Mythbusters tests "golf ball" dimples

[MetroMPG]

That was pretty darned entertaining!

FYI, you can view the video here: http://www.megavideo.com/?v=PMB935S8

When I saw the first test (dirt vs clean), I was sorely disappointed by their fuel measurement technique: note the fuel level in the sloshing tube by eye and mark with magnetic arrows!

I was relieved to see them ditch that method for the smooth vs. dimpled tests and opt for weighing the fuel cell each run.

Originally Posted by botsapper

The now famous 'dimples' car should be retested w/ ecomodders' test suggestions.... Any other suggestions?...

Mine are mostly about the testing methods, either things I saw and took issue with, or things they didn't show which leave big questions.

1) They need to do A-B-A. Return the car to its original state to see if the observed effect disappears. Admittedly not easy to put the dimple material back in and smooth it again, but they should have tried.

2) It seemed to me they were relying on Jamie to hold the car's speed. That's a no-no! Need to remove the human foot from the most sensitive input in the experiment - the gas pedal. They should have used cruise control or some other speed control.

3) It's fair to assume weather conditions (temp, particularly) would have changed between the two tests - it would have taken some time to carve out those 1082 dimples. They didn't take that into account. (We don't even know for sure if they did it on the same day due to the magic of editing.)

4) We don't know whether the car was equally warmed up for all tests. (Equal engine coolant temp doesn't count.) We're talking full drivetrain temps, including tires.

Their results are also really big (% improvement), which ratchets up the skepticism a bit more. They claimed 26 mpg (US) for smooth vs. 29.65 for dimpled. A 14% improvement! Yikes.

Has anyone double checked their calcs?

smooth clay:

10056 g to 9553 g gasoline
or 503 g burned
26 mpg (US) their figures

dimpled clay:

9450 g to 8972 g gasoline
or 478 g burned
29.65 mpg (US) their figures

Gasoline weighs 6.073 lbs per US gallon (says wikipedia). 1 pound = 453.59237 grams, so 1 gallon = 2754.7 grams.

smooth: 5 mi / 503 grams = 5 mi / 0.182597 gal = 27.38 mpg (US)

dimpled: 5 mi / 478 grams = 5 mi / = 0.173522 gal = 28.81 mpg (US)

That's a 5.2% improvement by my calcs.

So their calcs seem fishy too. Unless I just screwed it up. (Plausible!)

[Old Tele man]

...hm-m-m-m, wonder what the temperature of the gasoline was at each test intervals

...cold at the start, cool by pre-noon, warm by noonish, hot by end of day?

...temperature will affect weight by volume readings.

[pgfpro]

smooth clay:

10056 g to 9553 g gasoline
or 503 g burned
26 mpg (US) their figures

dimpled clay:

9450 g to 8972 g gasoline
or 478 g burned
29.65 mpg (US) their figures

Gasoline weighs 6.073 lbs per US gallon (says wikipedia). 1 pound = 453.59237 grams, so 1 gallon = 2754.7 grams.

smooth: 5 mi / 503 grams = 5 mi / 0.182597 gal = 27.38 mpg (US)

dimpled: 5 mi / 478 grams = 5 mi / = 0.173522 gal = 28.81 mpg (US)

So their calcs seem fishy too. Unless I just screwed it up. (Plausible!)

I pick up on this also but didn't want to write anything because I thought I might have heard them wrong.

[MetroMPG]

I'm going to save any further comments of my own for the other thread on the same subject because it seems to have had more people's analysis in it... I didn't check it before I replied here.

Too late to merge the threads now - it'd make a mess.

Other thread:

Mythbusters "tests" efficiency of dimples vs. smooth, and clean vs. muddy car