Mileage different than the on board Computer

I have a 2009 Mini Cooper Clubman who’s onboard computer indicates it is getting 35.8 MPG which is at the top end of the estimated mileage for the car. Having owned vehicles before the computer was king, I have always kept a log and computed the mileage manually (Miles driven divided by gallons to fill the tank). The manual computation is 33.8 MPG which is a significant difference. This is based on 4100 miles driven. Why would there be such a discrepancy?

Well, for one, are you resetting the computer after every tank or is that a cumulative average?

Secondly, the computer doesn’t really know how much fuel you’re using. None of the values the engine control unit uses to meter the fuel flow strictly deal with actual fuel volume and so the MPG computer has to use these values to sort of guesstimate what your gas usage is. If you’re feeling cynical, you might think that the car makers intentionally make these err upwards since most people don’t actually check their MPG the old fashioned way and to them what the computer says is their MPG.

Are you in the habit of always buying gas from the same place? Try a different gas station and see what happens. It may be that the gas pump is shorting you. It’s more common than you might think.

First the computer needs to learn about your car and your driving habits. Somewhere in the owner’s manual there should be some instructions about this and how you need to tell it how much fuel you are buying and it likely is assuming you are filling it up each time, which may not be the case.

After it learns and adjust for your car and your driving habits it should be close, but don’t expect it ever to be really accurate. Maybe in a few years they will become more accurate but for now, they are not.

However while they may not be really good at telling you want mileage you are getting overall, they can be good about helping you learn to drive for better mileage.  They will show you how your mileage goes way down when you hit the gas.  It will show you how it goes way up when you let your foot off the gas.  It will likely show you zero when you are idling at a light, when you are actually getting negative mileage if it could be computed (division by zero problem prevents a meaningful calculation). 

Frankly that small of a difference is not bad.  It might even get closer as it learns your driving style.

I’m surprised it’s that close! Once the car is broken in and you accurately measure your gas mileage by tanking at the same PUMP and stopping at the first click it should be closer. In any case, if you average several tanks, your own figure willl be the better; the computer does not actually measure the gas flow.

Thanks all for the replies. They have been informative and helpful.

I do not reset the computer after every tank, so the numbers are cumulative. I have figured it manually on a cumulative basis as well. I did not know how the computer calculated the mileage and had assumed it was using some sort of flow meter. Seeing that is not the case, the difference makes more sense, and yes, I was feeling cynical.

I had thought that the difference would become closer as I added miles, but the difference has been consistent with each tank full. ( I do fill each time and use several stations)

You were feeling cynical!

Good for you. A lot if cynicism isn’t healthy, but a little “show me” is a very good thing.

If you ever used a personals section to find a date, you will find the accuracy of the Minis fuel consumption to be pretty much acceptable!