Why is MID Always Optimistic?

**Moviemike** · 03-20-2010, 08:49 PM

Why is the MID always optimistic is a question that has been bugging me for a long time. Optimistic by 1-4 mpg per thankful, e.g. higher than hand calculation. Its a question for which I still don’t have a satisfactory answer, after 21 fill ups and 8.2k miles.

In search of an answer I’ve looked closely at gas tank filling issues (a major source of error in my prior car) and possible gas pump read-out errors. Someone (zurk I think it was) said they did not have optimistic MID readings, and once had a MID number below the calculated number. An out of the ordinary practice, of this person, was to fill up within a mile of the MID reporting a Range = 0. Doing this gave him a remarkably consistent amount of gas pumped with each fill up ..9,5xx gals.! Could this have an effect on MID mpg accuracy? So I gave it a try with the last two fill ups. I discovered my highest ever MID error of +4.1 mpg (I used the same pump, and pumped 9.534 and 9.586 gals.).

Some have suggested that car computing mpg is complicated and the technology is not up to the task. I reject that explanation, because I and many other 2 door I1 owners have reported consistent computer / hand calculation figures. At a recent fill up of my wife’s I1, the computer reported 51.9 mpg. The hand calculation at the gas pump’s auto turnoff was 52.05xx mpg!

Why can the I1 computer do the job, while the I2 computer can’t? The I1 and I2 are very similar vehicles except that the I2 has an automatic fuel cut-off that engages when you take your foot off the gas pedal. Also the I2 has an “ev mode” in which the battery and the little 13 hp electric motor propel the car forward while fuel is cut off. The stock I1 can’t do either of these things. If this is the source of the computer error, than you’d expect those I2 drivers that use a lot of ev mode to have high MID error, and those that rarely use ev mode would have less. Well that is exactly the case for me! The more ev mode I use, the higher the amount of MID error I‘m seeing! See my data at Blue Bird (Honda Insight) | Fuelly

If this is the source of the error, a small computational adjustment / computer patch could fix it!

bish79 · 03-20-2010, 09:01 PM

Quote:

Originally Posted by Moviemike

If this is the source of the computer error, than you’d expect those I2 drivers that use a lot of ev mode to have high MID error, and those that rarely use ev mode would have less. Well that is exactly the case for me! The more ev mode I use, the higher the amount of MID error I‘m seeing! See my data at Blue Bird (Honda Insight) | Fuelly

If this is the source of the error, a small computational adjustment / computer patch could fix it!

I hate to burst your theory, but, while on our vacation last year we had many stretches where we were driving 80+ MPH on the Interstate almost 100% of the time. Therefore there was almost no time where we were coasting or running on electric only. Guess what? SAME 2-4 MPG error no matter what. The only explanation is that Honda knowingly made the estimates optimistic for whatever reason.

Gimpster · 03-21-2010, 01:42 AM

Dumb theory, but are you using ethanol enriched fuels? Wondering if thats the case your MPG is suffering and the mid isnt smart enough to adjust itself to fuel trims.

noseph · 03-21-2010, 08:04 AM

Something to mull over. Why isn't the Miles to Empty calculation as optimistic as the MPG calculation? If anything it is pessimistic.

jkbrushaber · 03-21-2010, 01:33 PM

I find my I1 MID is usually about 2% off my hand calculations. The difference is (of course) more pronouned at higher mpg's. I have also found the % difference is also lower at 65 mpg vs. 85 mpg. So, getting 51.9 MID and 52 actual does not surprise me.

joecvt · 03-21-2010, 06:53 PM

Quote:

Originally Posted by Moviemike

Why can the I1 computer do the job, while the I2 computer can’t? The I1 and I2 are very similar vehicles except that the I2 has an automatic fuel cut-off that engages when you take your foot off the gas pedal.

The gen1 Insight also has automatic fuel cut-off that engages when you take your foot off the gas pedal. Other cars do as well.

JoeCVT = Just your average CVT owner

Gimpster · 03-21-2010, 08:22 PM

This I can actually understand.

My last vehicle when the range got down to 50 miles it would just say "LOW FUEL RANGE" and no give any more advice. I'm sure some lawyer or exec said to engineering don't cut it too close.

Quote:

Originally Posted by noseph

Something to mull over. Why isn't the Miles to Empty calculation as optimistic as the MPG calculation? If anything it is pessimistic.

**Moviemike** · 03-21-2010, 10:19 PM

Thanks for all of your thinking on this.

JOECVT: "The gen1 Insight also has automatic fuel cut-off that engages when you take your foot off the gas pedal. Other cars do as well."

REALLY?? I didn't know that. Do these other cars also calculate mpg, and do it to an accuracy = to the I1? My understanding, from reading Honda's I2 customer stuff nearly a year ago, was that the fuel cut-off was new to the I2.
If what you say is correct, it further supports my contention that: "If this is the source of the error, a small computational adjustment / computer patch could fix it!."

BISH79: "....while on our vacation last year we had many stretches where we were driving 80+ MPH on the Interstate almost 100% of the time. Therefore there was almost no time where we were coasting or running on electric only. Guess what? SAME 2-4 MPG error no matter what."

Was your mpg error always on the optimiistic side? You could have done some coasting as you slowed from 80 mph to zero, or while going down a long hill, or slowing due to a slower car in front of you. You might have unknowningly slipped into ev mode while creaping along in heavy traffic or in large parking lots.

Still you make a good point. While supporting my view that Honda could fix it, IF they wanted to!

In my old car, which also had a 10.x gal gas tank, I found that I could fill the tank to the point that the gas level was just equal to the lip of the filler pipe. I could then run the car to the point that it needed 8.5 - 9 + gal to fill the tank. At the time of fill up I could calculate mpg at the gas pumps' automatic turn off and then again at the point of fillup just equal to the lip of the filler pipe. The difference between the two hand calculted figures was 3 mpg! I think of this as "gas tank filling issues." An mpg calculation error, from this source, will be reduced in a mpg figure that covers many fill ups. The existence of this type of error does point out that one or two fill ups is not enough data to make the case for an explanation of MID optimism!

JKBRUSHABER: "...So, getting 51.9 MID and 52 actual does not surprise me."

The 52 was at the gas pump auto turnoff. A few clicks on the gas pump would have added a little more gas, probably lowering the calculated mpg to the car computer's figure.

GIMPSTER: "Dumb theory, but are you using ethanol enriched fuels? Wondering if thats the case your MPG is suffering and the mid isnt smart enough to adjust itself to fuel trims."

Yes. The switch to ethanol did reduce my in-town mpg, did NOT have a noticeable effect on highway mpg, but did reduce my overall mpg from 36mpg to 35.5 mpg in my old car ('02 Saturn SL1).

My I2 mpg is NOT suffering. I just want CONSISTENCY between hand calculation and MID. I'm NOT asking the MID to "adjust to fuel trims." ...speaking of dumb

zurk · 03-22-2010, 01:05 AM

mine is dead on.

joecvt · 03-22-2010, 10:42 AM

Quote:

Originally Posted by Moviemike

JOECVT: "The gen1 Insight also has automatic fuel cut-off that engages when you take your foot off the gas pedal. Other cars do as well."

REALLY?? I didn't know that. Do these other cars also calculate mpg, and do it to an accuracy = to the I1? My understanding, from reading Honda's I2 customer stuff nearly a year ago, was that the fuel cut-off was new to the I2.

Yes, Really.....This is not a new idea even to the gen1 Insight. Take a look here:
InsightCentral.net - Encyclopedia - Honda Insight Fuel Cut Mode

It mainly describes how the MT fuel cut works but the CVT version is just nearly the same except method for the clutch stuff....In fact, the CVT can enter fuel cut mode at high speeds and stay in that mode all the way down to AutoStop (even more than the MT model)....I checked it out myself testing the voltage supplied to the fuel pump. During my test, once you left off the gas pedal, about one second later fuel cut mode begins. My volt meter showed no voltage supplied to the fuel pump all the way down to AutoStop. Then as you let your foot off the brake (for AutoStartup), voltage was supplied.

It is not just for Honda either. Just a quick Google search of Fuel cut deceleration shows a PDF from Toyota in 2001:

http://www.autoshop101.com/forms/h43.pdf

There are tons of others found with Google showing the ECU / ECM cutting fuel during decelleration for fuel economy and emissions reasons.

I think that perhaps you are thinking of cylinder deactivation (something the Gen2 Insight can do but the Gen1 Insight can not)

Cylinder deactivation was already a function on some Honda V6 engines and Honda brought it over to the Insight 4 cylinder engine. Instead of fuel cut to all cylinders (as in fuel cut mode via fuel pump disable), the intake and exhaust valves remain closed of the cylinder deactivated and no air or fuel enters that cylinder. In the V6 engine, 3 cylinders are deactivated at once under light engine loads.

JoeCVT = Just your average CVT owner