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This Labor Day weekend, I drove from Sacramento to a friend's in Sebastopol, and systematically tracked my MPG.

While what I am reporting here is pretty obvious stuff about the relationship between speed and MPG... I wanted to systematically gather data from my Karma to help me understand the highway MPG (since ranges people were posting were pretty variable).

Conclusions: 26MPG on HWY is very reasonable statement. (I got this at 65 mpg)
Faster you go, the worse gas millage. At 80 MPH, I got about 20 MPG. At 55MPG, I got about 28MPG.

Grade of the highway of course factors in -- that explains most of the variability in 55 to 65 speed... the terrain around Vallejo and Marin is rolling hills.




Here is what I did: I entered a cruise control speed, hit reset on the trip computer, then recorded the MPG after about 3-4 miles of driving. Then I varied the speed, reset the trip counter, and repeated the data collection. (it made the drive go by faster). I recorded 30 data points.

I did it for both the drive down and the drive back... and noted the outside temp, AC settings. You too can add to the dataset, if you want. I'll see if I can figure out how to share the excel doc on google, or something.
 

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The main problem with this analysis is that you are relying on the Karma's MPG calculator. I have done a few extended highway drives to/fro San Diego and LA and while the tripometer MPG calc tells me I'm getting 25-27MPG, my own gasoline and odometer calcs show I'm getting more like 21-22MPG (i.e. I fill it up, drive for 120-130 miles, fill it back up, divide the miles I've driven by the amount of 'new' gas I've added to fill the tank back up). So I think the Karma's onboard calc does not give accurate MPG readings.

Anyone else try to verify this?
 

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SoCalGuy said:
The main problem with this analysis is that you are relying on the Karma's MPG calculator. I have done a few extended highway drives to/fro San Diego and LA and while the tripometer MPG calc tells me I'm getting 25-27MPG, my own gasoline and odometer calcs show I'm getting more like 21-22MPG (i.e. I fill it up, drive for 120-130 miles, fill it back up, divide the miles I've driven by the amount of 'new' gas I've added to fill the tank back up). So I think the Karma's onboard calc does not give accurate MPG readings.

Anyone else try to verify this?
That's a problem with pretty much every car's MPG calculations, mainly because the sensors measure fuel pressure and estimate the flow, rather than measure flow directly, which tends to cause the errors to accumulate over time.

The KW meter is probably much more accurate since it is a direct measurement but we don't have access to most of that data.
 

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Discussion Starter · #5 ·
Hi SoCal, you raise an interesting point, and I calibrated the trip mpg with my mpg using Brent's spread sheet - over 3 fill-ups/almost 3000 miles, and the overall figures tie out exactly (103 mpg overall in both systems). Brent, do your numbers tie out too with trip computer?

I could do one other verification, which is to take the weighted average of mileage observed and compare to gas consumed... I will try that on my next long trip, but I am satisfied with the overall cross check that the instrumentation bias is minimal.
 

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rex said:
Brent, do your numbers tie out too with trip computer?
I'm happy to say I really don't know; I haven't been to a gas station since April and prior to that it was February. Most of the miles on my current trip counters have happened since my last fill up and I have no independent way of knowing the actual fuel consumed. I suppose I could go fill up the tank to measure actual use and calculate it, but I'd rather see if I can manage to go 5 full months (then maybe 6) without stopping for fuel.

Plus, when people ask "What kind of mileage do you get?" they seem to be even more impressed with "It's hard to say, I haven't put any gas in since April" than with the calculated number :)

I am using my spreadsheet of course, so when I do fill up I'll be able to answer the question better.

Brent
 

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I went about 620 miles down to Zion Canyon (and within Zion a bit) and back on Sunday. (I forgot to record the mileage on return; Trip A now reads about 640 but that includes today's ~20 miles of errands.) There were no charging opportunities in between, and I started with 50 miles of charge and ended with 6 (still haven't let the computer turn on the gasoline engine on its own, at 0 miles).

I put 13.439 gallons of gasoline in the tank (in two fill-ups) on the way down (about 300 miles) and 15.431 on the way up (includes the in-park driving). Dividing 620 by the total (28.87) gives me about 21.5 mpg. The Trip A computer read 24 mpg on return, so it's obvious that the computer and "actual calculations" differ significantly.

Note that both trips involved a whole lot of 80 mph driving (that's the speed limit for many of those miles), so 21.5 mpg is pretty reasonable. I may have gone faster than that too. :D (I had one point, climbing up and down some mountain passes between trucks and such in the dark on the return trip, where I had to take it out of cruise for a while and later noticed that I was, well, let's just say "significantly over" the 80 limit, without realizing I'd gotten the car up to that point!)
 
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