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MAX Update No. 84: Speed Kills (Gas Mileage)

11/18/2011 2:49:20 PM

Tags: MAX, 100 mpg, Jack McCornack, Seven Springs 2011, Mother Earth News Fair 2011

In my 7,500-plus mile adventure in MAX this summer, one lesson really stands out: Haste Makes Waste.

To make a long story short, when George Voll and I drove our high mileage cars (that’s George’s car in the background) from his home in Indiana to the MOTHER EARTH NEWS FAIR in Pennsylvania, we drove various legs of the trip at various speeds. By driving in formation and scrutinizing each other’s fill-ups, we got two-heads-are-better-than-one confirmation of our mileage.

The above photo shows my side of the fuel pump, just east of Columbus Ohio, after our 100.4 mile, 45 mph run from Cleveland. I confirmed speed and distance with my GPS, but for most of the trip George’s diesel Metro was the pace car.

We had driven that leg at 45 mph for three reasons; first, George had never driven a hundred miles that slowly and didn’t know what his mileage was down there; second, 45 mph was the minimum speed when the Auto X Prize did its oval track “race” last year and we wanted to measure ourselves against that benchmark, and third, it was raining hard when we left Cleveland and it’s uncomfortable to drive MAX fast in the rain.

MAX and Diesel Geo Metro at Gas Station 

Mercifully it quit raining as we approached Columbus, so I was able to pack up my motorcycle rain suit (it’s in the black Triumph bag sitting on MAX’s “trunk”) and wring the rainwater out of my earplugs (no, I’m not kidding — the rain suit keeps my body toasty but rain blows in between my helmet and my ears, and if there’s lots of rain, my foam ear plugs soak it up). After this ordeal, my rain suit stayed packed and my ear plugs stayed dry for the rest of my trip — to Pennsylvania and all the way back to Oregon afterward. But I’m drifting off subject; I’ll write about rain some other day.

Anyway, back to the fuel pump. George and I looked over each other’s shoulders to verify we couldn’t squeeze another ounce into our tanks, and if you’ll look at the fine print on the pump you’ll see MAX sucked up 0.891 gallons, which works out to 113 miles per gallon. I generally expect 115 mpg at that speed, but it was cold and raining and neither cold nor rain help mileage. George got 94 mpg, which is spectacular for a four seater in any conditions, but I’ll save that for another story. This post is to show that I only have to do one thing to get MAX into the hundred-’teens on the freeway: drive at the low end of the speed limit instead of the high end of the speed limit. No hypermiling, no coasting down hills in neutral, no drafting, no waiting for clear warm days — just driving as slow as the law allows instead of as fast as the law allows.

Now I’ve had better mileage results in the heat of competition and so has George (he won the diesel class at the 2011 Green Grand Prix at 106 mpg) but we’ve also had worse results. In MAX’s case, mileage drops about 10 mpg when speed increases 5 mph, and my biggest challenge on long drives is keeping my speed from creeping up.

Since it’s an easy target to remember (and since people always ask), I shoot for 100 miles per gallon in my travels. 115 mpg at 45, 105 mpg at 50, 95 mpg at 55 miles an hour, it’s simple: I keep my cruising speed between 50 and 55, and if I get less than 100 mpg on a fill-up, I slow to 50 until I’ve got my mileage back up to three digits.

Or at least that’s the theory. In practice, well, if I’m getting 100 miles per gallon at 52-1/2 miles an hour, it’s hard to resist going 55. In the rarified atmosphere of extreme high mileage, reducing mileage by 5% doesn’t feel like it hurts much, and indeed it doesn’t; the difference between 95 mpg and 100 mpg is modest — a gallon every 2000 miles.

But at 60, MAX’s mileage has dropped into the 80s and I’m starting to feel some pain, and by 65 mph (where MAX gets roughly 75 miles per gallon) I’m feeling like a hypocrite, and at 75 miles an hour — yes, I did do a measured two hour blast at 75, gunning across South Dakota — mileage was deep in the 50s and I was giving myself the same pursuit-of-knowledge rationalization that I’d probably use if my job involved testing eye shadow on bunny rabbits.

Clearly I’m getting too emotional about fuel economy…or am I? My obsession with conserving fuel in MAX has taught me good lessons, lessons I can use no matter what I drive. Percentage-wise, my work van’s fuel economy suffers similarly from speed — every extra five miles per hour costs me about two miles per gallon, and two miles per gallon doesn’t sound like it’s worth bothering with — but thanks to MAX’s good influence, I’m driving the van at 55 instead of 65 nowadays, and it does makes difference.

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Jack Leffler
12/9/2011 3:13:49 PM
Although I believe there is a drop in MPG when your speed increase it is something that is not accurately assessed on th open interstate. The highways traveled on this trip vary to much in terrain. And running across South Dakota (East to West) you have a steady uphill climb from the Missouri to the Rockys. This and the normal summer headwinds from the west greatly affect the fuel mileage (trust me, I've driven the route often). All this being said: avoid being a hazard, drive the speed limit. And I'd like to have either of the vehicles for my long trips.

12/8/2011 1:40:21 AM
Jeff (this is in reply to the reply below--I couldn't reply to it directly 'cause this blog program only allows 5 links in the reply to a reply to a reply etc etc chain) re excess power @ 55 mph, with the old body I'd say "yes" and with the new body I'd say "yes indeed", thanks to the streamlining. I'm sure I could shift up into 6th gear if I had one but I wouldn't expect huge mileage gains from it. MAX is geared a bit taller than before (by final drive ratio and by tire diameter) and heavier with the full body so I use 1st gear for all my starts now. And when I'm accelerating from 45 mph, I normally leave it in 5th gear...unless I'm passing or something else where haste is in order; then I'll downshift.

12/8/2011 12:15:38 AM
Jack, thanks for your reply. How does Max "feel" when driving? I recall you writing that first gear was hardly necessary, that you could start in second with no problem. Does max have any excess power at 55 mph? Do you think you could shift up (if Max had a sixth gear)? Is there plenty of power going from 45 mph to 55 mph in fifth gear, or do you need to down shift?

Jack McCornack
12/7/2011 8:20:43 AM
That is a very interesting question! I've never run MAX at its limits for long enough to get a high-end fuel consumption measurement, but if your target was 35 miles per gallon for some reason (for example, mocking the folks who say the future CAFE standards will force us all to drive slow boring slugmobiles) I'd guess 85 to 90 miles per hour at 3400 to 3600 rpm. Will MAX go 90 mph? Yes, Officer, I do know how fast I was going, that's why I have a GPS.

Jack McCornack
12/7/2011 8:01:50 AM
Yes, in my experience fuel consumption is improved by using a higher gear PROVIDED that higher gear doesn't drop engine rpm down where its efficiency losses (caused by extremes of high throttle and low revs) are greater than the gains it gets from its longer stride (distance per rev).

Jack McCornack
12/7/2011 7:52:25 AM
I don't think so either, Jeff. 2200 rpm is probably the most efficient rpm for 20 horsepower, but if all you wanted was 1 horsepower I think you'd want rpm to be just above idle; 1000 to 1200 rpm or thereabouts. As extrapolated from the tests done in MAX Update No. 70, it takes about a quarter of a gallon an hour just to keep the engine running at 2200 rpm; that is, with zero power output, just keeping itself going at that speed. The same engine can idle at 1000 rpm on about half as much fuel (1/8 gallon an hour), so if you want a teeny little fraction of a horsepower, the horsepower you'd get if you added just one tablespoon per hour to the fuel consumption, 1000 rpm is about twice as efficient as 2200 rpm. HOWEVER, if you had a car that had so little drag that it could drive at that teeny fraction of a horsepower and you put that car in top gear, 2200 rpm would be slightly more efficient because it would travel 22 miles for every 10 miles the car traveled at 1000 rpm. That's 2.2 times farther for only twice as much fuel. It's complicated, and gets even moreso when you add the option of shifting gears.

12/7/2011 1:04:50 AM
So, if Max is more aerodynamic now with its new slippery body, then it takes less hp to cruise at 55 mph than it did with the original body. Therefore rpm can go down and mpg up. The increase in mpg due to lower rpm, won't be as much as better aerodynamics. But to optimize mpg, I think rpm has to be addressed. Maybe 2200 rpm is the most efficient rpm for any horsepower requirement, but I don't think so because idle rpm is much lower

12/5/2011 11:54:31 PM
You're right. MPG is more affected by air resisitance (related to cube of velocity) than to engine parameters at typical driving speeds. Bowling's MPG Estimator: Obviously, fuel consumption is improved by using a higher gear: longer distance traveled per rev of engine.

12/5/2011 5:45:17 AM
So what you are saying is that driving at 45 mph, you should shift into 4th gear to get the best mpg? Would that also work at 50 mph or even 55 mph? What would be the best gear/rpm at 35 mpg?

12/2/2011 7:22:21 AM
Thanks George, and Jeff, I think it's all air resistance. At 45 mph in 5th gear, the engine is turning 1800 rpm. According to the Kubota 1105-T spec sheet (you can download it at engine efficiency improves slightly as rpm goes up from there and it's at its highest efficiency at 2200, which is 55 mph. So since the engine apparently isn't losing efficiency and air resistance is increasing at the square of the speed, I'm prepared to blame air resistance.

George Voll
12/1/2011 6:00:23 PM
thanks for the update Jack.We really enjoyed having you over, hope we will see you again.

jeff dean
11/25/2011 4:35:50 PM
Jack, Looks great. How much mpg loss is due to air resistance and how much to higher rpm? Maybe you could test by driving 45 in high gear, and 45 in next gear down. Results would be interesting.

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