Well, by doing a simple calculation...

250 miles divided by an average 15.9 gallon tank (or fillup) would yield an average mpg of 15.7 which is absolutely horrible.

at 275 miles per tank, you're still getting 17.9 which is still about 5-6 mpg off from the average expected city mileage.

have you check o2 sensor? fuel filter? you obviously have new plugs/wires. Also, it would make sense that if you have an aftermarket intake/filter, you would be running more air. to compensate, your ECU throws in more fuel. This will cause a loss of gas mileage. you bought performance parts to go fast, right?

btw, on hondatech, there is a thread reporting gas mileage. People have generally gone 280 at a low, to 350 at a high. on all highway, some guy went almost 400!

And Vassil, I think its not right to compare an 88 lude to a 00. i saw a graph one time in statistics class showing the rapid increase of gas mileage due to better technological advance in fuel injection systems. also, your accord v6 is just that, a v6. So you would probably see less fuel efficiency in that as well.