Measuring Fuel Economy by Weighing the Car

[BrandonMods]

I think this would also be an issue for people who drive in areas where it snows a lot. I'm curious sometimes to how much mass my car picks up in slush and snow just on the drive to work each day.

[sendler]

Well this is only something that would be done in competition or one time test and is not even any good for that.

[euromodder]

It's rather ridiculous to measure a small weight difference by measuring 2 heavy objects then subtracting one weight from the other.

[niky]

True enough. But once you control for extraneous weight, it works... but with limitations.

Best really is to use fuel cells and weigh those, but that's a difficult ask with cars you can't modify.

Originally Posted by sendler

Well this is only something that would be done in competition or one time test and is not even any good for that.

I have a few questions... was the pre-comp weight taken the day before or right before each run?

Perhaps it was mentioned later in the thread, but I didn't catch it.

[sendler]

Why even consider this?

[Fat Charlie]

Originally Posted by niky

The big problem was the fuel sponsor would not allow refueling at the track... at all, which was maddening.

Originally Posted by niky

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

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

If a stupid plan is the best you're allowed to use, then that's the plan you use.

[niky]

Originally Posted by sendler

Why even consider this?

As said, and as cited by Charlie, we were prohibited from filling on-site by the sponsor. (the other year's sponsor allowed it, though)

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All methods are inaccurate. What's important here is deciphering to what degree they are inaccurate, and why.

I've seen some wonky fill-ups in my time (and yes, the gas-bladder Prius was one of them).

I've seen cars take a gallon over "full" on fill-up, another quart or two after sitting for an hour, and then another half gallon the next day.

I've seen cars magically get a bonus of nearly a gallon at the pumps, even with all the controls, simply because they sat in line too long waiting for their turn to fill. And this is at an event organized by an old hand, who knew all the tricks and cheats. His next event closed some of those loopholes.

By and large, if done right, a fill-up can be accurate down to half-a-liter. But when you're measuring road cars, there are always jokers. If you've ever seen the obscenely complicated assembly that is a Honda Fit tank and filler tube...

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We all know weight is inaccurate, but the question is why and how bad can it be?

If the problems at the race were due to rain, we can (and did) control for that.

If they were due to moisture condensing in the car due to AC use and sweaty drivers, we can control for that.

If they were due to overnight condensation, we can try to control for that.

If the natural variation in weight due to ambient humidity at the event (sans rain), or the natural loss of rubber, or even dust collecting in the sills over 100 km... if any of those is big enough to bugger the scales, then we can scrap the idea, altogether.

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My personal preference is to go with manual fill-ups, with the car on crocodile jacks to eliminate most of the air bubbles, audited by weight, both times, using weights to round off the weight to the nearest half-pound.

I'd need evidence to show that this is necessary... and to call for re-validation of the method. (I didn't do the validation tests, and I have issues with them)

[sendler]

In an event where the best competitors only use 1 gallonUS/ 6.16 pounds of gas, an error on the scales of only 1 pound out of a 3,000 pound car makes a difference of 16% calculated fuel consumption. Weighing the whole car to determine how much fuel was used in a three hour event doesn't work.