partly to my driving habits (I get passed a lot).

I think the average mileage for the current Prius is ~48 mpg overall, which is "only" 10 mpg over what I get now. If I were to keep my Prius for 200k miles, like I have my Corolla, that should give me a savings of (200k/ 48) - (200k/ 37) ~= 1200 gallons. At *current* gas cost, that would be a savings of ~$3600. If I can also take the low emissions tax break (that may expire, though, right?) of $3500, the savings would be around $7000. Add that to the price of a Corolla, and it comes to just about the same as the price of a Prius. Of course, the price of gas could go up or down...

So my question: because I get higher than average mpg on my Corolla, am I likely to get higher than 48 mpg on a Prius? Or are all you Prius owners such careful drivers that you would also get 36-38 mpg on a Corolla? Or is this an impossible question to answer?