Re: Better gas mileage when tank is half full?? See my scenario
 

Thanks guys again for the responses. I'm not disagreeing with the function, calibration, reliability, etc or the gas gauge. I made an observation that based off of the gas gauge, I appear to get better gas mileage when my tank is fuller vs. it being empty. My first two tests of that theory appear to support that. I was 33.50 MPG this morning when filling up with my tank being about 5/8 full. My question is is there any logical reason for this to happen? I'm going to keep tracking it just to see.

BEEF, I live in the Raleigh area and commute to Rocky Mount.

Re: Better gas mileage when tank is half full?? See my scenario
 

I'd be interested to see a gaslog showing it working over the course of 100 gallons or more. I cannot think of a logical reason for it to help.

Re: Better gas mileage when tank is half full?? See my scenario
 

shrem, I live in reidsville and commute to greensboro.

I still think that it is dependent on other factors. it has been abnormally hot this week (minus the weird snow last sunday night). we have seen 60s and 70s. I think if you were to run an entire tank again, you would probably see similar mileage. realize I am assuming on that one.

also, if you like this job, you may want to consider relocating. 75 miles is a lot.

I would say to start a gas log. they are handy little things and tell you a lot about trends over time.

Re: Better gas mileage when tank is half full?? See my scenario
 

75 miles is a lot. My wife and I have considered relocating but we're not quite sure. There's a lot of variables that go into that. There are a lot of people that commute from Raleigh so I'm hoping to maybe be able to car pool.

I will do a gas log and share it with everybody. I'm sure I can find an app on my phone. I will keep everybody up to date.

Re: Better gas mileage when tank is half full?? See my scenario
 

It seems to me that one would possibly get better gas mileage the more empty the tank is. Simply put, if all conditions are exactly the same, the tank would be less full and thus weigh less. Now, I don't know how much gasoline weghs by the gallon, but it stands to reason that your first gallon on a 14-gallon tank would not perform as well as your last gallon simply because there would be 13 less gallons of fuel and weight to transport. Makes sense to me, but would this much weight reduction for the final gallon or two even be worth 1 extra mpg?

Re: Better gas mileage when tank is half full?? See my scenario
 

I noted getting better fuel economy when under half a tank driving the Prius. There were likely other factors at work than just weight differences. The gen 2 Prius had a fuel bladder. When full and expanded, the bladder could be applying pressure to the fuel. This extra pressure could squeeze a little bit of extra fuel in whenever the pump turned on and/or the injectors opened. More than what the system called for.

While I don't know of any other car with a fuel bladder, there is column pressure. The weight of the fuel at the top of a full tank compresses the fuel down by the pump pick up. Whatever the cause of the pressure, if it had an effect, it likely isn't noticable until 55+ mpg.

IIRC, a gallon of gas is 6.4 pounds.

Re: Better gas mileage when tank is half full?? See my scenario
 

I thought of the pressure thing, but the computer will adjust for that based on O2 sensor readings...or it'll throw a code.

The weight issue would matter to weight-sensitive cars but I don't believe most cars on US roads are sensitive to weight fluctuations less than 15% or so.

Re: Better gas mileage when tank is half full?? See my scenario
 

While the sensors involved in running the car are more accurate than the tank gauge, they do have a tolerance range before throwing a code in which extra fuel can squeeze by. Tiny amounts that won't be noticed in lower mpg vehicles if it is happening.

The other factor in play is driving style. Unless the computer learns, and takes the tank level factor into account as soon as the pedal is depressed, pulse and glide may amplify the effects of the tank pressure. With more times going from off to on with accelerator, there is more chances for the pressure squeezed gas to slip by before the computer can adjust. It's miniscule amounts at each event, but P&G allows more of them to add up over a tank.

Re: Better gas mileage when tank is half full?? See my scenario
 

Alright guys. Here's my update. It took me a little longer for various reasons but the data is accurate. Just in case anyone is wondering, I filled up at the same gas station (not always the same pump) and did not top off. I use this car to drive to and from work only and I take the same route every time. There shouldn't be any variability. I deleted the data for days when I knew I drove the car differently ie. running a lot of errands after work. The data is the date, odomoter reading when I filled up, amount in gallons put into the tank, and the MPG.

3/6 - 153872 - 9.42 - initial fill up
3/12 - 154140 - 8.77 - 30.63
3/14 - 154382 - 7.53 - 32.13
3/18 - 154708 - 10.07 - 32.4
3/19 - 154862 - 4.16 - 37.04
3/23 - 155170 - 9.97 - 30.83
3/27 - 155324 - 5.45 - 28.27
3/28 - 155479 - 4.89 - 31.74
3/31 - 155697 - 6.93 - 31.49

As you can see, I was a happy man on the 19th but couldn't duplicate those numbers. That seems like a pretty big jump so I don't know what happened. I have done some work on my car here recently. I kept getting a P0125 fault if I'm not mistaken which deals with the coolant system. I replaced the coolant temp sensor because I knew that was messed up. I got the error 2 more times so I replaced the thermostat this weekend. Based off of how my temperature gauge was reading, I think it was stuck open or coolant was getting through the gasket or something. The temp reading is much more consistent now. Besides something related to that, I don't know what would have caused my MPG to jump so drastically. :-?

Re: Better gas mileage when tank is half full?? See my scenario
 

What was the weather like on those days?