How Much Fuel Did I Really Get?

[Green GX]

I filled my GX recently. The pump was pumping at about 4300 psi. per the gauge on the side of the pump. The total fill was 5.414 gge.

When I started the car all of the fuel gauge segments lit up but the top one. And, within about a half mile the second segment from the top went out. I've used this station once in the past and when finished the top segment on the gauge didn't light up that time either. Despite the pump stopping, it appears as though its not filling my tank completely. The first time I used the station I waited about 10 minutes and tried to top off the tank. The pump only squirted another .08 gge's into the tank. So, the pump thinks my tank is full.

I've noticed other people post their experiences here using some sort of way to adjust the gge using the pressure. How does that work?

If I do the math, 238.7 miles/5.414 gge= 44.1 mpg. This is more than I normally get, so it doesn't pass the smell test.

Bill

[MrComment]

Re: How Much Fuel Did I Really Get?

4300 psi cant be the output pressure to your car's tank, but its probably the current storage pressure at the CNG station.

Your dispensed pressure shouldnt be more then 3600psig max (assuming its not freezing cold outside).

Also you would need to know at the starting pressure and volume of your cylinder/tank to approximate your dispensed amount of gas from the station. You would have to assume that your getting a full 3600psig charge, and the temperature change between start and stop during filling is about the same. The dispenser doesnt need to know all this info because it is probably measuring the amount dispensed directly, via a coreolis flow meter, which possibly might be out of calibration.

If you know exactly what kind of mileage you typically get you can work your problem backwards to get an answer for how much gas you bought. However doing so means you actually got more gas then you paid for. So thats a good deal right?

[Curtis]

Re: How Much Fuel Did I Really Get?

I've noted the same thing when I use my station (set to 4200 psi) to fill my cars. I was a bit perplexed too. The range went up, as you experienced. I chalked it up to the pump being temperature compensating, as well as the GX's fuel guage. Obviously it's getting a little better pressure but it's hard to determine exactly how much.

A while back, the gas co inspector told me that in the heat of the summer I should have it cranked up to 4500 psi to compensate for the 100+ degree temps.

[freedml]

Re: How Much Fuel Did I Really Get?

I've come to the conclusion that it's not really possible to calculate mileage based on GGE dispensed because you never really know how 'full' the tank is. The pressure indications on the dispensers are unreliable.

[TheBundo]

Re: How Much Fuel Did I Really Get?

I too notice one light going out fairly quickly on the fuel gauge after a fill. I assume this is from the fuel cooling and hence the pressure reducing after the fill

[Green GX]

Re: How Much Fuel Did I Really Get?

I too notice one light going out fairly quickly on the fuel gauge after a fill. I assume this is from the fuel cooling and hence the pressure reducing after the fill

After "filling" the tank, I've actually let the car sit for about 10 minutes (73 deg. F. outside) then start the pump up again to top it off. It only put about .1 gge more in and stopped. (so much for the cooling off theory) And, when I drove off the top segment on the gauge still went out within .5 miles.

So I'm not sure how the fuel gauge really works, or how full the tank is.

I work with a Hydrogen wizard, I'll ask him what effect the hot gas has, how long to let it cool down, etc...

Bill

[freedml]

Re: How Much Fuel Did I Really Get?

The fuel indicator is not a simple (here's the pressure) indicator. It factors in temperature, so the temp change after filling (whether standing or driving) might be doing it.

[Curtis]

Re: How Much Fuel Did I Really Get?

10 minutes may not have been sufficient to get the temperature down significantly. Being in the insulated environment of the trunk, it may take longer than anticipated. I'm sure that someone out there has played around with cooling times vrs additional filling capabilities. In fact, some of these maniac's probably have spreadsheets on the topic. Come on guys, how long does it take to get another gge in there?

[jetboatjohnny]

Re: How Much Fuel Did I Really Get?

Every time I top-off, I do it the next day, and I drive 8 miles (city) round trip and always get between 1.0 and 1.5 gge. After that I can usually go 65-75 miles untill I hit the full mark instead of 35-45 miles.
Usually my tank pressure shows between 2800 and 3000 before the fill.
One time I filled with the 3,000 psi pump then filled about 10 min later with the 3,600 pump and got only .7 gge.
(99 GX)

[Yroc]

Re: How Much Fuel Did I Really Get?

I think the only real way to figure gas mileage is to get a gascard. I got one a while back and it asks for your mileage at every fillup. Then at the end of the month you get a bill with your mileage calculated. This seems like the best indication of mileage. My 2000 escalade averages about 14-15mpg each month, city and hwy combined... I'm going to get a card for my new 04 f150 and test it out.