CarTalk.com Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Car Mileage meter - how accurate

I bought a new hybrid car that calculates the mpg I get. I am wondering if anybody has compared the mpg the car says it is getting with the actual calculation done by dividing total-miles by gallons-of-gas consumed. Did you find the car’s calculation to be accurate?

They can be accurate, some are optimistic by a few percent. You’ll need to check.

My daughter has a 2012 Cruze LS, and she calculated the mileage by dividing the miles between fill-ups by the gallons added. It matched the car’s mileage calculator so well that she stopped calculating it.

I have found the Avg MPG readout on my 2006 Sienna, 2010 Cobalt, and 2013 Equinox to be 1 - 1.5 mpg (4 to 5% ) higher than the calculated mpg. I calculate the mpg every 3 to 4 tanks of gas (~1k miles). My coworker has a 2010 Prius with 100k miles and is confident the mpg readout is close the the actual mpg.

Ed B.

The MPG indicators on my Lincolns have been accurate to within .1 MPG when compared to the miles driven/gallons method.

The mileage calculator on my CRV was a bit optimistic; I would say 1-2 MPG for each tank.

Now I am testing the one on my Mazda.

I’ve never checked mine but I think they are fairly close. Its more for comparing driving style and one period of time to another to see if something is off. When I need gas I just fill it up. Wouldn’t use any more or less regardless of what the computer said.

Initially, I was skeptical of this gadget, but, like both ok4450 & jtsanders, I found the mpg readout on my car to be extremely accurate. My calculations found the readout to be optimistic only to the tune of .1 to .4 mpg. With accuracy that close to reality, it just isn’t worth using a calculator any longer.

I think the MPG calculation is very accurate based on some of my initial observations. That is, while the vehicle is actively moving. My 2012 Odyssey is not factoring idle time into the reported number, I know this for certain. I have sat and idled for extended periods and the displayed MPG for that tank does not vary from the tanks where I was driving most of the time.

If they used a simple calculation that included all engine run time / miles driven, there would probably be hordes of people complaining that the display did not match the manufacturer claims.

If you have a similar situation and spend lengthy times idling, you would think the display was wildly optimistic compared to your calculations since it would be nearly impossible for you to exclude the gas consumed while idling from the calculation.

That’s really odd TT. My understanding is that it adds up the fuel injected and divides by miles traveled. Mine drops while idling.

I agree with texases that TT’s observation is…odd.

My gas mileage is unusually consistent, simply because I tend to drive the same roads, at the same speeds, under the same conditions, on a regular basis. The avg mpg readout always seems to fall somewhere in the 22.5-23 mpg range in these local “mixed” conditions.

However, I was recently stuck in stop & go (almost entirely “stop”) traffic for about 40 minutes, and I saw the avg mpg readout plunge to 17 mpg. So, while TT’s Odyssey may not factor idling time into the calculation, I can tell you from experience that my Subaru does factor idling time into the calculation.

My Magnum MPG calculator has checked out as accurate

I figured it was an improvement. Why skew the calculation with idle time? My idle times are 3-4 major periods of anywhere between 45 minutes to 90 minutes on a single tank. Tried both leaving the running avg alone and resetting prior. Almost exactly settles on same value. Full tank, reset, no excessive idle times- virtually same mpg.

I’ve found the mpg (average) to be a good and accurate indicator of mpg. I often zero mine out when I fill up and find the computed mpg to be on the mark. Mpg can vary significantly per tankful. In my SUV I can get just 20 mpg on a pure interstate highway trip, 12 mpg when pulling a trailer, and 15 mpg or less when just doing stop and go city driving.

I have found the mileage meters in the vehicles I have owned to be reasonably accurate. The first vehicle I owned with this meter was a 1990 Ford Aerostar Eddie Bauer edition. At first, I thought that this feature was rather worthless as I have no problem calculating gasoline mileage. However, I found it useful for giving an indication of the overall health of the engine. If the mileage meter indicated a real drop in overall miles per gallon, I then look for the reason. It also is a good indicator of how weather affects mileage. My best mileage is in the spring and fall when the weather is moderate and I am not using the air conditioning. In the winter, I sometimes see a drop of 4 or 5 mpg when the temperature is around zero. In the summer, I observe the effect of running the air conditioner.

I had a Ford Explorer for a few months as a company car. The actual and “metered” gas mileage varied considerably. I have a handy mileage calculator in the glove box and using the trip meter and gallons pumped I get instant mpg actual. But you need to use the same gas pump to get an acurate figure; pumps differ considerably as to when they shut off.

Texases, just thought of one possible factor- I’m idling w/transmission in park.

I believe that idling is taken into account when the computer calculates mpg. Remember at idle the motor is only running at 800 or so under almost no load, therefore it uses only a minuscule amount of gas, compared to traveling down the road.

I find them to be very very inconsistent. The systems aren’t that complex to be very accurate. Some may find it accurate…some may not. I don’t even look at mine any more.

If you drove from Detroit to Denver with a warmed up car at legal highwway speeds, the overall figure would be very close to actual.

The algorithms in the program cannot take all factors into account.