Tracking MPG/Savings ETC...

Nice spreadsheet. I can't see the formula's, but I see some mistakes.

A good formula for comparing mfdmpg and pumpmpg is;(mfdmpg-pumpmpg)/mfdmpg. For that formula, a positive number means that the mfd showed higher mpg than the pump.

The cost savings seems incorrect too. A good formula would be; ((miles/22)-gallonsin)*pricegallon. That tells you the cost for the extra amount of gas your old car would have required to travel the same distance.

I personally don't like to use an mpg average for anything. I have decided the mfd is more accurate than the pump calc, so I use that one.

I also like to use miles per month and miles per year.

Other than that, great spreadsheet and great mpg.

 

I personally don't like to use an mpg average for anything. I have decided the mfd is more accurate than the pump calc, so I use that one.

I also like to use miles per month and miles per year.

Other than that, great spreadsheet and great mpg. [/quote]

hello,
could you explain why you feel that the mfd is somehow more accurate than using exact figures, like the exact amount of gas used and exact miles driven?? My mfd always shows that its a better mpg using the mfd's figures, However, exact figures can't be wrong, Thanks for the insight, Norman.

 

Hi Norman. I think I can answer this one: since the fuel tank of the Prius has an expansion bladder, the amount of fuel going into a "full" tank may be different depending upon the environment at the time. In other words, you might be able to put more gasoline in if the weather is cold than you could if it's warm.

This kind of puts the "exact figures" into question for the short term.

The MFD readings, however, are theoretically calculating mileage based on actual usage of gasoline to the engine (I say theoretical because I have no verification of how or how accurately the machine does that, and there may be some magic math in there for all I know), so it should be a more precise measurement.

 

I'm not sure I buy this. Gas in the tank is gas in the tank. I may have pumped more today and less last week due to bladder stiffness differences. My thinking is that over time "underfill" and "overfill" should average out. So would differences in pumping "technique" or actual pump hardware.

I suppose I need to track this in the summer as well to make a conclusion, but for now, my MFD has *never* shown a figure less than my "pump" mileage. It has often been way too optimistic though.

Anyway, my wallet does not run on "theoretical" calculations. What I pay for is the real expense to me and the amount of gas that I actually buy seems to be the best indicator, however unprecise it might be. Of course, I can be wrong too

Anyway, here's my chart that covers every fill-up since I bought the car in October '07. You can see the difference can be as much as 10-15% b/w the MFD and the pump and it is always in-favor of the MFD.

Now, I would not mind you proving me wrong - the MFD figures are better

 

Can't seem to be able to save my edits, so a new post here...

Added Google Spreadsheet for the same data so you can see it. I did not know Google had this "Google Docs" feature - pretty cool. I'll stick however to my Windows Mobile 6 PDA/Phone for this - more flexible and actually allows me to plot the MFD vs. the MPG on a single chart (could not figure out how to do two series in one chart on Google)...


Google Docs - Prius

 

I quite agree with you, kocho, which is why I mentioned the short term. Once we get a significant number of measurements (I'm thinking at least a year's worth), we can start looking at a real-world trend in mileage, and maybe even be able to calculate the MFDs fudge-factor.

 

I suppose before disagreeing, I should have read your signature!!! - Yes, I'm looking forward to the warmer months of the year as well to hopefully up my fuel efficiency, though then I would need to use the A/C...

I suppose a factor would also be the size of the tires - larger diameter tires would result in worse (?) reported or calculated FE as the odometer would be off - shorter calculated distance traveled for the same amount of gas because the tires would rotate fewer times.

 

I'd buy that if I weren't currently running brand new, stock wheels/tires. But then, I have only filled the tank twice, so I feel it's too early to tell if there is a significant amount of exaggeration on the MFD.

 

Hi All,

One way to explain this is fuel volume. In winter the Prius measures the volume of cold gas. So, cold gas has less volume that standard temp gas. Gas stations pump standard temp gas (out of the ground). Which does not vary as much as gas in the Prius tank. So the volume the Prius measures has less volume being cold for a given energy content which directly relates to miles.

I know one thing. As temps get warmer in the spring, my tank takes more, and the disagreement between the pump and MFD has a spike each time the temp warms up. And the reverse is true in the reverse direction in the fall. One extreme situation I had a 56 mpg MFD, and a 63 mpg pump. The difference was 85 F the previous fill, 45 F during that fill.

So wait till summer and see if the MFD average runs below the pump, with upward spikes on the pump curve.

Also, can you correlate those downward spikes with type of gas, or particular gas station/pump ?

 

Not really - I tend to fill-up from the same 2-3 stations, usually Exxon or Chevron. More likely - I drove more short trips with cold starts or I was rushing the Prius more than usual. Longer trips are however clearly correlated with top spikes.

My last fill-up yesterday (not on the sheet) was probably a fluke of some sort as I measured 56 mpg or so (50.2 MFD) - only after about 30 miles I am one bar down on the gas gauge - probably the pump was not calibrated as I did my usual 1 click top-off. I expect the next reading would compensate and be very low in the low 40 if not worse...