Gas Mileage

[Yardsale]

I think Richard understands the inconsistency of the automatic shutoff (he notes this and is why he uses a visual fill line).



But ...



I think there may be confusion between determining the number of miles driven on one tank full vs the number of miles driven for some quantity of fuel (perhaps 40 or 100 gallons). In the first case, one would want to refill the tank to the same level, in the second, it doesn't matter because no matter how full is the tank, it is the fuel pump register value that is used for the calculation.



Significant digits: The result cannot claim to be more accurate that the least number of significant digits in the data. In the case of fuel mileage, most use the vehicle odometer, which is typically limited to tenths of a mile (e.g. 436.6 miles/30.226 gallons=14.444517964666181433203202540859 mi/gal rounded to significant digits is 14.4 mi/gal). I think this is plenty accurate for fuel mileage of a truck.

 

[Gavin Allan]

:supercool:

 

[Richard L]

Yardsale,

I think there may be confusion between determining the number of miles driven on one tank full vs the number of miles driven for some quantity of fuel (perhaps 40 or 100 gallons). In the first case, one would want to refill the tank to the same level, in the second, it doesn't matter because no matter how full is the tank, it is the fuel pump register value that is used for the calculation.

The more miles you drive and the more times you fill up will tend to negate any minor errors in the pump reading or miles driven. However, on a short trip and no more than one tank of gasoline allows minor error to create significant errors. Multiple tanks of gas is more accurate then one tank, or a partial tank because repeatedly filling the tank will tend to average out any small errors made.



If you use anything less than a tank of gas, or only one or two tanks full, you may not be filling the tank to the same level as when you started the test. Since auto pump shut off can very from station to station, pump to pump and even the same pump can shut off at different fuel levels. So it is possible to use 10.0 gallons of fuel but the pump may shut off at 9.0 gallons. Those few 10ths of a gallon of gas on a 200 mile test run may not seem like much, however that variance can create a 2 MPG difference in MPG. That's a 200 mile test. a 50 mile test run might show even a bigger margin of error.



Anyone who uses the fuel gauge in any part of the calculation is not getting an accurate MPG calculation



Anyone who assumes that the gas pump's auto shut off will always shut off when your tank is filled to the exact same level is also likely to be getting an inaccurate MPG calculation.



My method is only used as an occasional MPG test for a short run of say about 40-100 miles, or no more than one tank full of gasoline while on a long road trip.

gnificant digits: The result cannot claim to be more accurate that the least number of significant digits in the data. In the case of fuel mileage, most use the vehicle odometer, which is typically limited to tenths of a mile (e.g. 436.6 miles/30.226 gallons=14.444517964666181433203202540859 mi/gal rounded to significant digits is 14.4 mi/gal). I think this is plenty accurate for fuel mileage of a truck.

The problem with your calculation is that pumps only read to the 10th or at most 100th of a gallon. Over a 436 miles (one tank of gas) that caluclation is probably accurate enough, but will very from day to day, depending upon weather, traffic, etc...all of which the driver cannot control or even estimate the impact on fuel milage.



If you are running a quick, 40-50 mile mileage test then a 10th of a gallon or a 10th of a mile can skew the MPG by a significant amount.



To make it simpler for you, the amount of gas used is more accurate if you can see the fuel at the restrictor plate in the fuel filler neck, than relying on the gas pump's auto shut off.



If I fill my tank to the restrictor plate, I will have a much more accurate reading of the amount of gas I put in my tank than relying on the gas pump auto shut off. I'm sure you and everyone else has encountered that pump that only pumps about a cup of gas before the auto shut-off kicks in. This tends to happen on the coldest, windiest day of year:angry:



I think you are confusing my method of performing a short 40-50 mile MPG test run with a 400 to 4000 mile, multi-tank full mileage average. The shorter mileage test requires far more precise methods to assure accuracy.



...Rich

 

[Igor Smetaniuk]

Hey guys, you're out in left field with the last posts because the OP is noticeing a drop in his own way of calculating the numerical drop over the years. I would assume he's being consistent in the way HE is measuring each time. He's talking about noting a difference and not exact calculations.

 

[Johnny O]

Ethanol will muck up your injectors. Bad enough I got a check engine light and the engine suddenly ran like crap. Just a few things off the top of my head that can affect mpg.



Dump in a can of BG CF5 and a can of BG Fuel System Drier to clean the tank and injectors. I do that once a year now on all my vehicles.

New plugs and wires.

New air and fuel filters.

Some lubes can get thicker over time and not always thinner. Change the rear axle lube, also the front and the t-case if it is a 4x4. When i changed the axle lube in my daughter's old Jeep, the first thing I noticed was that that automatic transmission upshifted faster. No kidding. Had to be a mess of drag coming from that cruddy old axle lube.

Flush the tranny. If the tranny is slipping that will hurt your mpg.

Tires. When I switched from the OE all-seasons to all-terrains in the stock size I instantly lost 2 mpg on the highway. Need A/T's for snow though.



$0.02.

 

[Dean Hedin]

Keith, I don't believe you indicated what your mileage is now.



I'm not a big believer in engine wear being a big contributor to "loss of fuel economy".

Unless you have really high mileage or are blowing oil.



First off 10% ethanol is a 4-5% drop in fuel economy vs. pure gasoline.

Secondly, fuel economy always sucks in the winter.



You may want to consider replacing the O2 sensors. Also, Catalytic converters can start to disintegrate over time and leave chunks of material that can cause back pressure in the exhaust. This happened on my brother's Escape. It did not issue a ECM code.



Rear brakes dragging? Won't throw a code and will definitely kill mpg.



Bad coolant sensor. It's not a totally failed sensor, but enough that the ECM thinks the engine is running cooler than normal. ECM compensates by running a richer. MPG drops.



New plugs and wires if you have not already done so. Although the ECM is pretty good at detecting a misfire.



Can't think of anything else. I have a 2003 with 95k and I still can get just above 22mpg highway.



Around town has always sucked and can be anywhere from 14-17mpg.

 

[Richard L]

toptobottom,

Hey guys, you're out in left field with the last posts because the OP is noticeing a drop in his own way of calculating the numerical drop over the years. I would assume he's being consistent in the way HE is measuring each time. He's talking about noting a difference and not exact calculations.

That was the whole point of my post...:grin: And they key word you used was "I would ASSUME". You cannot assume anything that someone else is doing is correct or accurate. When you make assumptions, you are only guesstimating, and you also ASSUME something that may not be true. You must insure that your measurements are accurate and consistent, or you are just spinning your wheels and chasing ghosts.



...Rich