Thread: Better MPG when filling up sooner?

This may be a stupid question but I'm just curious... I always have noticed that the sooner i fill up the better mpg I get....

For example: I went to go buy my car, it was about 100 miles away and I filled up after I bought it for the ride home. After about 70 miles I filled it up and I got 30 mpg (putting about a little over 2 gallons in)

Example 2 (this is what I always do) I drive my car until I need to put gas, I have a 12 gallon tank and always drive it to where I have to put about 10 or 10.5 gallons of gas and I get 23 mpg.

Recently I lost my 5th gear due to 5th gear pop out so I've been getting 20mpg and I went to go fill up and put only 7 gallons of gas in and I got 24 mpg. What gives?
I figure that because the less weight that is in the car it would technically make the car a little get a little more mpg's...

What's going on here? It happens evreytime so is it better to just fill up after every 150-170 miles instead of like 220-240 like usual?

I know with my NX the first half goes really fast and the last half tank goes forever(got 250 miles on half a tank labor day traffic and all) because B13s have faulty fuel guage sending units.

are you filling up at different stations? Variations in gas mix, such as the ethanol content, will easily do this to you.

Nope, Always filling up at the exact same station, have been for the past year or so.
and it's not just random whether or not i'll get better gas mileage, It's every single time.

I only ask because there's no mechanical or logical reason different gas levels in the tank will change your MPG so long as the gas isn't sitting for prolonged periods of time in the tank. It makes no sense to me.

the car seems to have more power when you have more than a 1/2 tank of gas. it makes no sense at all. well, this was the case for me and my b15.

my daily seems to act the same way.

If nobody minds, It would be nice if one of you can do the same test and see if it's the same for you... Nobody knows why and I'm curious... I mean, 4mpg's more for not putting in 3 gallons more? weird...

There are a few things that come into play here, some will be the properties of fuel, others will be the driving style.

First, if you fill up at 100 miles, sounds like you did highway miles only, while full tank probably had a bit of mixed driving. Now doing a full tank highway miles should yield to similar numbers as doing the first 100.

Onto the fuel it self. Fuel tends to vaporize fairly quickly with warm weather. When you have a full tank of fuel, it doesn't have anywhere to turn into vapors. Once you get to half full, then you have that much more space for it to vaporize.

Now you could have a slow leak too, would cause fuel vapors to get out or actual fuel. Emission equipment issues will usually do this. On my car, I would get 33mpg highway if I did 100 miles or 300 miles. But if I would let the car stand for a week, I could almost watch the gas needle drop. Fuel would vaporize, escape, etc. etc.

Well I fill up every 2 days or so so I don't know how bad that it could be and a majority of my driving is highway