Miles per gallon is important info for vehicles. I think MPC or miles per cache is more importatnt. It is easy to figure out. Take how many caches you have found on one tank of gas and divide by how many miles your drove. Many factors play into he number. Did you park and go get a few caches, or did you do a geo art and had to drive to each one? Is it a multi and you have to travel around or are you on a trip and stopping every so often for a break and a cache.
Here is a cache that you can stop and stretch your legs, take a break, and best of all find a cache. I like stopping at Flying J's since you can walk around and they have several bathrooms and space to take a break. Cache should be pretty straight forward. Happy caching and many caches per mile!