Hello again,

just finished my fourth tank for a hand-calculated mpg of 48.3, my highest yet!! I seem to be able to get high mpg's much easier now that I'm approaching 2k miles. very happy. This is the first tank that I zero'd out the trip on the car's computer........is the car underestimating MPG's by 2.4mpg?? Which is correct?? Anyone else experience this?
Jason in VERY cold Chicago

 

Yes, I am getting about 5% more than the computer says also. Always a nice surprise!

 

Consider it a common thing for the HCH-2.

The HCH-II instrumented mileage almost always underestimates the FE performance... which can be much better than the over-inflated FE performance other well known hybrids offer to their owners.

Cheers;

MSantos

 

Mine underestimates by 1.5 mpg across the board. Interesting...

 

The Car's ECU computes MPG by measuring the fuel injector open dwell time and deriving the amount of fuel that passes through the injector. There is a margin of error in this method because of load factors, how much coasting the car does, etc. that will increase over the measurement cycle. This is why measuring how much gas that is put in the tank and dividing the mileage will come up with a different and more conservative answer. The other issue is the actual amount placed in the tank will vary from fill-up to fill-up thus affecting the final figure.

Short of filling the tank up to the filler each time, I don't know of a way of consistently filling the tank to the same level every time to get an accurate MPG measurement. The auto-shutoff valves on gas filling nozzles is not consistent.

 

I belive my calculations at the pump have averaged 1.4 MPG higher than the dashboard reading over about 25 tanks, which is when I stopped keeping track.

 

I find that if I'm above 50mpg, the computer isn't off by as much as when I'm below 50mpg. On my highest tank of 59mpg, it was exactly on (which was very disappointing since I figured I was in the 60mpg range)

Although on my second to the last tank (~44mpg by hand), the computer was calculating 46 so every once in a while I get a whammy...

 

My two Cadillacs mpg computers tended to be off by about 5-7% around town...and almost dead on in highway driving. The explanation I was given had something to do with measuring the fuel usage from the number of fuel injector pulses as opposed to the actual fuel pulsed. It was a way technical answer that I never really understood.

Has nothing to do with the HCH but thought I would share that little nugget of joy for no apparent reason

 

I just always figured it was some sort of built in safety feature to help prevent you from running out of fuel. 1MPG over 10-12 gallons, could mean the difference between making it to the station in the car instead of walking to the station to get gas for the car. Then again, I guess it is pretty hard for a car to be exact in that reading anyway. As people stated, the pump shut offs vary by station and probably even by pump slightly. Not to mention, the ground isn't always perfectly level and the shut off may happen with less going in or more going in depending on the tilt.

I think the only way to truely measure the MPG, is to have a set amount of fuel sitting somewhere and completely running the tank until the ICE stops running and filling it with the same amount everytime.

 

The most accurate mileage calculation will be by dividing the mileage by the amount of gas filled. Any inaccuracy will be averaged out over several tanks.