You have to correct for it. Easy for you to do, hard for your car to do. Just zero the trip odometer at some convenient mile marker and drive 10 miles or whatever and compare readings. That percentage will close until the tire wears down after 20000 miles or so. I have to add 4% to my odometer reading to be correct.
Originally Posted by Textual Innuendo: “I can buy that it’s a case of tolerance stack-ups, although injector open time+delivery pressure * Hz sounds an awful lot like fuel volume pumped into the engine to me.”
Only if you think like a programmer. In the real world some can stick and open slowly, fail to close quick enough, have dirt in one or more of them, etc. Hell the computer probably doesn’t even know if one wire fell off. Calculating things is most emphatically not the same as measuring it.
On a somewhat related issue, whenever I fill up the tank, it tells me 400ish miles to empty. I fill the tank again at 300± miles, because it’s almost empty, sometimes low-light empty. Every time, even when I go away for the weekend & the entire day is highway driving.
I also track MPG in a spreadsheet; it’s within a couple or three miles; within the range difference for local vs. highway driving. Then why the 'ell are they overestimating by 1/3 when I have a full tank?
Let me add that the range to empty comes down with the gas gauge; IOW it’s not saying 85 miles left when the low gas light comes on (which ½ the time is as I’m pulling into the gas station.) In short, it’s so freakishly off at the high end, I don’t dare to trust it at the low end. I’ve thought about testing it by intentionally running out of fuel while having a filled gas can in the car but that’s just too much effort; it’s much easier to keep an eye on the trip odometer (&/or trip odometer) & fill up accordingly.
I’ve thought about that myself, but I’ve been warned that it’s not a good thing to do to your car intentionally. Like I said, with my car, it’s extremely conservative, with the “miles left” counter going to zero with at least two gallons of gas left in the tank, which for my car is a range of at least 50 miles. Just look up the size of the tank and how much gas you need to fill when the counter gets to zero. As I said above, my car has a 13.2 gallon tank, and when I reach 0 miles, it’s still only about 10.5-11 gallons to fill up, and that’s with “topping it off” just to see how much I could squeeze in there. (And I run it to zero frequently around town. Maybe about a dozen times a year. Even when I start getting a little nervous for having run it at zero for a couple of days, I always discover I have plenty of gas left. I think the most I’ve ever filled it was around 12 gallons. Note: This will vary by make and model of car, of course.)
MPG gauges are a convince feature put in because it is easy to do with the data they need. Fuel injectors are rated typically in volume per time interval, typically an hour.
They know how long those injectors are open, and in most cars the unused fuel just flows back to the tank. In fact it is pretty much required to maintain that excess flow to make sure that the fuel pump stays cool and has a long life.
They need to know exactly how much fuel they are using to meet emissions, power and mix requirements. But there are a lot of potential sources of error that may cause issues with the added MPG calculation, and they also dampen the displayed value to make it more user friendly.
But to be clear, they don’t have additional subsystems in place to track the MPG, they just leverage the tools required to have a functional fuel map and EFI system to provide a utility function.
You would think that is the case, but that’s not my experience at all. My car’s speedo reads about 2-3 MPH high at most speeds. I drive past one radar speed display in a 25 MPH zone and another on the highway where the speed limit drops from 65 to 55 MPH. In both places, my speedo reads about 2-3 MPH high, so it’s not the percentage difference you would expect.
As to the OP’s conundrum, every car I’ve owned that has a mileage computer over-estimates my MPG, as compared to my manual calculation (fill gallons divided by miles driven). What I wonder is whether the higher speed indication correlates with more odometer miles than actually driven. I’ll have to do some measuring with my GPS when I get a chance.
Yesterday, for instance, I pulled into a gas station shortly after my display was showing 0 miles and the display had no bars left on it, indicating empty. I jammed in as much gas as I could, ignoring the auto shutoff as much as I could, and got 10.8 gallons into a 13.2 gallon tank.
I’ll try a similar experiment with MPG when I get a chance, but I don’t expect the difference to be much. I have a feeling my car might slightly underestimate MPG.
The factory MPG readings are based on brand new cars in perfect condition driving at exactly the optimum speed (for that car) without any breaking or accelerating. As already pointed out in this thread, there are many factors that can adversely affect fuel efficiency.
Interesting. I drive a 2012 Mazda3 and my car’s estimated range tends to be very optimistic. Granted, this is DC so even on the highway there’s really no such thing as “highway driving,” thus my actual MPG is always going to be on the low end of the expected range.
With a full tank the computer is quite confident I’ll get well over 400 miles.
In reality, I’m filling up at about 300 miles, give or take about 25, putting in about 11.5 gallons, and the range reads 50-60 miles when the fuel light comes on (it comes on when the gauge is down to the last tick).
Huh. Interesting indeed. The computer might be a little optimistic at the start and then it gets very pessimistic at the end (so much so that I’m confident the original mileage estimate was pretty much spot-on.) My 2004 Mazda 3 was the same, though I didn’t have a computer with “miles remaining” on it. When I ran the fuel down to completely empty, it had a similar “slop factor” built into it. That was a 14.5 gallon tank, and, no matter how long I braved running it on empty (I’m not talking about the fuel light coming on, but the needle being buried with nowhere to go), I’ve never filled up more than 12 point something gallons on that thing. For the longest time, I thought it must have been a 13 gallon tank because 12.5 or so is the most I could get at a fill-up. Nope. 14.5 gallons, apparently.
Wife’s 2005 Ford Focus is similar. I was panicking a few weeks ago, as it was run down to E and driving for a bit and I had no idea as to how conservative the fuel gauge was. Same type of deal. That’s a 14 gallon tank and, when I finally got to a gas station in a bit of a panic, only filled it up 12.5 or so gallons.
Different points though valid being mixed together. My previous car’s trip computer gave optimistic mpg’s. But the ‘actual’ (taking fuel pumped and odometer readings as accurate) still averaged higher on highway trips than the EPA highway rating. On my current car I don’t keep track of what the car calculates for cumulative mileage but ‘actual’ mpg is ever further above the EPA rating. Car is rated 33 highway, but I only get below that driving hard on winding roads. Just cruising on big highways it’s 36-40 depending on terrain and temperature, best in warm weather, steep hills also a negative.
Here’s a graph of the EPA highway cycle with further link to the city cycle. It does seem at first glance the highway cycle is friendly to high mileage though it’s not actually at a constant speed or speeds optimized for the particular car. It looks kind of like driving a two lane rural roads, sometimes you’re 40-50 sometimes 50-60mph but slow down further for a town every once in a while. But I notice in real life my car gets better mpg 65-70 constant on interstates than even when I cruise nice and easy on rural roads but average slower and occasionally have to slow down further. You can access mpg curves for some cars showing an mpg advantage at 50-60 v 70 and that’s the general conventional wisdom but it doesn’t seem to be practically true of my car if accounting for other typical conditions of 50-60mph roads (as opposed to going 50 on a 70 limit interstate, which isn’t optimal safety wise).
Likewise for most cars the car-computed mpg doesn’t use the speedometer or fuel gauge as inputs (ie. odometer and speedometer are independent). The accuracy of speedometer and fuel gauge are interesting but generally unrelated issues to the car’s own mpg calc.
Owning a 2005 Matrix and a 2008 Prius, I think this is a systemic problem with Toyota. I did calculate though with the Matrix at one point (and I don’t remember the exact specifics), the fuel tank listing online and in the manual was 13.5 gal. The light came on at 10.5 (another 100 miles are possible!?!)- but that is another whole issue. However it took 230-270 miles to go from full to 1/2 and then <100 miles for the next 1/2. But if I just assumed that the other 3 gallons + the volume of the pump were in the 2nd “half” that would explain the apparent rapid decent. So the real issue is that Toyota does everything in its power to make sure that you don’t run out of fuel.
The Prius is just as bad with 2.5 gallons remaining when the light comes on. So instead of simple math, think of the toyota gas gauge as an algebra problem and you have to solve for X (safety fuel reserve) and Y (volume of gas pump in the tank).
Correct; they analyze the test fuel composition beforehand (to calculate the hydrogen:carbon ratio), and then during the test they measure exhaust emissions to see how much carbon went through the engine. From that, they can calculate the total fuel mass flow for the test. Since they’re measuring emissions anyway for the test, this method of measuring fuel flow is much easier than trying to rig up an actual flow meter on the car’s fuel system.
Sure. However EPA ratings are only partly based on measurement. They are partly based on calculated corrections from test to road conditions, and/or extrapolation from a tested model to a similar model which wasn’t actually tested. And the EPA is using results produced by the manufacturers which they only partly audit. Bottom line being that in some past cases (like Ford with some of its hybrids a few years ago) downward revisions have been made to EPA ratings, spurred by customer complaints, that were probably bigger than if the EPA rating had been based on the trip computer.
However Consumer Reports found not long ago that EPA ratings were more accurate than they used to be:
And again the last two cars I’ve had do better mpg in real highway driving, driven for economy, than the ratings, my BMW 328 significantly. OTOH a Ford Expedition EL I rented for a long cross country trip a few years ago fell at least a couple of MPG below (an already pretty low) highway mpg rating in loads of pure highway driving, never reached it on any tank. I was surprised given recent experience with own cars. I guess ‘YMMV’.
