Incorrect MPG either mfd or bladder tank

[djphill]

I'm still on the same tank that I got my car with. Total average has been about 46 mpg (normally drive short routes) and i had about 6 pips left of gas when i only had 140 miles on my car (36 were already on the car when i got it so i suppose they refilled the tank). So either my tank isn't actually carrying around 11.9 gallons, or my mfd is way off in its mpg calculating. Can anybody give me a solid answer/guess to this problem.

[San_Carlos_Jeff]

The mfd is usually pretty darn accurate, mine is about 1.5% too optomistic after 45000 miles. What isn't linear is the pips for the gas guage. They will start disappearing more quickly as the tank get lower.

[djphill]

thats what i thought, but ive only gone 140 miles on half of my tank. That just doesnt make sence

[Bob64]

Make sense now? The gas tank isn't a gas tank, its a gas BALLOON (bladder). Thats why the top pip seems to last forever, and the bottom pip goes so fast.

[miscrms]

The MFD is generally found to be much more accurate than anything else. After taking a lot of tanks and averaging them together, people almost always find the MFD is right on, maybe 1% high. Any given tank may be off by 10% or more in either direction based on the pencil and paper method.

A lot of people like to bag on the accuracy of the gas gauge, which I think is unfair. Its pretty accurate, probably as good as any regular car, but what its trying to do is more complicated. Most people find they routinely get about 9 gallons into a tank on a fill up if you fill when the last pip just starts to blink.. Sometimes 8, sometimes 10, occasionaly 7.5, occasionally 10.5, but usually averaging out to about 9. Part of the shortfall is because there is still some fuel in the tank, and part is due to the bladder taking up space. Part of the variability has to do with the gas pumps, part has to do with the Prius' sealed vapor system, and part has to do with the bladder changing size and flexibility with temperature.

Since the tank size is basically variable, the gas gauge has a tough job to do, trying to determine what exactly full is. It doesn't want to tell you its not full when it really is, as then you might try to overfill it which can do very bad things to the vapor recovery system and spill fuel all over the place. So it dynamically tries to estimate when the tank is full based on a number of sensors. This means that on top of the normal pump stop variation, and the fact that the Prius may make this a little worse due to its more closed vapor system, you have the computer adding a bit more uncertainty onto the top. The upshot is that 1 pip can not be equated to any constant amount of volume of fuel, or number of miles. The gas gauge is generally vary reliable. When it tells you its almost empty and you need to add fuel, its almost always right. Where a lot of people get into trouble is when they second guess the gauge, by assuming they know how much gas is in the tank. Especially if they actually believe the tank holds 11.9 gallons.

In general if you just don't worry about it you'll be fine. If you live in a city with a gas station on every corner, fill up when the last pip blinks like I do. If you live in the burbs, and gas is generally pretty available, fill up sometime during the last pip. If you live out of town, maybe fill up when you get to two pips. Just pick a safe buffer for your circumstances, and you'll be fine. There is really no value to squeezing 500, or 600 miles out of a tank beyond bragging rights. Not worth the risk to the car IMHO.

There are of course times when you need to have a rough idea how much fuel you have, and how far you can go. In those circumstances here's what I do. I assume that however much fuel I put in last is all the fuel that I have. I divide that by 11 (I add one pip as the gauge often seems to pin off the top for about 1 pip) to get the rough current gallons per pip. I then multiply pips remaining, by gallons per pip, by expected mpgs for the rest of the trip. Note this may not be the same as your average mpgs. This gives a rough idea of miles remaining, and should be on the safe side. I'd still play it a little conservative though. Especially if I didn't fill at the blinking pip and am trying to project beyond the number of pips I filled at.

Here's an example. Lets say I last filled up with 8 gallons right at the blinking pip. Divide by 11 is 0.727 gallons per pip. I have 5 pips remaining, and expect to average 45mpg (fast highway with some uphill). So 5pips*0.727 gallons per pip = 3.64 gallons * 45mpg = 163.6 miles. In this case that ends up being about 32.72 mile per pip. That would actually even be lower than your 140 miles in 4 pips or 35 miles per pip. It can be much better than this with a bigger fill and higher mpgs, but it does show your case to be very feasible. I also wouldn't be surprised if a dealer fill is on the light side, and if they didn't top up at 36 miles. If often takes 60 miles or more to drop the first pip.

I think this is where people run into problems. They think in round numbers and say, ok the tank is about 12 gallons, and there are 10 pips, so 1.2 gallons per pip. At 50 mpg I should get 60 miles per pip! We've already shown above it can be half that under some conditions. Even if you know about the bladder and say 10 gallons, that would be 50 miles per pip. Either assumption will get you into trouble sooner or later.

For the most part you can just ignore all that. If you just drive the car and put fuel in when it says "Add Fuel" all will be well.

Rob

[morpheusx]

you will get about 390 to 410 miles from that tank (assuming your average stays the same). Normally a full tank carries about 9 gallons.

[qbee42]

The gross capacity of the N.A. Prius fuel tank is 11.9 gallons. The usable capacity is less. At most the usable capacity is somewhere between 10 and 11 gallons. I always figure 10 gallons as it is safer.

As the other posters have noted, you never really know how much gas you have until you don't have any. Out of gas is the only sure state. When the gas gauge tells you to get gas, get gas. That's all you need to know.

Tom

[djphill]

thanks so much for the help. The one this is that my tank was filled by the dealer, so i would expect that they would get it pretty full. Also if i continued this fuel consumption, i would only get about 280 miles from this tank. I understand the bladder problem but i dont understand how a full tank would only give me such a small amount of miles.

[morpheusx]

I would assume the opposite. I would always assume the dealer will almost certainly never completely fill the tank and only fill high enough to move the gauge to show full.

[djphill]

though that was what i would normally think, my first pip lasted the longest.