I can't figure my car out, I'm like a cow, follow the same path.. lol.. here's my question.. after driving the same route, a nice 4 lane stretch that's 10 miles long..setting the cruise at 60 at the same place each time, when I get to the end of the road, sometimes my average mpg is 41...sometimes it's 52.. is it my engine? My computer? My fuel? Or is this normal? Thanks!
How accurate are the mpg calculators?
- Thread starter Blkcloud
- Start date
Tdijarhead
Top Post Dawg
- Joined
- Nov 10, 2013
- Location
- Lawrenceville PA
- TDI
- 2003 TDI Jetta Daughters Car, 2001 TDI Beetle, Wife’s car, 2005 Golf TDI Mine, all 5 spds
If you really have to know your mileage get a pen and paper and keep track. Those little mpg gizmos are purely for entertainment purposes.
ZippyNH
Veteran Member
They are only as accurate as your speedo.....If you are running nonoem sized tires, it can skew the results...
At the basics, it fuel used and miles traveled....The instant read ones are not so useful...You kinda need to do it over a period of time...Most...The OEM, and the club in o es that use odb2 info do both.
At the basics, it fuel used and miles traveled....The instant read ones are not so useful...You kinda need to do it over a period of time...Most...The OEM, and the club in o es that use odb2 info do both.
Terrific-In-Tahoma
Veteran Member
- Joined
- Jun 4, 2012
- Location
- East-of-Toronto, Ontario, Canada
- TDI
- '01 ALH Jetta M5 / 05 BEW Jetta Wagon A4
Variations in On-Board MPG calculations
.
IF you travel 800 Kms and then fill up again (480 mi), and have filled in 40 Litres (10.5 Gallons) then your average MPG is (Canadians use inverse L per 100 km) 5.0 L / 100 km or 48 MPGs(approx).
Now if you slice that into 500 slices, and then travel say 50 Kms in one direction (For example East), then take the same route, but now are facing in the opposite direction (Travelling Westbouund), you now are going to be pushing against the prevailing winds in N.America and your MPGs will drop down as it burns perhaps 5% more fuel (or going uphill), this then is why the variation in the MPGs one way and then the other.
When going uphill, you will generally burn more fuel, than downhill, so your mileage may vary, however, if you add all of those 500 slices back up, on par, you will get the same reading with pen and paper.
PS. One other part I forgot to mention is the use of the Difference in Pressure between the fuel volume going into the Fuel Pump, and the fuel volume going back into the return line back to the fuel tank. Depending on how accurate the two sensor(s) are, this will impact the accuracy of the MPG calculator in the dash, since it is a difference in the two values (IN-Out over the time(seconds?) that give the number in mL compensated for by temperature. This difference yields the fuel consumption at any given moment.
Just for fun , hit reset one day, while stopped at a long stoplight. See how much fuel is being consumed over what distance? If you are stopped the distance traveled is zero point zero tenths of a mile (km) , so the calculator *should* approach 0 MPG, or go to 99 L /100 km then read 'over-range' or similar since it cannot divide by zero, a mathematical impossibility.
The computers LIE
.
Unfortunately, it is something called Integral Calculus. Think of it this way, When you Fill up your vehicle with fuel, How much fuel do you dump in ,and over what distance is it? (Pardon me I am in the frosty North so for me all the math is easier? in metric)
IF you travel 800 Kms and then fill up again (480 mi), and have filled in 40 Litres (10.5 Gallons) then your average MPG is (Canadians use inverse L per 100 km) 5.0 L / 100 km or 48 MPGs(approx).
Now if you slice that into 500 slices, and then travel say 50 Kms in one direction (For example East), then take the same route, but now are facing in the opposite direction (Travelling Westbouund), you now are going to be pushing against the prevailing winds in N.America and your MPGs will drop down as it burns perhaps 5% more fuel (or going uphill), this then is why the variation in the MPGs one way and then the other.
When going uphill, you will generally burn more fuel, than downhill, so your mileage may vary, however, if you add all of those 500 slices back up, on par, you will get the same reading with pen and paper.
You are welcome
PS. One other part I forgot to mention is the use of the Difference in Pressure between the fuel volume going into the Fuel Pump, and the fuel volume going back into the return line back to the fuel tank. Depending on how accurate the two sensor(s) are, this will impact the accuracy of the MPG calculator in the dash, since it is a difference in the two values (IN-Out over the time(seconds?) that give the number in mL compensated for by temperature. This difference yields the fuel consumption at any given moment.
Just for fun , hit reset one day, while stopped at a long stoplight. See how much fuel is being consumed over what distance? If you are stopped the distance traveled is zero point zero tenths of a mile (km) , so the calculator *should* approach 0 MPG, or go to 99 L /100 km then read 'over-range' or similar since it cannot divide by zero, a mathematical impossibility.
Last edited:
ZippyNH
Veteran Member
PS
Your readings are NORMAL...
Some days, due to temperature, winds, varrarion in fuel, etc will carry from trip to trip...
So stoked you use more AC, sometimes spend more time behind other cars or trucks, it will vary.
Its best just to look at totals from tank to tank.... Long term averages...Even then it will vary seasonally.
Your readings are NORMAL...
Some days, due to temperature, winds, varrarion in fuel, etc will carry from trip to trip...
So stoked you use more AC, sometimes spend more time behind other cars or trucks, it will vary.
Its best just to look at totals from tank to tank.... Long term averages...Even then it will vary seasonally.
Btravelen
Veteran Member
My 'trip computer' reports roughly 2.4 mpg better mileage on a tank of fuel then the manual calculation, and has since new. Makes me wonder if it was programmed for different sized tires. When I bought it, the changeover from 15's to 16's had just taken place.
pdq import repair
Veteran Member
With VCDS you can tweak the calculation to make it more accurate. I have mine reset pretty close to actual calculated mpg.