I think the dash board display is a bit more accurate in the HCH II than it is on mine. I've compared the display to the tank calculation for the last 37 tanks (~19K miles). My average error is +3.1 MPG (display is higher than calculated). The low error was +.2, most it's been is +5.1. It's never been correct, and always high.

I think consistency is the key to any comparison. As mentioned, using the same station and even the same pump is about as good as you can do. However, there will always be some variability no matter what you do - that's why a sample over 10+ tanks will tell you more.

 

An "engineered MPH error of about 3% too fast"??? Is this documented anywhere? As far as my speedometer, I drive past radar signs at least twice a day and my speed is always dead on. Our previous car was about 10% off. Regarding the calculated FE vs. the dash, people should always go with the calculated. Even though this has the potential to provide more variation from tank to tank, it is definitely accurate in the long run. My calculated averages have always come out higher. They have ranged from 0.7-1.4mpg off and have averaged 1.15mph off. Most HCHII owners have reported similar findings. As an aside, using the dash value will hurt the database average for the HCHII, although this will inflate the average for some other vehicles. Does anybody here even use the dash for reporting?

 

I agree, digital speed can be calibrated to the car's actual tire size much more accurately these days than it used to be. Still, it is good to compare the measured verses the actual. Plus, replacement tires of the same "size" have as much as 5% height difference from factory tires, which will change the measured speed. You can check this out over at the Tire Rack by comparing the aftermarket tire specs.

 

Yes. I use the dash exclusively, as one of the perks where I work is having our own gas pumps dishing out 89oct at wholesale cost. The upside is that our gas is usually about $0.20 cheaper than the 87oct down the street. The downside is that our pumps have the old dials that prevent you from getting anywhere closer than about .1gal accuracy. I keep track of both in my spreadsheet, but so far only have about six tanks worth entered. As of right now, it would appear that either I'm getting more gas than the dial reports, or the dash thinks I'm burning more than I really am.

 

Hmmmm---your post made me think about something....OCTANE!

I know you are using 2 above recommended of 87. I do know that with my old Acura Integra I gained about 2 MPG when I filled with 93 vs. 87 but at a little over $1 per gallon, it was not worth the difference in cost.

Has anyone tried putting in 93 octane to see if it affects MPG and also overall performance? I know that with electronic ignitions the higher octane will cause a hotter combustion. Ironically, the Integra had a distributor but still showed a real improvement (I did this over a number of tanks for each octane and measured the results).

 

Exactly my point. I'm not trying to say that all cars come from the factory set to precisely a 3% inaccuracy, I'm saying that the manufacturer usually errors on the high side and you need to compare the reading to an independent source to calculate your real gas mileage.

Speedometer accuracy changes over the life of a tire. The Honda Civic Hybrid OEM tire has a new radius of 12.1 inches with a wear depth of 0.28 inches (this is assuming you have the Bridgestone Insignia SE200-02 tire and not the Dunlop SP31 A/S tire -- I do not have the precise specs for the Dunlop tire -- however, the effect of tire wear on speedometer accuracy applies to all tires in general). So as your tires wear for the full life of the tire you will see a drop in the height of the tire by about 2.3%. Even if your speedometer reads precisely correct now, it will be reading fast by slightly more than 2% by the time you need to replace your tires.

 

At least on new tires, my speedo (and odometer) are dead on accurate. I've confirmed this with a GPS over a several hundred mile trip (FAR more accurate than going by a radar sign!).

 

Octane rating is useful to determine a fuel's resistance to compression before detonating. Higher octane fuel actually tends to have less energy, resulting in slightly worse MPG. Higher octane gas can be run in high performance, higher compression engines hence the association of higher octane fuel to higher power.

See:

http://auto.howstuffworks.com/question90.htm

http://www.waukeshaengine.com/intern...&numPageID=378

http://en.wikipedia.org/wiki/Gasoline

 

Some of this stuff is probably more confusing because cars have variable timing now. Some Honda's have a very high compression ratio(11to1) yet still use 87 octane. Yet I have a '91 Eagle Talon which is turbocharged and it required premium octane with a (7.8to1). Octane and horsepower go together by generally the hotter the boom in the combustion the higher the octance needs to be to keep it a reasonable temperature in the cylinder. Octane up to an extent can help regulate temperatures during the combustion cycle. From there you are ensuring ratios of fuel, timing, boost(if turbocharged) are all happy and if not the engine will not run properly. Lucky if it runs with less power on the wrong octane and unlucky is detonation do to serious knock<--engine go boom). For some reason I want to get my car out of hibernation and get some 118 octane fuel and see if I can finally break into the 12's in the quarter mile. OK got it out of my system now. $9+/gallon not good. 9mpg not good. Driving around in your hybrid using less gas and getting 50+mpg priceless.