With all the talk of MPG and gas prices are currently on the rise. I was going to ask the community how do they calculate the gas millage manually instead of using what is listed on the computer in the vehicles. I then turned to google, which directed me to a U.S. Govt website, http://www.fueleconomy.gov/mpg/MPG.do?action=calcMPG.

Odometer Method

MPG can be calculated in four easy steps:
Step 1. Filling the vehicle's gas tank completely and writing down the vehicle's odometer reading (mileage).

Step 2. When it's time to refuel, filling the tank completely and writing down the number of gallons it took to fill the tank and the vehicle's new odometer reading. Once two odometer readings are taken, MPG can be calculated.

Step 3. Calculating the distance driven by subtracting the previous odometer reading from the new one.

Step 4. Dividing the number of miles driven by the number of gallons it took to fill the tank. The result is the vehicle's MPG for that driving period.



Alternate Method

If you use your vehicle's trip odometer, MPG can be calculated by:
Step 1. Filling the vehicle's gas tank completely and re-setting the trip odometer.

Step 2. When it's time to re-fuel

• Filling the tank completely
• Writing down the number of gallons it took to fill the tank
• Writing down the mileage on the trip odometer
• Re-setting the trip odometer

Step 3. Dividing the number of miles driven by the number of gallons it took to fill the tank. The result is the vehicle's MPG for that driving period.


Question for the community, do you use either of these methods if not how do you calculate it manually. Also how often do you do it?

 

On my RX330 I use your Alternate Method. Note that there are two trip readings, A and B, on the odometer display -- at least on the RX330. Normally, I reset both when I fill up the tank. On long trips I will keep one as a running total and reset the other at each fill up. BTW, if you print a receipt at the gas pump it will always print how many gallons were dispensed along with the price per gallon.

I think that you will find either method you listed is technically correct. Pick the one that works best for you. Just do not overfill the gas tank.

 

Same on the 400h, for the records.

Worth mentioning also that neither methods give a 100% accurate mileage, because they are both based on the odometer reading, as it is the onboard computer, of course.

To get a more accurate number, you should measure the odometer error with a handheld GPS, and consider that error in the calculation.
I didn't do that (yet) on my RX, but I've yet to find a car where 100 miles on the odometer are exactly 100 miles of road.

 

thats depends if your hand held gps is measuring point to point straight line or route taken.... if you drove a 100 mile circle your gps would say you went zero miles if in point to point mode.

presuming you don't have some bizzare tires on your car, odometers these days are inside of 1 decimal point accurate minus turning errors. and as we say... if you are figuring MILES per gallon... the dif between 22 mpg and 22.49754 isn't what anyone is asking.

 

That's not my experience, though I must say that I never checked that on any US car.
But with most European cars (Audi, Alfa Romeo, M-B, BMW, Porsche, Volvo, Maserati, etc.) I've always seen from a 2 or 3% difference, up to 6 or even 8. And mostly with stock tires.

Otoh, I take your point on GPS not necessarily following the exact path of the car, but if you set any decent GPS in tracking mode and with a high "resolution" (be it in terms of distance or time), the error is really negligible. Also because whenever I made these checks, it was always on pretty straight highways.

 

Don't forget to factor in adjustments for tire wear and its affect on the number of rotations per mile from new to needing to be replaced and thus the accuracy of the odometer.

Any accuracy beyond one decimal place and you have to be kidding yourself. Besides, what is the purpose of regularly checking a vehicle's MPG? Does anything more than one decimal point accuracy even matter? No method is high precision.

 

has anyone ever checked the accuracy of the onboard system ? it uses the same fuel volume transducers that i used on the race cars i believe so it it knows pretty precisely how much fuel actually goes into the motor. add to that the sophistication of the electronics monitoring the gbox and i wouldn't be surprised at all if it was very very close. i would be curious to know what it can actually do.

 

Every tank.

I also use this method on all cars as it's very easy. Jot down the trip odometer reading on the receipt and reset at fill-up. The receipt will already have gallons filled. If you're obsessed about accuracy consider that the accuracy of the particular pump is unknown and can vary.

I guess the importance is subjective. The accuracy of one tank isn't all that important to me as much as the trends and changes across multiple tanks.

Haven't checked into which is more accurate but it hasn't been uncommon for my calculations to be up to 2 MPG less. I guess for the RX 400h I also jot down the trip computer's MPG reading and hit reset on the info screen as well.

 

well thats about 10 % error... more than i would have guessed. interesting. but the comment about trends is correct. single data samples are not going to tell you whats really going on.

 

This is what it says in the Owner's Manual:

(3) Average fuel consumption after refueling (“AVG MPG” or “AVG L/100 km”)
Average fuel consumption after refueling is calculated and displayed based on total driving distance and total fuel consumption with the engine running.
The displayed value is updated every 10 seconds. The calculation is reset when the vehicle starts after refueling.

However, I have never seen any indication that the system truly resets itself and as indicated above it always seems to be 1-2 MPG off for whatever reason. Perhaps, also as noted, it is correct and the odometer is actually off causing the difference. Still, it is the trends we are looking for and as long as you are consistent in whatever method you choose you should be fine.

Do you think Heisenberg drove a Lexus?

 

almost uncertainly

 

I've also read that in the owners manual. After I top up, the info value spins like the wheel on price is right. Starts at zero, then goes high then goes low and high again then will eventually stop at a number I don't like so I don't make the bonus round. lol.

That's the reason I started looking into doing it manually since reading so many people here in the forum write about it.

I do mainly city driving with a lot of red lights, red light cameras, quota seeking law enforcement and now with more trucking companies and vans installing gps in their vehicles to micro manage employees in the field, and can't forget bike lanes, traffic in the city has become a nightmare especially during rush hours.

 

I use the alternate method to check mpg. When I fill up the tank, I divide the mileage in trip "A" by the number of gallons taken to fill the tank. I also use an app on the iphone called "AccuFuel" to help me keep track of how often I refuel and I get a nice pattern of what my mpg usually are, it also shows me a graph of avg mpg for the entire life of the car.

I use trip "B" on the odometer to tell when I need to change the oil. I like to change my oil every 5,000 miles with full synthetic. When I get an oil change I reset trip "B" and once it reaches 5,000 again I know its time for a new change.