This is more curiousity than anything. My car has a mind of it's own when it comes to gas mileage. On average I get about 32-33mpg in the summer higway driving at 70-72mph. But on occasion my car will get very good mileage for a while (as high as 35-36 highway and 32 around town) but sometimes it will go way down for a while (26-27 highway) and this is the same driving, same roads, same weather, same gas stations. Just wonder if anyone has had anything similiar? It's a 2000 ex 5spd 2.3 VTEC

 

How are you calculating your mileage?

Unless you either have a OBD-II based computer or you average your "Miles driven divided by Gallons consumed" data over several tanks, you really cannot know for sure if you car's fuel economy is artificially high (or low) or if it is due to the fact that fuel pumps, even the same pump, can click off at different points which can vary by as much as a gallon and a half.

 

My '92 5spd is pretty consistent in gas mileage, but does drop down 10-15% in wintertime (grease, oil and tranny fluid being thicker I guess).

 

% of Ethanol mixture at various gas stations can decrease mileage 10%+

 

Also keep in mind that while winter blends of fuel do result in a minor reduction in fuel economy, the larger issue is the cold air itself. Why? Because cold air is the enemy of atomization/vaporization of fuel, and as a result, the engine must run a bit richer to assure a stable flame front.

 

I realize that the winter fuel does reduce the mileage a bit. It is a little lower on average in the winter. As for the pump clicking off at different points, I always top it up so I always have the same amount of fuel. And I calculate miles driven divided by gallons. Also, when it's getting very good mileage, it seems to run a little better. And I do have an extensive mechanic background, and have done thorough maintenance/testing. Like I said I'm more curious than anything. Never had a vehicle do this before

 

Gas stations vapor recovery systems might suck liquid back if you top up the tank too much. That can affect it more or less strongly depending on level ground, how many times you click the nozzle, & probably other things related to how the recovery system is running at the moment. But that shouldn't affect how the engine runs.

Saturate the charcoal in the EVAP canister by topping off the fuel too often? If you do, then you might get richer fuel from the EVAP purge valve. I didn't expect this to be a strong effect, but ???

I don't really understand the supply-chain for a gas station. Independent station might sometimes get shipments from different terminals, different additive packages?? Especially if your favorite gas stations are located where they might be able to choose alternate suppliers?

 

As JimBlake said, this is very bad for your car's evap system, frequently bad for fuel economy, bad for engine operations (probably why your car runs poorly sometimes), and if all of the above isn't "bad" enough, topping off your tank is very bad for our environment.