Trip computer (in)accuracy

[mkaresh]

I recently had a 328i automatic as a press car for a week. The trip computer reported some outstanding averages. Using "Eco Pro" and driving it like I might a Prius, I averaged 37-38 mpg for the 8.5 suburban miles to my kids' school, and even managed 40.6 once. Driving the car normally, the trip computer reported 30-35. Push it hard and the numbers dropped to 19-22.

Wondering if these figures could possibly be correct, I asked the press fleet company to let me know how much gas they put it in and the total miles traveled. The calculated result: 21.9 mpg, about 10 mpg lower than the numbers reported by the trip computer.

Trip computers are often off by 1-2 mpg, but about 10 is unusual. Are other people observing similar inaccuracies?

If you can, please post the powertrain (four or six, manual or auto), calculated number, and the one reported by the trip computer. Type of driving and how hard you pushed the car would also be good to know.

[mryakan]

The average you saw is consistent with numbers BMW claims at least (and some independent testing done). Without knowing the exact amount of fuel that was put in, I would not doubt the OBC. From what I understand, it measures the consumed fuel by calculating that actual amount of fuel that is injected into the cyls, and with the injectors used in these modern cars, it is extremely accurate. I would doubt the fuel pump, or even suspect fumes escaping before doubting the OBC.

Don't forget that these are very efficient cars. With Start/Stop, brake energy regeneration, electric water pump and steering pump, etc, parasitic losses are at a minimum. Combine that with a frugal 2.0L engine when you are not hard on the gas (combined with smart engine management and Valvetronic), and you can expect very decent mileage. I am not surprised by what you saw at all.

[mkaresh]

I didn't doubt the OBC...until the fleet company told me how many gallons they put in the car!

Any owners with their own observations?

[mryakan]

I didn't doubt the OBC...until the fleet company told me how many gallons they put in the car!

Any owners with their own observations?

Just thinking, maybe they told you what what they put in a while before they gave you the car and maybe they drove it for a while in the city then reset the OBC. This would make the actual mileage since their initial fillup lower than what the OBC reported to you since the reset.

I am also curious to verify if others see such mileage.

[mkaresh]

They gave me the additional miles they drove after picking up the car but before filling it back up.

There is a lengthy thread at another BMW forum where a couple of people have reported discrepancies in the 1-1.5 mpg range, nowhere near the 8-10 I seem to have observed.