Project: Geo Metro boat tail prototype - 15% MPG improvement @ 90 kph / 56 mph

[3-Wheeler]

Originally Posted by Nerys

ahh so Cr is coefficient of rolling resistance or something like that?

Hi Nerys,

Yes, that is correct. Crr stands for Coefficient of Rolling Resistance.

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Note that Crr does not simply imply tires, but all the moving components in the drive train and so on.

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As our vehicles are optimized in aero shape, the Crr will stand out more.

I expect more manufacturers to offer low Low Rolling Resistance (LRR) tires.

Same thing about motor oils; they have been getting thinner in newer vehicle releases.

Jim.

[Christ]

You also have to remember that the difference noted between 45mph and 55mph is ONLY at those speeds.

It doesn't account for speeding up, slowing down, etc. It's a steady state test, designed to find the difference between two sets of data. That's all.

If you're trying to measure the same difference on your daily commute, you're going to fail at it, and that's all there is to it.

The main point is that there is/was a 15% difference between 45MPH steady state, and 55MPH steady state, using the SGII or other instrumentation to determine instant economy data.

[Nerys]

I do not doubt the data. but part of good science is being able to reproduce data which is what I tried to do.

steady state or not 15% improvement "should have" shown up as more than a 1 or 2% difference in 400miles of driving where each leg is 54miles 1:20 minutes (basically that times 6)

if even 50% of my trip is relatively steady state (its much more than that but I don't have hard numbers) that means I should see at least a 7-8% difference.

then again It could also be my car. I might have something failing thats dragging me down (lately I have been maxing at 40-42mpg but its also been REALLY cold and really windy the last few weeks which might explain that.) so I have not tried to do it again till I have more consistent figures. because I damned well would slow down 10mph if it meant I could gain even 5% FE improvement :-)

[Christ]

Originally Posted by Nerys

I do not doubt the data. but part of good science is being able to reproduce data which is what I tried to do.

steady state or not 15% improvement "should have" shown up as more than a 1 or 2% difference in 400miles of driving where each leg is 54miles 1:20 minutes (basically that times 6)

if even 50% of my trip is relatively steady state (its much more than that but I don't have hard numbers) that means I should see at least a 7-8% difference.

then again It could also be my car. I might have something failing thats dragging me down (lately I have been maxing at 40-42mpg but its also been REALLY cold and really windy the last few weeks which might explain that.) so I have not tried to do it again till I have more consistent figures. because I damned well would slow down 10mph if it meant I could gain even 5% FE improvement :-)

That statement isn't true, because there are still times where you're accelerating as part of your measurement. If you're not using instant data, you're not getting a proper indication of the difference, and you can't assume that because you're doing the right thing for 50% of the time, that you'll actually see 50% of the result.

Also, you have too many other variables to accurately reproduce the test. Part of science is indeed the ability to reproduce the results, and that means using the same parameters, as well.

Darin's car is much more optimized, as is his driving style, than yours, so there is likely a bunch of unaccounted for variables messing with your data, including the likelihood of cross winds in multiple directions, changes in road surface, and speed variability over such a long distance.

[MetroMPG]

Christ: thanks for pointing that out.

Nerys: that's the problem trying to compare controlled-as-possible (test) & uncontrolled (commute) data. Apples and oranges.

If you want to do a meaningful comparison of only your commute results, you have to log a very high number of data points, like palemelanesian has done.

Originally Posted by 3-Wheeler

Not to belabor the point, but Metro would certainly want to do his mileage test on an almost calm day to make deciphering the data easier.

Yes. That's always a goal.

In the ~2 hours it took to do the A-B-A boat tail test, the wind speed varied between calm and 2 km/h according to an online federal weather station a couple of km away.

[Christ]

Darin - I only try to help.

[MetroMPG]

Originally Posted by Nerys

I damned well would slow down 10mph if it meant I could gain even 5% FE improvement :-)

In comparable conditions, you absolutely would. You need instrumentation in your car. (An MPGuino would work.)

[Nerys]

No that statement is true. why would you reply telling me my statement is not true without any data to support that.

I posited that over 50% of my trip IS AT SPEED logically (ie my brain works) I deducted all time spent from decel to accel back to speed again. (maybe on the way home tonight I will put a clock to it my guess is 60-70% of the trip is relatively "steady state"

Data points. I have driven this route in excess of 450,000 miles over the last 15 years. I would say thats a pretty decent set of data points to work with :-)

I am not saying I am right. I am just saying my INFORMAL logging seems to contradict what is being posted (about the 10mph speed difference NOT the improvements gained with the tail)

This caused me to become curious. My issue is NOT who is right. my issue is WHY does my results not "jive" with the math.

that is all. it could mean the math is wrong (not likely) though it could be there is more data involved that the math we are using suggests (such as the addition of CRR)

it could be my specific commute. It could be MY CAR.

the last one being the issue. IF ITS MY CAR. I want to know about it I want to know why and I want to fix it.

hence the questions. trying to figure out how the data works so I have some way to attack the problem.

I can not really test anything right now. way to great a variation in temps and winds. Went from 30+mph gusting the last week to almost NO WIND last night and this morning and I can already see a visible difference in FE just by visual indication on the fuel gauge! So that leaves me with chatting about it and figuring out what I need to do in the mean time.

[Christ]

I said it's incorrect because even though the majority of your route was steady state, your FE reading would still incur hits from acceleration and deceleration, because you're calculating from tank to tank.

You, of all people, should know that a single tank is never enough to accurately decide whether something was an improvement or not, especially when tank to tank is your only testing method.

If you had some real time data, that you could read as you're driving, it would be a completely different scenario.

For posterity, I'm not going to argue the point. The information is on this site, as well as at least 3 others that I know of. If you're that interested in the data, you'll look it up.

[Christ]

Darin -

Any further iterations of the boat tail on your mind for future (read: Spring) testing?