TechnoMage said:
mdacmeis said:
Toyota's official statement is the displayed MPG is very accurate.
I'm somewhat inclined to disagree with Toyota. :wink:
I agree with TechnoMage. I think improvements were made in the '04 model's displayed mpg accuracy, but many people have reported that the NHW11 models consistently display an overly-optimistic mpg.
I experimented with mine shortly after I bought it. Over 10-12 tanks, I recorded tank mpg according to the display and according to the pump. The two average values fluctuated quite widely for the first few tanks, due to the variability induced by the bladder. However, after the final tank in my experiment, the difference between the two averages had settled to a quite stable 5.6% error, with the display reading higher than the pump value.
The difference between the two readings is a function of the measurement method. Pump measurement is legally required to be accurate, because it uses volume from the pump (which is calibrated and tested regularly by the government) and distance from the odometer (which is required by the government to be accurate).
Display measurement is based on fuel injector data: given that the fuel injector sprays fuel at a fixed rate when it is open, total up the amount of time it has been open, and multiply by this rate to get the amount of gas used. However, this rate has tolerances such as +-3% (i.e. some injectors will spray less, others more). It has been hypothesized that (for marketing reasons) Toyota chose to adjust the computation algorithm so that all cars will report
at least the accurate value; maybe more but never less. So if the tolerance is indeed +-3%, the displayed value for any given Prius should be 0% to 6% high.
Toyota can still call this "very accurate", because an average error of +3% is still very good by most standards. Most measurement systems (digital or otherwise) use a few op-amps and a few 1% resistors to digitize the input data, and these 1% errors accumulate through several input stages, so a 3% error isn't unusual.
What does this all mean? The displayed average values are useful for comparing one trip to another or one tank to another (i.e. relative measurement), but they are not useful for long-term absolute measurement, which is probably why the '04 resets the displayed average every time you fill the tank. The only way to be sure of your lifetime mpg is to use pump measurement.
Douglas (2002 Silver, Wisconsin)