I have watched the mpg readout on my 2011 Toyota Sienna and my 2003 Toyota 4Runner and have seen them lose .1 mpg at a stop sign right after I have filled the tank. I think the same was true with the mpg readout on my 1990 Ford Aerostar.
When I have manually checked my mileage, the indicator on these vehicles seemed to be accurate. I think the mpg readouts are useful for indicating the overall health of the vehicle. If the mpg drops rapidly, I find the reason for the decrease in mileage.
On that aspect I totally agree, @Triedag. Those Ford Aerostar mileage meters were reasonably accurate but totally correspondent to the actual mileage. Several years dealing with a fleet of those vans and comparing the drivers and routes and weather with respect to mileage convinced me that the system was somehow on track. A failing thermostat, a week of extremely hot or cold weather, a tank of poor quality gas, or a change of drivers made a noticeable difference in the actual mileage and the indicated mileage.
I use mine to do as Tridaq described, only as a historical perspective to gauge if something is amiss.
I posted on this aspect previously- I think some of the newer cars are omitting idle time from the calculation.
I think the meters are more commonly being used to gauge actual versus advertised mileage and being used as a means to contest the manufacturer’s claims.
To level the playing field and get consistent results across the entire population, why not omit non-driving usage? It’s not really relevant to the actual efficiency of the vehicle but more indicative of the application it’s being used in. Using extreme case by comparison, would you expect a soccer mom to get the same results as a regional salesperson? No. And what happens when the soccer mom shows up complaining that the mileage is dismal compared to the advertised figure?
Prior to these meters being so common, most people did not bother to do any tracking. Now it’s at your fingertips, even shining brightly in your face. It’s in their best interest to eliminate sources of error from the calculation and report what is statistically relevant. However, doing so makes it more difficult for the driver to make comparisons…maybe that’s also not a bad thing from their perspective.
Omitting idle time? Yeah, I doubt they’d bother and I’m sure my L/100km increases dismally in traffic. I see the reading as being intended for entertainment value only. The only info the computer needs is the total injector “on” time per distance. Applying the dynamic flow curve which is essentially linear (with a small zero offset which I’ll bet is ignored) and you have the number. Since the manual calculation is based on the same distance reading, the only reason for a discrepancy other than the flow calibration is the variation in fuel pressure. I track both manually-measured and readout fuel consumption and find that the readout error is pretty consistent, while the biggest contributor to tank to tank variation by far is introduced by the exact level that I top off the tank.
I don’t trust either of the calculations. Both are estimates because your tires are probably slightly worn, and they never wear evenly. Since you start with an odometer reading that is an estimate, your MPG calculations would also be estimates.
Omitting idle time? Yeah, I doubt they’d bother
It’s not like it would take some extra feat of engineering- simply skip the calculation if the vehicle is not moving.
I see the reading as being intended for entertainment value only
I suspect lots of people use the reading to gauge if there has been some major shift in performance that needs attention. Like I said, just based on some people posting here, I believe more people are actually using this displayed reading to dispute mfr claims and not just as a source of amusement. I would find it hard to believe that any manufacturer would prominently display a calculation like that for the purposes of entertainment when so many people are concerned with the price of gas and their relative fuel economy. When they first came out- maybe, but not now.
BTW- if you search on the prior discussions you will find that I am basing my conclusions on the empirical evidence I have regarding the readout in my 2012 Odyssey. I never said they all do it but based on some of the discussions lately and what people are reporting vs usage profiles, it seems it may not be the only one doing it.
"Drive the car for a mile, then leave it idling overnight. What is the MPG figure? "
Very low. I’ve watched my mpg drop as I sat idling, shortly after a fillup, where the miles traveled is small.
How many are on this three year young Prius?
Did you get the advertised mileage when it was new? Is this a change?
Does it have the original low-rolling-resistance tires?
Is the maintenance all up to date?
I realize that you’re comparing your measure mileage to your car’s computer and not to the EPA estimate, but I’m thinking that a variation in tire rolling diameter from the original tires could affect the computer’s calculations.
The only complaint I have about the mileage minder on my Lincolns is the length of time it takes to get back to speed so to speak.
If the car is in town and getting say 18.7 MPG and I hit the open road the car may actually be doing 28 MPG but it takes a while for the display to get to that figure. Every mile or so it will add a tenth with the increases becoming slower and slower as it gets closer to the actual MPG.
There’s a 3rd method to back up the display and the fill/odometer method here anyway. It’s flat terrain and all of the section line dirt roads are spaced exactly 1 mile apart for infinity.
My cars have two read outs-instantaneous and average. So sitting at a stop light it will read zero on instantaneous but the average will always lag behind because it keeps recalculating the average all the time.
Just remember that car odometers and speedometers are seldom accurate. While they tend to be consistent they are often in error.
I would also suggest that most anytime you are computing less than several tank fills. you are likely not very average.
The first thing one should do is check your speedo against a gps which I find more reliable. If the (speedo) is off, I feel your odometer may be as well and even your manual computations using the adometer will be suspect. When you look at some speedos being off a few percent, it’s a guess as to which is more accurate. I use the gps for computing mileage for both cars and boats. If you think SUVs are bad, compute mileage for a boat…scary. Makes you want to take up kayaking and sailing.
@rawee2000 makes the good point that a single entry is insufficient especially considering varying driving conditions. A little head wind on one measure compared to another makes a significant difference. IMO, like most comparison measures, change is more important then absolute measure over time. Even your BP is not definitive, and must be taken several times and conditions driving vary as much as human activity.
We have an '11 Prius with 17K mi. on it and live in the S.F. Bay Area. We have averaged 52 mpg (calculated) since new. We noticed our milage increased after our car was broken in (~5K mi.) Our cars computer is usually shows about 1.8 mpg more than we calculate.
The speedometer in my Accord matches the GPS exactly.
There is no reason to believe that an inaccurate speedo affects the odometer. We all know speedos are often set slightly higher to keep us all out of trouble, and for the manufacturers to avoid lawsuits. You can read the speed the ECU actually thinks you are doing off the OBD-II port and in my case I’ve found it to agree to a GPS-measured speed within 1 km/hr at 100, while the speedo is 5 higher at all speeds. That’s plenty accurate enough for me.
As for the anecdotal evidence regarding not measuring fuel use while stopped, why post the hard data (over many tanks of course) so we can make up own own minds.
@KiwiME so when you’re pulled over for speeding, you can say
“Officer, my speedometer indicated 100mph, but I don’t believe I was actually doing 100mph”
The officer is going to look at you funny and toss you the ticket all the same.
@asecular - you don’t understand how many mpg meters work - they are cumulative averages since they were last reset. So if I drive a mile at 30 mpg, then sit and idle, the average mpg will continue to drop as the gas used increases while the miles stays the same.
“As for the anecdotal evidence regarding not measuring fuel use while stopped, why post the hard data (over many tanks of course) so we can make up own own minds.”
Hard data? First off, I really don’t care if you believe me or not. If I did, I could spend 5 minutes typing anything to support that position into a spreadsheet and hitting the plot data button.
Here’s the issue I see. You’re basing your conclusion of NO cars do it based on your limited sample of cars you have owned. I’m saying some do because I own one that does. I wonder who has a better chance of being right?
@TT, I’m not suggesting that you have don’t have some oddball vehicle that does that, I’m just suggesting you haven’t shown us any evidence at all to support your hypothesis that other manufacturers would bother with such a thing. And if you don’t care then stop posting about it!
@db4690, are you not familiar with kilometers per hour?
Back in the old days, I kept a notepad and pencil in the glove compartment. When I would fill the tank, I recorded the mileage and the number of gallons that went into the tank. At the end of the month when I would fill the tank, I would add up all the gallons that went into the tank from the beginning of the month. I would subtract the mileage at the beginning of the month from that at the end of the month and divide the number of gallons used over the time period into the distance traveled. Sometimes, if I was doing highway driving, I would calculate the mileage over the interval that I was on the highway. I didn’t need a calculator to do this.
I guess the more accurate way to do this would have been to use a fifth wheel that accurately records the mileage and use a special calibrated tank hooked directly to the fuel pump. I think Consumer Reports used this procedure on the test track. As I remember, the calibrated tank was in the car with the driver. I didn’t have the overwhelming urge to be that accurate.