Gas Mileage

bubbaleenc

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For the /TLDR/ crowd, the gist is: the Distance to Empty calculator is odd!

I've noticed the distance to empty calculation is not terribly accurate in my car. Generally the low-fuel alert (which, according to the manual does not trigger until the tank has 2 gallons left) comes on with the DTE showing around 40 miles. Given that I usually see very low 40s in mpgs, the DTE should show no less than 80 miles when the alert goes off.

To test it, I carried a gas can and drove it past DTE of 0 miles for about 10 miles. I stopped for gas at a station (didn't need the gas can thankfully) when the blue line next to the E on the gas gauge disappeared. I filled with 11.7 gallons.

In another test, I drove 95 miles on a tank of gas then refilled it with 2.2 gallons. Adding so little a volume did not automatically reset my trip odometer, so I reset it manually. The DTE also did not increase to account for the top-off as it does when I fill up with a larger volume (typically around 9-10 gallons on a refill).

Even on my regular fill-ups, the DTE only shows about 95% of the actual distance to 12.4 gallons used, averaging out the last 3 tanks of gas mileage. Then, the DTE creeps downward (that is, it doesn't go down mile-for-mile driven, but slightly less) and, after about 75-100 miles, it represents a good estimate of the total mileage to using 12.4 total gallons (e.g., if I drive 100 miles, the DTE shows 420 which is about right for my fuel efficiency and using 12.4 gallons). This usually stays true to the 500-530 total miles available (DTE + miles on tank) until I've driven about 375 miles on the tank - then the DTE starts to drop off faster than the miles driven, no matter what my trip mpgs or tank mpgs are.

I know that cars often have a dead-space in their gas tanks that volume of fuel is not functionally usable (most modern cars' fuel pumps shut off at that point to prevent pump damage). I have no idea if the 12.4 gallon listed fuel capacity accounts for the dead space (meaning the tank itself can actually hold closer to 12.9 gallons assuming 0.5 gallons of dead space), or if the 12.4 gallon capacity is the total size of the tank (meaning the tank can only functionally access closer to 11.9 gallons of fuel). Either way, the DTE and the low fuel minder don't match up.

Any similar observations?



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Baconstrip

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Any similar observations?
I've only had my civic for 4 fillups, but yeah, I notice the DTE is not terribly accurate either. Low Fuel came on around 35 miles left... which technically should be 1 gallon if my MPG is accurate. When I fill up my car says 410 miles DTE, but I'm probably getting closer to like 330-350 miles off a tank.

Granted, out of all the cars I've driven with a DTE, it's never seemed any more accurate :D
 

bubbaleenc

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Granted, out of all the cars I've driven with a DTE, it's never seemed any more accurate :D
That's the TRUTH! My Toyotas are pretty miserable too. At least they are inaccurate in a consistent way though.
 

UberCivic

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I'm not sure that I have noticed the tapering off thing, but the zero mark is consistently at 11 gallons to fill. I've pumped 12.6 gallons into the tank and gone 90 miles past the zero mark without running out of fuel.
 

                           































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