Thursday, May 01, 2008

Hypermiling - If you do it, I swear to God I'll smack you

This article greeted me this morning in my round-up of the web. Interesting read. Interesting ideas. Only one of the ideas is a moderately good one: that of over-inflating your tires. The other ideas are REALLY REALLY BAD.
Among the other ideas were running low-viscosity oil, "drafting", and a curious technique that must be intended solely for hybrids called "pulse-and-glide".
I've come across others in the past, like accelerating to speed, briefly lifting, and then holding the accelerator very lightly, which supposedly leans out your fuel mixture. This, if true, would reduce the amount of fuel burned. But a lean fuel mixture runs much hotter than a stoichiometric (~12:1 air:gasoline) mixture, so engine-temps skyrocket. In front-rear mounted engine designs, the rear-most cylinder takes a beating from this type of driving, and over time that cylinder will lose compression, which means your engine will ultimately need expensive service. The lean mixture technique thus falls into the VERY BAD IDEA category.
Low-viscosity oil dovetails into the whole lean-mixture argument. As engine-mileage rises, greater tolerances are introduced. There's fatigue and scoring in the metals. Things wear down. Higher-viscosity oils tend to stay where they're supposed to, and help shore up weaknesses in older engines. They are highly recommended in higher-mileage trucks. Mine, for instance, is ticking. I've done a bunch of research on the issue, and most of the folks who've experienced mild engine trouble in high mileage trucks recommend switching from 5W30 to 10W30.
I have a coworker with a 200K-mile Toyota Avalon. He switched from 5W30 to 0W30 and picked up 2mpg. But at 200K miles, that motor is tired. It's never been over-bored or sleeved, so he's begging for blow-by, a condition where pressurized oil under the cylinders seeps (or sometimes blasts) up over the cylinder, filling the chamber and hampering combustion. This makes those beautiful blue-smoke clouds you see behind old Chevy Caprices and Novas.
Drafting is illegal. Period. It's stupid and dangerous, and as a result, it's illegal. Don't do it. If your fuel economy improves by 100% but your potential for a life-threatening accident increase by 1000%, you lose. Plain and simple.
Over-inflating tires is something I've experimented with. I've had marginal results, generally picking up a mile or two per gallon, but nothing amazing. There's a danger here, too: abnormal tire-wear. Over-inflating your tires can quickly lead to bald-spots on your tires, or if you inflate them to the maximum, you can wind up with bulges on the tire, which indicate that a moderate bump could explode the tire. I generally inflate to 10psi over the recommended cold pressure. The ride is a bit stiffer than normal, but that doesn't bother me too much, and the wear-patterns don't change.
But consider the "savings" of inflating to maximum versus the cost: if you inflate to save 2mpg, but run off 20K worth of potential mileage from your 50K-mile rated tires, then assume you'll need tires every 30K miles.
If your car generally gets 30mpg, then over 30K miles you're putting 1000 gallons in the tank. That's roughly $3,490 in today's prices. Improving that by 2mpg saves 62.5 gallons, so you'd spend $3271.88 on fuel. That's $218 in savings, or about half the price of a set of tires. Not worth it.
You'd have to gain a real improvement of 5mpg before you'd actually save money, and even then you'd have to keep your tires in good condition for at least 50% of their intended life-span. That's not going to happen.
Re-run the numbers with el-cheapo Pep Boys 80K-mile rated tires: in 60K miles, you'd put 2000 gallons in the tank at 30mpg: $6980. You'd put 1875 in at 32mpg, again using $3.49 as a standard gallon, you'd spend $6543.75. That's a fuel-savings of $436.25 (this number actually surprised me!). This time you come out about $100 ahead, but only if fuel prices remain constant. And it will have taken 3 - 5 years to have saved that $100.
Meanwhile, you can save $600 - $1800 / year by getting rid of cable TV. 4 times the savings in 1/5 the time!
I watch my fuel-economy on every single tank of gas that goes into the MINI. The on-board computer tells me that I generally get about 31.3 mpg. Quick back-of-the-napkin calculations tell me that the OBC is not always right: last night I got 10.008 gallons and had gone 330.1 miles. The OBC said 31.3, but it doesn't take a genius to realize that 330/10 = 33mpg. That's combined city & highway, and the car's rated 28/37 (32 combined)*, so it's right in line with what it should be.
A month ago, Amanda and I drove the MINI about 150 miles to and from the Northern Neck of VA, and keeping a constant speed of 64mph (engine turning at 3000rpm, a happy and fuel-efficient speed for the MINI) the OBC reported 39.8mpg average. I didn't get to finish that tank on highway driving, and it came down considerably, but at 39.8 (if accurate), I could have gone 517 miles. I suspect the economy was actually closer to 42mpg. Not bad. And the tires weren't even over-inflated.

*These numbers are based on the old EPA estimation method. Based on the new EPA ratings, the car is actually rated 25/33 (28 combined).

Edit: Evelyn didn't understand it, and I got distracted, so my conclusion was forgotten. Hypermiling is retarded and dangerous. It introduces unnecessary risk to your vehicle for the possibility of saving a few hundred dollars on gas. Those savings are lost in the replacement of other consumables and potentially monumental repair bills (and/or hospital bills). Just drive reasonably, stay in the slow-lane, take the more efficient vehicle if you have more than one and can choose, and do not, for the love of God, run fuel in your tank that's not intended for your motor. More tips:

1. Have a steep driveway? Back out with the car off and in Neutral.
2. Don't start the car until you're buckled in and ready to drive away.
3. Here's another surprise: accelerate quickly but reasonably to speed and then maintain speed. You'll burn more fuel accelerating, but for far less time than by driving like a grandma.
4. Use your cruise control; it's more stable than your foot.
5. Coast down hills.
6. Keep your speeds up on exit ramps. You'll save fuel by not having to accelerate into traffic, and it's fun!


Ev said...

I understood not one iota of this! Do I want to understand it - not so much. This is why you have your car friends!!!
Glad you found something to amuse you? intrigue you? bother you? I'm not even sure if you liked what you read!

amy =) said...

ok, i got it the first time, but thanks for the extra tips. and i completely agree with what you said, it is stupid, much like people drive severals miles (of God forbid, half way across town) to get cheaper gas. you know, gas that is 2 cents cheaper per gallon. how dumb!

tmarchut said...

OK, but you forgot a few not so dumb, not particularly dangerous techniques that easily add 10-20% to my mileage.

1. Turn off the car at stop lights. With fuel-injected engines, you will save gas if you are stopped for more than 10 seconds.

2. Park and walk into McDonald's, the bank, Starbuck's, etc, rather than sitting in the drive-through.

3. Park toward the entrance to the parking lot, rather than driving around, hunting for the space closest to the door.

4. Coast in neutral to stop signs and red lights, rather than driving full-speed until the last second and then using the brakes to slow the car to a stop.

5. Plan trips to Target and the gorcery store, etc, so that you take fewer trips and call a friend to see if they want to tag along, saving them a trip. Of course, this last one saves them gas, but next time, you can ride with them.

ahamos said...


Turning off your car at stoplights is not a recipe for success. You might burn more gas idling, but it's not a 10-second equation. Your car is extremely inefficient at starting, and in the USA, we don't get the industrial starters that our European friends have. Ours fail after a few thousand starts. That might sound like plenty of starts, but if you drive every day, thats a minimum of 730 starts in a year.

You're also placing undue strain on the battery and charging system.

Again I return to this notion that saving fuel isn't worth trashing the rest of the car.

Calculate the cost of a new starter, battery, flywheel, and possibly even alternator against the fuel saved.