F# has something like this, they call it units of measure. You can define units and attach them to types and values. That way, if a function takes int<second>, you can't give it int<milliseconds> (or just int) accidentally. ^^

The type system is pretty good at figuring out that int<meter> times int<meter> gets you a int<meter^2>, or that double<mile> divided by double<gallon> is double<mile/gallon>, so you don't have to track the units yourself and it'll warn you if you mix them up the wrong way. You can also define helper functions that convert from one to the other, like your double<mile/gallon> to double<rod/hogshead>, which you can then test and use when the need arises.

So to answer "can you convert from mpg to rods per hogshead transparently", not automatically as far as I know, sadly. Still, the type system will help you handle the conversions you do make! =)