I use fuelly.com and I noticed that the CPU seems inaccurate many times. Most of the time, the CPU gives me 0.1 to 0.2 L/100 km. less that Fuelly, but lately with Petro-Canada 94 octane it's very strange... Yesterday I drove 575 km. at 110 km/h and the CPU gives 5.9 L/100km but on Fuelly it's 5.0 L/100km.

Last time the CPU was giving me 6.2 and Fuelly 7.9.

Anyone experienced that ?

Usually, with Shell it's working fine. Maybe it's the gas type that's creating this problem ?

 

Tank to tank averages can vary because of a bunch of things. It's hard to fill up to the same point each time. I assume you don't have an 09, correct?

 

No I have a 11. I know if can vary a bit, but still, 5.0 to 5.9 or 6.2 to 7.9 is a HUGE difference !

 

Depending on where the pump stops and where you top it off will affect it tremendously, mostly because you are talking about a small tank capacity (small differences add up on small tanks). Also, already mentioned are differences in pumps, stations, and the time of day you fill up (filling up in the heat of the day will yield less fuel compared to filling up at night/in cool temps). I've found filling up in the morning and rounding to the nearest *50¢ nets the closest calculation to the computer result.

 

Thanks... But still, huge difference... I always double top the tank... and always between 20 and 30 celcius in the last weeks... Strange

 

I agree with you that the difference is huge. But I also know that those CPU's are not accurate. They were never designed to be used as an precise analytical tool. Rather they were designed to provide the driver with an instant feed back on driving habits. Honda put it in your car so that you wouldn't mash your gas pedal at every stop and go light. Any other use is well....unintended.

 

Gasoline can lose up to 3 percent from just burning and sitting. When a fuel vaporizes it will lose some of its volume from the chemical reactions usually 1-2 percent and when sitting from permeation(evaporation) from the tank and lines. Then there is the temperature variations and the gas station inaccurate pumps that go in their favor.

 

What is the concern with it being an 09?

I have one and my computer mpg is off.

 

Honda had the data maps wrong on the 09's so they consistently read 10-15% better MPG than actual. There's a TSB and a ECU flash to correct. (see the sticky at the top of the forum).

 

I never top off and my computer is always within +-1%.

 

I fill up at all times of the day and it is pretty accurate(ly off by 1mpg on every fill-up). I have an '09 with the reflashed maps so accuracy is pretty good. Time of day will have very little to do with the amount of fuel that goes into your car as fuel temperatures vary very little during the course of a day. The pumps are underground so temp differences in fuel between 7am and 7pm will be negligible and will only noticeably increase over days or weeks and not hours (with the exception of if a Tanker truck sitting out in the sun all day fills up the station tanks). Like someone else said, it is most accurate for me when I fill up and don't top off the tank at all (or more than one click).

~SB