# Accuracy of MPH Display

Our 2015 Town and Country, 3.6L I believe, with 20,000 miles posted a 31.3 MPH over 800 miles RT between Phoenix and El Paso. We did have a nice tail wind one way. How accurate are those displays? Friends say they mislead the owner into thinking performance is better than expected

I assume you mean MPG. Those displays are noteworthy and can help you spot trends, but are not necessarily accurate… The best way to measure your mileage is to record the miles driven and the gallons needed at each fill up. You have to do this over several fill ups to average out any differences in how the pump shuts off.

An algorithm in the computer is used to calculate the fuel mileage.

This means that all the inputs into the engine management system are used to estimate fuel mileage.

So these systems are pretty accurate.

Tester

I also hope you mean Miles per Gallon. That read out is just a guide, as Steve said the only accurate way is just basic math.

In my experience they seem to be pretty accurate. If you do the pencil and paper method, then you are relying on the accuracy of your odometer and the shut off on the pump stops at precisely the same place when you fill up, plus the temperature of the fuel. Seems like its good enough for me.

Consumer Reports said they found them pretty accurate. Mine are.

The ones in my Lincolns have been accurate to within less than .1 MPG so I’d say mine at least are near dead on accurate.

Mine is about 1.2 to 1.5 mpg off. I do a paper and pencil method with each fill up. The furthest off it has been is 2.1, the closest was .6 mpg. Most are 1.2 or 1.5 mpg higher than calculated. Since most pumps are pretty accurate these days, as are most odometers, comparing the MPG readout to calculated over 30k miles, I’m pretty confident in the trend. The MPG display changes in .3 increments instead of .1.

Edit: On several road trips, I checked the odometer to the mile markers on the interstates. In several tests of over 100 miles each, I got 100.4 miles on the odometer vs 100 miles on the mile markers. This has been consistent on about a half dozen tests on road trips. I did one test of the speedometer on I-10 in New Mexico. On a very low traffic time early one morning, I set the cruise to 80 and got 79.6 miles in one hour. Its an analog speedometer readout so the needle could have been off ever so slightly, but it looked right on.

It is easy enough to check the accuracy of mile markers on the interstates but watch out in Ohio, they seem to plant them wherever they get kicked out of the truck.

If the odometer is off though, then the pencil and paper calculations will be off too.

They use convict road crews in Ohio too. Visiting our son last summer the crew was out mowing the ditches. Coming back 4-5 days later, they were still out there mowing the ditches. And they had weed whackers and what not to do the job, plus of course the security guards and transport trucks. It was like a big event. In Minnesota, its one guy with a tractor and an orange vest. Maybe its the same crew putting the mile markers in. Nothing against Ohio but wish he’d move back.

I had a Honda and a Ford with this feature and they were off by 1-2 MPG. On my Mazda and Hyundai, they are accurate to the point that I have stopped doing the math.

0.4 miles in 100 miles is not off by very much and will not have much affect the calculated mileage. On a 500 mile interval between fill ups at 30 mpg, that 2 miles spread over 16.7 gallons of gas.

500/16.7 = 29.94

498/16.7 = 29.82

But my point was that it is very consistent over 30k miles of driving. With that many samples, there will be few outliers but mostly all the little variations average out.

If the OP had also checked his fuel mileage the old fashioned way then he would have had his answer.

If the OP had also checked his fuel mileage the old fashioned way then he would have had his answer.

You mean actually do something… man, are you old fashioned or what !!! Here in the 21st century anything that is not done for us just can’t be done. DOH !

(I really hope the sarcasm is noticed)

PvtPublic, you can use html and

(joke)

I can’t speak to the accuracy of the MPG readout on Chrysler products, but I can attest that–similar to what others have reported–the readout on my Subaru is only slightly optimistic.

Since I know from repeated paper and pencil calculations that the digital readout is only “off” by .4 mpg, I don’t bother with paper and pencil (or a calculator) anymore. Being .4 mpg “optimistic” is close enough for me.

Thanks to you who have compared the two methods and shared your results. My car doesn’t have this feature, but I would have guessed the two MPGs would be within 5% of each other. Looks like that’s about right.

I guess my point was that if your odometer is off, then every calculation done will not yield the true mileage. True, doing a hundred calculations over thousands of miles would provide a good average, but only as good as the accuracy of the odometer. Unless there is some other way to actually calculate the true miles driven which may not correspond to the odometer. You know the bike wheel mounted to the rear bumper would give a true readout of the miles driven. Otherwise its just based on an inaccurate odometer.

In the end though who cares? I think the important factor is if there is a change from one time period to the other that would indicate a problem. More important probably is the total dollars spent or total gallons bought over a year.

I agree that if the odometer is off, then the MPG readout will be off as well.

Car makers always had a tolerance in their odometer accuracy. Years back, when our Mazda was off, the answer I got from their corporate office was they guarantee it to within 7%.

I wonder if those tolerances are now tighter than they used to be, given how it’s tied to MPG accuracy.

When I saw the discussion title I had comments on the accuracy improvements of modern speedometers verses a couple decades back. When I realized the discussion concerned MPG not MPH I just sat back and observed. My 2002 and current 2010 speedometers appear to suffer less than 2% error (plus 1 MPH @ 60 MPH). The 2010 is my first MPG display. For the first 6 months I used my calculator but it was so close the calculator has been gathering dust for 5 years. I was amazed when I discovered the MPG display reset automatically after every fill-up. How do it know? The first time I had to accelerate very “briskly” from a stop at the gas station’s driveway onto the 55mph highway with an average speed of 60-65mph I looked at the display upon reaching 60mph and it read a little over 5mpg! I quickly remembered near wide open throttle is not an effective fuel saving technique. Within 5 miles it was reading 36mpg which is average highway mileage. One thing I don’t like is that Winter is depressing enough without the display reminding me that the combined average mpg is now 4mpg less than Summer.