This is a subject I've been giving a lot of thought lately. Motorcycles are known for getting great has mileage — in the 50-60mpg range for the larger 600cc bikes, and up closer to 80mpg or more for the smaller 250cc bikes. So I was surprised to hear people on forums talking about getting 12-15mpg out of their 600cc ATVs. I figured those posts must have been exceptions, but I've kept track of my own gas mileage on my ATV, and I've been averaging about 13.5 mpg. That's a lot worse than even my Buick, which averages 22, much less the Civic with its comfortable 31 mpg.

As I thought about it, I realized that it makes sense that off-roading would be inherently less efficient than rolling on a surface that someone has carefully prepared for you in advance — you don't spend a lot of time above 15mph on a trail, and you're using a lot more power than an on-road vehicle would need to simply maintain 70mph down the highway. I don't know what kind of mileage jeeps and the other "full-size" offroad vehicles get while driving trails, but I'd imagine it's even worse than I get. When you take that into consideration, I guess it's surprising that I can get close to the mileage that an SUV gets on the highway, even while I'm crawling over rocks. But still, that doesn't change the fact that you're using a whole lot of gas per mile.

But I realized that that's not the right way to look at it, at least in my case. On an ATV, unlike in a car, I'm not trying to get to any specific point — it's the activity itself that I enjoy doing, so if I set out to ride for the day and don't make as much progress as I'd planned, I'm not going to extend my day until I ride every trail on my list. I'll get tired of riding after four hours or whatever, no matter how much distance I've actually covered in that time.

So it's more informative to use time, rather than distance, to compare fuel usage. I added a field to my Excel spreadsheets to convert my fuel usage numbers from miles per gallon into gallons per hour. The ATV has an hour meter in addition to the odometer, so I can do the calculation directly, but for the cars I had to estimate the number of hours driven based on an assumed average speed of 50 mph.

In this case, things look more like I'd expect. The Buick consumes 2.3 gph, the Civic uses 1.6, and the ATV uses only 0.4. It's possible that the ATV's hour meter counts the time that the ignition is on but the engine isn't running, which would make its number look a little better, but just like motorcycles, ATVs have small batteries, so you have to minimize the amount of time you've got the key in with the engine off. I think that figure is pretty accurate.

At first I was surprised by how low the ATV's hourly consumption was, but I realized that, when comparing fuel usage per hour of operation, you'd expect the usage to scale roughly linearly with respect to engine size, regardless of the distance traveled, because the engine will be operating in the same range of revs in either case. It's the transmission that takes care of converting that output into various rates of forward motion — the engine doesn't know or care whether you're going 10mph in first gear or 70mph in fifth. It will be spinning at 3000 rpm in either case, and thus using about the same amount of fuel. I'm curious to calculate the gallons-per-hour figure for a motorcycle. I'd expect that an on-road vehicle with a similarly-sized engine (or smaller) would have a similar (or better) consumption rate, and I'm curious to see if that's actually the case.

Anyway, regardless of the numbers, there's plenty of reasons why offroading is not a very ecologically friendly thing to do, but at least I'm now reasonably confident that jumping on an ATV and riding for three hours (and covering 30 miles) uses about the same amount of gas as getting on a motorcycle and riding for three hours (and covering 180 miles). And that's one relief, at least.