93 civic vx mpg varies

[onioncasserole]

I have a 93 civic vx. I drive 180 miles a day and have been averaging two distinctly separate numbers. Sometimes between fill-ups I avg about 34 to 36 mpg. Then for no apparent reason my mpg jumps to around 44 to 46. Anyone have a suggestion?
The car has been recently tuned up. New timing belt,water pump. I know I have a small vacuum leak around my brake booster, my radiator fan relay is kaput but it doesn't seem to affect the car temp (mostly highway miles). The previous owner put on an after market exhaust manifold (o2 sensor in one exhaust line instead of at the collector).

Jason

[1993CivicVX]

I also have a 93 Civic VX and my name is also Jason and I also have the same issue. (Look at my gas log!) If you are measuring your FE the same way I am, by calculating the mpg after each fill up, my hypothesis is that for whatever reason our cars cut off the fuel at different time each time we fill up. So one time I fill up it stops before the fill up sound starts to rise indicating its near the top of the tank. Next time I fill up the rising sound ends in a gurgling of overspill. So it seems to me you are actually averaging somewhere around 40mpg.

[onioncasserole]

That seems plausible but I fill up at the same station every time.

Jason

[1993CivicVX]

I think it may be the gas tank as much as the pump. Listen for the rising sound when you fill up. Do you hear it every time you fill up at the same pump? If you do --see if you can tell if you hear it stop at about the same time. If you only sometimes hear it, that could be the reason. I think, at least with my car, that the gas tank is finicky with the nozzle--even if its the same nozzle. I've only once or twice had the gas tank fill up to the top twice in a row. But I do not fill up at the same pump each time. Gas stations around here are way too mobbed to have that luxury.

[onioncasserole]

I will check for that from now on. Thanks.

Jason