# Yet another reason to switch to gallons per mile

Pop quiz: Say you split your driving evenly between a 21 mpg SUV and a 75 mpg Vespa scooter. What would your average miles per gallon be? If you answered 48 mpg you're not alone, as that's what Vespa came up with in their infographic we posted last week (reposted above). The only problem is that that answer is wrong. The real average is 32.8 mpg.The reason for the large difference is that in order to correctly calculate average mpg, you can't simply add both numbers and divide by two. You have to first convert to gallons per mile (gpm), then average. We've discussed gpm before and have featured a graph on the subject here, but both of those posts are a few years old so we thought this was a good time for a refresher.

If Vespa is having trouble with the math, the average consumer isn't likely to fare much better. Of course, if we just switched over to the metric system and measured in liters/km, like most of the rest of the world, we wouldn't have this problem. Fat chance, we know.

In any case, it seems someone informed Vespa of this error after the infographic was released. The company quietly changed the infographic values on its website.