I bought a one-year old Subaru Impreza with 5000 miles on it. The sticker was still in the glove box and it said it should get 25 mpg city and 36 mpg highway. I’ve only been getting about 21 mpg combination city and highway. It has an “average mpg” reading* that works the trip odometer. I have driven the car about 1200 miles since I bought it.
I discovered today that there is a 2nd trip odometer that has never been re-set, and that one shows 25.5 mpg. So before I started driving it must have been even better than that.
What can I be doing to affect the mileage this way? I drive normally, I don’t race or rev the engine or anything like that. I’m willing to take it to a mechanic, but I don’t know what they can do. Any ideas?
*this roughly matches the amount of gasoline put in compared to the miles since the previous fill-up.
The largest single factor in gas mileage for a given car is located between the accelerator pedal and the seat.
The second biggest factor is location of your driving.
With your car if you are driving in downtown SF where thee is a stop sign or red light on every corner and I’m impressed with 21MPG. Drive in suburbia where the stop lights are two miles apart and the land is flat I’d be underwhelmed with 21 MPG.
It can’t possibly be any more accurate than actual miles driven divided by actual gallons consumed…
(As far as trip computers, in my 2015 Mazda the accuracy of the range calculation drifts from ~accurate to ~70 miles off, depending on how much gas is left in the tank.)
Is your car a manual or automatic? I’m sure the difference is negligible but I wonder if the sticker is for one or the other. I drive a Subaru WRX and man that thing can eat some gas because I love to drive it fast. It’s a manual and I find that if I drive it more conservatively it gets decent mileage on gas and it has the little meter to tell you when you are getting maximum fuel efficiency. To me it’s too fun a car to buy if you’re worried that much about gas.
I bought my new car in May and consistently had a couple mpg over the factory rating. Now it’s cold, we’re on winter blend and I’m a couple mpg under the rating. So, I think it’s a yearly average on the sticker.
Colder weather is going to be taking it a bit longer to warm up, but not sure it would explain that kind of drop. I imagine SF is going to have a lot of stop and go and sitting their idling going nowhere. You don’t know what percentage of miles previous driver did with city and highway driving to know why he ended up with 25.5 mpg. You didn’t say what percentage of city and hwy mileage you think was in your 21 mpg sample, but gosh to not even meet the 25 mpg for city that you should be getting is pretty bad. Are you sure you are a normal driver? My brother thinks he is one too, but gosh, he accelerates fairly damn quick to speed limit. If it isn’t that responsible, maybe your area just has way more stop and go and idling than previous owner.
I keep a mileage spreadsheet for my car. I try to tank up at the same pump at the same time of day, and stop fueling when the nozzle clicks off. The MFD does not show tenths of miles, so I use the distance figure from the trip meter. Basically, I try to limit the variables as much as I reasonably can. The MPG figure on the MFD is about 3.7% more optimistic than the calculations using observed data.
I too have an MPG gauge function that is consistently wrong. It always reads 2-4 mpg more than calculated by the miles / gallons method, and I know my technique is accurate. The speedometer reads fast, too, according to radar speed signs. It makes me wonder if the odometer is accurate, but I haven’t bothered to really check it.
My guess is that your odometer is accurate. In every car and motorcycle I’ve checked against gps, the speedo reads 3-5% high while the odo is right on. My understanding is that regulations everywhere allow a speedo to read slightly too high but never ever too low. So the manufacturers overshoot for safety.
In my BMW, if you monkey with the service menus, you can find an option that shows a digital speed readout in the display below the analog gauge. The digital reading is correct, as is the odo. The needle reads high. Some tools supposedly can adjust that compensation figure as the percentage is otherwise fixed.
My Ducati specifically says in the owners manual that they take the computer speed value and always add 5% for no given reason. Odo is accurate though.
the “trip computer” gets its information from the PCM, which by its very job needs to know miles driven and gallons consumed. It knows at all times how fast the car is moving, and how much fuel it is injecting into the engine. the trivial calculation is “hmm, I’m going X miles/hour, I’m injecting Y gallons/hr of fuel, divide the two, hours cancel so I’ve got X/Y miles/gallon!”
the real reason people doubt the accuracy of the trip computer is because it averages/integrates over a time interval which hides tank-to-tank variations. OP is a great example. he is getting consistently lower mpg than the previous driver, so it stands to reason he’s driving it harder.
Define full with regards to the fuel tank.
When the nozzle clicks off? That will vary from nozzle to nozzle, the flow rate of the nozzle, the angle the car is sitting at as well as the temperature and level of remaining fuel in the tank.
Since you can’t accurately define full any calculation you make is an estimate.
The ECM on the other hand knows the distance traveled and the fuel consumed to the microliter.
The trip computer is more accurate.
I have said twice now that I drive pretty conservatively, meaning that I don’t rabbit from stops and race to the next stoplight, or that sort of thing. Given that, maybe you (and Rick) have something concrete to offer in the way of driving advice.
It is unlikely that you will get the stated mileage.
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Understood. However, the previous driver did get over 25 mpg, although I don’t know what kind of driving he did.
Well, of course it doesn’t matter whether the tank is ever even “full” or not, never mind what that means - perhaps it would be less confusing for you to think in terms of “gas purchases” rather than “fill ups”.
I know how many miles I’ve actually driven, and how many gallons I’ve actually pumped - tracking over several weeks and dividing the sum of the former by the sum of the latter gives you your real-world mpg.