Measuring Fuel Economy by Weighing the Car
 

Is this a good methodology for accurately measuring fuel economy? | TopGear.com.ph

Just wondering what everyone thinks.

The background... it was an economy test of around two dozen vehicles (only 19 showed up, but, whatevs). Previous testing had the technical crew spend two days filling the car before the test, just to make sure it was completely topped off.

Due to time constraints, this is what was done, instead.

Sounds stupid. Worked. I think. But still, a lot of questions. :p

Get a new technical committee. The way we do it here is better.

Getting to zero, filling, measuring the fill, then driving, then measuring what's left can be accurate. But every step is at best some added slop and a chance to get something wrong. At worst? A kindergartener could cheat and not get caught.

Better? Fill up, drive, then fill up again with as few variables as possible- same pump, conditions, method, etc. Measure fuel pumped on the refill.

You need a very precise scale and no fluid outflow from any driver or passenger. No adding of any weight during the test cycle (buy a soda or other drink-food).

Under those parameters I would prefer the weighing method.

regards
mech

Get on a Cat Scale and weigh at all four corners to be exact about tire pressures, plus correct any imbalances. Past that?

Scales were motorsports grade, unfortunately, they don't read to decimal places, even though they're accurate to fractions of a pound. I suggested correcting to whole numbers using small weights, but they didn't want to take the extra step... even though I tried to explain how quick and easy it would be.

Nothing in the cabin. No water, no snacks. No AC use, so water wouldn't condense around the AC evaporator, either. If the car got lighter due to evaporation from the radiator, then that's not our problem. :D

The first run was done that way. We needed several fills both pre- and post- to get exact numbers. Something that would be hard to do with two dozen cars.

-

The method I've been most satisfied with was doing multiple fill-ups over the course of the run... then you get the median fill-up, as the first few tend to be off due to issues with the tank.

-

Good if you don't have a line at the pumps. Last eco-run we had, the time spent waiting for the fill-up affected the numbers... which is how two diesel crossovers got 40-50 km/l on countback... because their high pressure fuel rails took a dump back into the tank while they were sitting parked.

-

Personally, I would love to do this with a fuel cell. Or even a plastic bottle or bag like Mythbusters uses. I've seen a run done with a bottle mounted to the hood on a modern diesel... so it's possible, even with a high pressure fuel rail, but that requires a modified hood, so not practical to do with a lot of cars at once.

Trying to calculate fuel consumption by weighing the difference of the entire vehicle is terribly inaccurate. Especially if only a few gallons are used. The Green Grand Prix used to rely on this using 4 race quality scales. Some of the cars actually made fuel.

Now that's interesting! And the kind of anecdotal data that would be useful to know. Have any links?

Also... What kind of run was it, and what were the parameters?

We avoided rain... all the cars were parked in the pit garages before the event, too.

Reading up... seems like a big issue with such small figures... 10 pounds versus 1 gallon is a pretty big level of error.

-

The big problem was the fuel sponsor would not allow refueling at the track... at all, which was maddening. In an ideal world, both methods would be used to audit each other.

So... I guess that leaves removing a washer nozzle, running a hose out to a bottle of fuel suction cupped to the hood, and running until the bottle is dry? :D