Confusing MPG readings

[bad.moshi]

So my first assumption is that our cars have really strangely shaped gastanks or that my gauge is just off. Here's the scoop:

I live in a very hilly area, and as the semester began the students have flooded back in. So there's a good deal of traffic as well. I filled my tank a couple of weeks ago and burned off nearly all of that gas within the town, running errands and seeing friends and such. I topped out the trip with 240 miles to the entire tank. When I refilled, I grabbed a brand new 15.5 gallons. This equates to me getting about a 15mpg. It sucks, I know... but I live with it.

I took a trip home this weekend and refilled those 15.5 gals as I was leaving town. I drove a pretty large amount over the weekend, having travelled 261 miles as I pulled back into my driveway. This is already more than I travelled on my entire tank of gas beforehand, and as it sits in my driveway, I still have a half-tank. My driveway is flat, and I have taken it out since and it still reads a sliver below half tank. That means I went 261 miles on 9 gallons of gas or does our "half tank" not actually equate to a full half?

If this is true, I went from getting 15mpg to 29 highway. I see the advertising stickers for a 93 bonne says that's not abnormal but it seems like an awfully huge jump to me. At the same time it always appears to take forever to get from full to a half-tank but takes no time to go from a half-tank to below a quarter. My real question is, does anyone else see this kind of fluctuation?

[harofreak00]

the 92-99 gas tanks are VERY deceiving. Half is not half. If your car is running properly, this is how your tank should reflect the miles.

You should get 100 miles before the tank even drops past the "F". You should get another 200-250 miles until the tank is halfway. You will probably get another 100-125 miles on the last half of the tank.

[bad.moshi]

Haha yeah I figured it was something like that. Thanks!

[99SilverSE]

On my way to Chicago last weekend I filled up and when I dinged I had 450Miles on it.

While I don't know what my milage was. I did watch the needle and thought it was wierd...
It was a little past full when I filled it slow and it stopped on it's own.

At the 3/4 line I went 200miles
At the 1/2 I was at about 300
and when it dinged(whats that about 1/8) I was at 450
If I kept driving I probably could have got 500miles out of the (18 gal?) tank.(thats only 27.7MPG :( )

[wjcollier07]

Its so inaccurate, it can hardly be referred to as a gauge, it's practically a Got Gas/No Gas indicator.

I've only been able to fit 15.3 gallons in my tank after driving for roughly 20 city miles with the fuel light on. It was 400 miles exactly from full to the light turning on. The first half of the gauge was about 2/3 of the driving at least. That was about a 27 mpg tank. The next tank was over 30 and that still didn't let me fit in more than 15.

[imidazol97]

I took a trip home this weekend and refilled those 15.5 gals as I was leaving town. I drove a pretty large amount over the weekend, having travelled 261 miles as I pulled back into my driveway. This is already more than I travelled on my entire tank of gas beforehand, and as it sits in my driveway, I still have a half-tank. My driveway is flat, and I have taken it out since and it still reads a sliver below half tank. That means I went 261 miles on 9 gallons of gas or does our "half tank" not actually equate to a full half?

My car would have been about 9 gallons at that point. At exactly half it was 8 gallons out of 18. or 18.5 gal. If your driving was fairly smooth and no hot rodding and didn't have lots of stops and starts--just cruising along at 55 or so, that's typical mileage at 29 mpg.

With the new fuel pump and sender, it seems more linear. So I am at 3/4 tank and I usually would have gotten in 6-7 gallons. I filled up on Sunday evening because of the wind storm damage and power outages, and only got in 4. The replacement gauges seem much more accurate.

[LeSabre in Buffalo]

That's odd.

The OEM sender design does suck. The float can't spin freely around the shaft that reports the level (at least on my 99), so it's not terribly accurate since the part that's floating changes in how much is actually floating. If you ever see an original sender you'll see better what I mean. The new sender recently put in had the float able to spin freely so it doesn't have the same problems, and may be a better indicator of actual gas volume when the tank is low. It's not doing the usual plummet from half to empty so far after filling up an empty tank with 7.5 gallons (about 1.5 gallons remaining, so 9 gallons starting off).

And yeah, that mileage is about normal for around town and highway.

[JBMark33]

My SE has the 3.05 gears and I got 35 mpg, no lie, on a 165 mile trip to visit my g/f in college.

I filled up when I got on the highway in Ebensburg PA, and filled up when I got off the highway less than a mile from my house in Edinboro PA. I filled up at Sunoco and ran 165 Miles on 4.7 Gallons. I couldn't believe it myself. The elevation change from start to finish was only a couple hundred feet. I thought it must have been downhill most of the way, but it was equally up and down.

My tank pretty much does what Haro says. I will get 80 miles before my needle even moves. Right around 100 miles right on the full line, then I get 250 miles to the first half tank. I usually fill up again once it gets a little below half tank, but if I let it go down, I will get about 450 miles with about 1/16th of a tank left. I have only run it that low maybe twice.

[harofreak00]

My tank pretty much does what Haro says. I will get 80 miles before my needle even moves. Right around 100 miles right on the full line, then I get 250 miles to the first half tank. I usually fill up again once it gets a little below half tank, but if I let it go down, I will get about 450 miles with about 1/16th of a tank left. I have only run it that low maybe twice.

On your 2000? I was talking about the 92-99 tank.

My 2000 gauge is VERY accurate. Normal city driving, I get 100 miles per 1/4 tank.

[JBMark33]

I generally get 100mpg to each 1/4 tank, but that initial 80 miles before the needle moves, offsets everything. Does that make sense? Mine is consistent, but if you compare mileage from the first half to the second half, it's off by 80 miles.

[bad.moshi]

Its so inaccurate, it can hardly be referred to as a gauge, it's practically a Got Gas/No Gas indicator.

Best thing I've heard in a while. I may have to start referring to it as that. But yeah, I've known for a long time that it's hardly a reliable source. My environment prohibits many stops on flat ground, so I have to average from the hills about how much gas I have. My driveway is one of the flatter spots. I get excited when I'm driving flat after having gone uphill because it tells me I have 1/4+ more than I really do . Never ceases to deflate me as it drops.

My bonking begins at about 3 gallons left. I do believe our tanks are 18 gal. I rarely let it get that low unless I'm terribly busy all week but when it does bonk and the light comes on, as it did just before I left town, I usually put in 15 gallons or so before it's full. Stupid uneven gastanks...

[danthurs]

I think the tank is shaped like a upside down pyrimid. I get about 150 by the time it goes past F, about 250 and 5/8, 300 at about 1/4, after that I can watch the gage move. I figure I can get at the most, 50 miles on the last 3/8 tank. Also if memory serves, parking with front up hill, my tank is fuller, parking down hill the tank reads low. Or is that the other way around, can't remember.

[bad.moshi]

Also if memory serves, parking with front up hill, my tank is fuller, parking down hill the tank reads low. Or is that the other way around, can't remember.

That's how it is. Uphill shows more gas, downhill shows less. As it were, I only get the low fuel light coming on as I'm stopped facing a steep downhill.

I always remember it by thinking "it'd be better to run out of gas going downhill" as the bonne' likes to make you think you are.

[willwren]

On the 92-99 tanks (87-91 also), the sender and pump are in the rear half of the tank, and the bottom of the tank is smaller and has radiused corners, so it holds less than the 'top half'.

[Bob Dillon]

The tank is "L" shaped, with the vertical bar on the "L" much thicker the the horizontal stroke. My gas gauge reads just as most others above.

I thought it was the weirdest shaped tank I've ever seen when I pulled it to replace the fuel pump.

[teleplayer]

I agree with the comments above - the gas gauge is close to useless on my '92. Every time I fill up, I reset the trip meter, and I use that as a guide. It also gives you a good way to easily calculate your mileage, every time you fill up. I get 18-22 MPG, depending.

[freneticburn]

It is an odd tank. I calculate my mileage almost every fill up. I take what's on the trip meter and divide that by how many gallons it took to fill up my car. I drive a pretty good mix of city and highway leaning more toward city miles and usually my car bongs at around 310 miles, in which I usually fill it up and typically that is about 13 gallons. My mileage ranges from 19-25, though most of the time its roughly 22 MPG. The way I look at it is that once my car hits 1/4 of a tank, bongs, and gives me the idiot light I have a good 50 miles left at least.

[harofreak00]

I agree with the comments above - the gas gauge is close to useless on my '92. Every time I fill up, I reset the trip meter, and I use that as a guide. It also gives you a good way to easily calculate your mileage, every time you fill up. I get 18-22 MPG, depending.

The 92 tanks are especially bad. You could update your tank to see a little more accurate gauge.

[LeSabreUltra]

Huh. My gauge is very accurate. You guys say yours won't drop off F until around 100 miles or so...mine drops off F between 30-40 miles and by 100 it's around the 3/4 level. I don't have any record of the fuel pump having been changed though. There's about a 10k mile gap between where the previous owner stopped recording maintenance (very meticulous before though) and when I got the car so perhaps it was replaced then, I dunno. If so, I'm very glad! I don't know how long these fuel pumps last, but if it were original it would have almost 148k on it. Holds good pressure either way.
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[willwren]

Don't assume comparisons can be made between similar models.

My 93 SSEi and 95SLE both react very differently on the gauge in those first 100 miles. Different clusters, different gauges. And through the years, different senders. Some cars may have senders set up specifically to take into account the odd tank shape by the sender not having a linear pot.