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Old 01-19-2009, 12:54 PM   #21 (permalink)
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I used data gathered by MetroMPG on the Blackfly and the 06 Corolla to find the optimum speed to drive each car, based on the premise that saving gas is good, getting to your destination sooner is good, and depending on how much you value your time and your fuel, there is a correct balance. So I assigned a dollar value to time and to gas.

I pay something like $2/gal, but I feel obligated to save gas like it was much more expensive. I value it at $6/gal. Getting there sooner also matters to me, to the tune of about $12/hr.

I plotted the cost per mile for each of MetroMPG's data points, and I had Excel fit a parabola to the result. The optimum speed to drive, considering only time and fuel consumption, is the speed where the cost per mile is the lowest. For me, that would occur around 67mph in the Corolla, and around 62mph in the Blackfly.

However, suppose I was running late for work and I'd be willing to pay $.50 for each minute earlier I arrive, but I still care to conserve gas to the tune of $6/gal. Under these conditions, I'd drive the Blackfly at 68mph, or the Corolla at 78mph.

This is very much back-of-the-napkin. A better optimization problem, like the one that runs in my head whenever I'm behind the wheel, would take in to account that some speeds are not as safe as others (whether too slow or too fast), cars probably wear out faster at higher speeds, and that it's fun to post large MPG numbers. Also, these data only apply to steady-state cruising in economy cars on flat roads with the A/C off, in top gear.

Bottom line: drive faster to save time. Drive slower to save gas. For me, the balance appears to be around 65mph.

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Old 01-22-2009, 10:01 AM   #22 (permalink)
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Quote:
Originally Posted by RobertSmalls View Post
Where can I get raw data for one of these cars in the high-speed runs?
I don't think the raw data is available. The chart came from here: Green Car Congress: Fuel Consumption at Higher Speeds
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Old 01-22-2009, 11:43 AM   #23 (permalink)
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Quote:
Originally Posted by MetroMPG View Post

I'm new here but is this graph saying at 70kph the vehicle gets 51.x9 MPG? And at 40kph it gets 57.2?

I own an 05 Corolla CE Automatic and it gets nowhere near these figures at those speeds... you stated you borrowed the vehicle so I would assume that means its stock (unmodified). I'm confused how you arrived at these MPG ratings, unless I'm just totally missing something here.
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Old 01-22-2009, 11:54 AM   #24 (permalink)
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Hi Milkman - yes, the figures posted are correct and the car is unmodified.

But note the test conditions the readings came from, in post #9. FYI, you can't compare real world (round trip, cold start etc.) mileage to these figures. They're simply "constant speed" numbers for a warmed up car under pretty ideal conditions.

Have you done a similar test (level road, cruise control, no traffic etc) and come up with different numbers?
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Old 05-10-2016, 08:23 AM   #25 (permalink)
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This is fascinating to me. I have a 2004 Matrix XR, FWD, Automatic. I had assumed that my best fuel economy would come in my highest gear, after torque lock-up, at the lowest possible RPM--this equated to about 37 mph. Then, empirically (without a scan-gauge), I found that a slightly higher speed than the lowest possible RPM speed in this condition produced a better result--45 mph or so yielded better fuel economy. Your test results concur with my rough finding. What is astonishing about your result is that 24.9 mph--which must occur prior to torque lock-up--is an even better efficiency speed! Remarkable. Please tell me if I'm interpreting your results incorrectly. I know have 25 mph and 45 mph marked in my mind as my efficiency target speeds. Thanks for the work. Steve
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Old 05-10-2016, 10:04 AM   #26 (permalink)
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Hi, Steve

Quote:
Originally Posted by Manualhybrid View Post
empirically (without a scan-gauge), I found that a slightly higher speed than the lowest possible RPM speed in this condition produced a better result--45 mph or so yielded better fuel economy.
I wonder if what you saw is the difference between optimal (flat road, constant speed) testing conditions vs. "real world" driving where it sometimes makes more sense to cruise a bit faster than the lowest speed at which top gear/lockup engages just to avoid automatic downshifts on hills, slight acceleration, etc.

Quote:
24.9 mph--which must occur prior to torque lock-up--is an even better efficiency speed!
I've seen this in a few vehicles -- but only ones with automatic transmissions. Even though it isn't in top gear, the aero load is reduced enough at 40 km/h / 25 mph that it comes out ahead. If you could manually shift up to the next gear, the "dip" seen in the next 2 data points would disappear.

Darin
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Old 05-10-2016, 05:47 PM   #27 (permalink)
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Yup, I see that same phenomenon in my Echo, too.

I'm confused though, it sounds as though you were saying it shifted into 4th AND lockup at the same time? That doesn't sound right. Should hit 4th at around 30ish and lockup at 37-38mph.
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Old 05-11-2016, 11:02 AM   #28 (permalink)
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You're right. It didn't change to 4th and lockup at the same speed.
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Old 05-11-2016, 10:00 PM   #29 (permalink)
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Cool. Did some digging and the transmissions are nearly identical for my Echo and the Corolla, the difference being final drive. Both are of the U3xx family.

Corollas and Echos are remarkably similar in their FE performance, nod to toe Corolla for carrying extra weight and displacement. And both positively rock with stick. Neat stuff.

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