Best Tank Ever

July 7th, 2008 by Potato

With gas prices so high, there’s even more reasons to try to get the most fuel economy out of your car. Tonight I had my best tank ever, 6.36 L/100 km (37 MPG). I was driving back to London and was in no particular hurry, so I “drafted” (note that I always leave at least 3, usually 4 seconds of reaction time/distance between me and a truck, so I’m not really close enough for drafting to have much more than a few percent boost — though every bit counts, and the trucks help set a “pace”) a truck up to about Guelph around 105-110 km/h. Shortly after that I got stuck in a bit of a construction backlog as the 401 went down to one lane, so there was a fair bit of cruising at 60 km/h. The second half of the trip I decided to just set the cruise control at 95 km/h and blast the stereo, but not the A/C. It was just cool enough out to get by with just the regular fan. My car is rated for 10.2/8.1 L/100 km (23/29 MPG), so to beat that is pretty good. My overall average, which you can see on my GreenHybrid database page, is 8.5 L/100 km — damned winter & city driving pulling the average up (where up is bad). If I can keep getting that kind of mileage on my London-Toronto runs, I’d save $11.80 for each round trip compared to my average mileage… though the slower driving would cost me about 36 minutes. With gas prices where they are, I think it’s probably a good return on my time.

So the constant, moderate speed on a pretty flat drive really helps contribute to getting the most out of the car. These days you can’t read 3 webpages without tripping across hypermiling tips, so I won’t get into any of the obvious ones, except to say that I keep my tires slightly hard at 40 PSI. The tips often say to keep your tires “properly inflated”, but the ideal pressure is usually some compromise between the figure given by your car manufacturer on the door jamb — in my case, 32 PSI, which is a compromise value between ride comfort and fuel economy — and the max sidewall rating of the tire, 51 PSI for my Nokian WRs.

Perhaps not too strangely, the fastest two vehicles I saw tonight were both SUVs, both tailgating and doing about 140 when they got an open patch of road. The cost of gas is nothing when you have a giant truck!

5 Responses to “Best Tank Ever”

  1. Wayfare Says:

    Oh dear god you have a mileage database.

  2. Potato Says:

    What’s so surprising about that? What did you think I was doing with all those gas receipts where I wrote the distance travelled for each fillup?

  3. Ben Says:

    I’d been meaning to ask you about hypermiling since I figured you’d know something about it. I was curious about the drafting since there are always tons of tuck on the QEW on my trips to and from Toronto. But I was never sure exactly how close I needed to follow them in order to get any benefit from it. Can you convert your 3-4 second reaction time into car lengths?

  4. Potato Says:

    Sure! To start with, you get a bigger benefit being closer to the truck: hanging just a few feet off its bumper can bump your fuel economy up by 25-50%! However, that is asking for death. It drops off as you go further back, to about 1 second behind, where it’s theorized that there might be some sort of threshold or turbulence issue. There you go from about a 10% benefit down to something barely measurable (2-3%) but that small benefit extends a fair bit behind the truck. A minimum safe following distance in general is 2 seconds: just enough time to see something, react to it, and hit the brakes. With a truck, since you can’t see as much around them as when following a car, and since they can kick crap up at you (like exploded tire fragments), a longer following distance is recommended. One of the better mythbusters episodes tested the benefits of drafting, which is where I’m getting my close-in figures from — I haven’t verified anything closer than 2 seconds experimentally myself!

    So, the math: 100 km/h is 100/60/60 km/s, or 27.8 m/s. So 3 seconds at 27.8 m/s is 83.4 m, or approximately 18 car lengths. At 120 km/h, a 3 second following distance would be 100 m or 22 car lengths. Personally, I like using seconds, because you need about the same following distance in seconds no matter what speed you’re travelling — your reaction time and safety factor is the same. Plus it can sometimes be tough to judge distance behind the wheel, but it’s not too tough to wait until the car in front of you passes say a sign post and then count 2, 3, or 4 seconds out until you pass that same signpost. And of course, it’s the method I learned when I was a young driver with CAA, so it’s what I stick with.

    Drafting from that kind of distance provides very minor benefits in terms of your reduced air drag. However, trucks do tend to set a pretty decent pace in the right lane. If you want to drive slow, like the speed limit for instance, that can annoy a lot of drivers behind you, because other drivers are largely douches. They can get into very bizzare and aggressive behaviours, like tailgating you, flashing their highbeams, etc, and I just don’t need that when I’m on the highway. However, if you’re following a slow truck, psychologically they tend to blame the truck and just pass you without all the drama… unless they’re the weird ladies in SUVs on cellphones who I see run up to tailgate someone, even when the passing lanes are open. If I move over to the fast lane, they’ll buzz by me in the right lane and then tailgate the next person. Very weird.

  5. Ben Says:

    I’ve been driving 100 in the right lane for a while now. I realized back in 1st year when I was traveling back and forth to Toronto weekly that going 120km/h would eat up about 3/4 tank on a round-trip, and I’d save about 15 minutes each way. At 100km/h I could do a round-trip with 1/2 a tank, so the savings were fairly noticable and substantial and the extra 15 minutes each way wasn’t a big deal.

    So far no one’s high-beamed me, I do get tailgaters from time to time, but then I just slow down to about 90 and eventually they figure out “Oh yeah, this is the slow lane, duh!” and go around me. It’s a little more frightening when it’s an 18-wheeler tailgating you… I think I’ll try drafting on my way back to TO tomorrow night and see what happens.