This is my first comment on the whole mpg issue. Maybe this subject has been discussed before, but when I fill my tank, I top off the tank till it is ready to overflow. Maybe it might be dangerous, but I get at least 5-6 more mpg. Am I crazy or what?

 

Well for a Prius, it is risky because if it overflows into the vapor control system in the tank, you will get a permanent error that can only be cleared by replacing the tank, ~$1,200, which is not warranty work. But it is your car.

Bob Wilson

 

I wouldn't be real concerned as I have topped off every tank and I have 20K+ on the clock. I have heard of reports of tank/evap system damage but they seem rare. As Bob mentioned though, it is your car so it's up to you.

 

Am I missing something here? I can understand why you would get more miles per tank (although I'm not sure why you would care) but more MPG? Why would tank level affect that?

 

Not missing anything, topping off the tank can only result in worse or negligible mpg. More weight = less mpg

 

Mathematically speaking, the fuel efficiency calculation (amount of fuel added and distance covered from previous tank) has a missing constant. That constant is the maximum fuel level, which for pragmatism is left as an unknown. This maximum fuel level becomes an implied factor over several tank fills; Whatever discrepancy there is between one refill and the maximum fuel level gets covered by the successive refills (it is not compounded, the difference is only on the latest tank). This is why calculating single tank fuel efficiency is inherently inaccurate, whereas getting the average over several tanks is reasonably accurate.

Example: If you super-fill one tank, say as a one-off experiment, then that tank's fuel efficiency will be relatively poor.

Example: If you under-fill one tank, then that tank's fuel efficiency will be relatively better.

(Both examples create an exaggeration to the fuel efficiency of the previous trip)

Going from gas station to gas station, pump to pump, there will usually be some deviation or inconsistency in just how full your tank becomes. The different nozzles may click off with different tolerances. But this doesn't matter when you take all of the above in mind about averages. This is why I take the tact that there is no point to being **** about having the exact same fuel level whenever leaving the gas station or giving individual tank fuel efficiency much weight; The degree of error gets eliminated by looking at more than one tank and there is a lot less anxiety involved.

 

This sounds more like something to do with gas vapors and minimizing the empty space with the most fuel jammed in as possible maybe?

I heard some gas saving tip on TV about not going to the gas station too often to just top off the gas tank...but rather, wait until there's only a 1/4 tank left and then go get gas. Something about losing the gas vapor every time you open the gas cap.

I'm tired and haven't given too much thought about this. Just throwing some things out there and maybe someone else can figure it out. Because this sounds interesting.

 

The most consistent method is to use the same pump at the same gas station everytime you fill-up. However, that is not possible/practical for many people, but your fuel economy will average out...One pump might cut-off sooner (giving higher MPG), and one pump might cut-off later (giving lower MPG), but it averages out.

 

I can't think of why you'd get better MPG... if you are figuring it by hand the best I can come up with is that you have some error in determining how much gas you put in each time... the fuel shutoff on a gas pump is far from a precision instrument... its only designed to prevent overflows and spills and I've had it fail to do even that!

 

Note too, it's not just about MPG but fuel consumed. If you're going to the gas station more often than you need to, it's as bad as making an extra run to the store because you forgot to get a gallon of milk.

I have two routes I take to work in my conventional Explorer. One is a mile shorter, but surface streets; the other on the highway with better FE. I'm still collecting numbers from my ScanGauge to see which takes less fuel (not which gets the best economy.)

Benton 22jul08