50% ought to be fine for most hypermilers in most vehicles. If you're towing a camper up a mountain in the summer, maybe not.
A bicycle derailer cable should work to operate it, and using a bicycle shifter it may be possible to get enough travel. If so, a modern bicycle shifter will offer indexed, partial movements, allowing you to adjust it quite finely. You may need to buy a length of bulk cable and housing from a local bicycle shop or online retailer instead of a pre-cut kit, in order to get it long enough.
If you're not familiar with modern bicycle shifters, I'm thinking one like this:
It has one lever that pulls the cable, ratcheting up to three clicks in a single push, and another lever (not visible in that picture) that releases it one click at a time. It's also got a display to show you what gear you're in. There are other designs, but I think that one (except maybe with a different display) would work great.
Come to think of it, if it doesn't provide enough travel, a bicycle derailer could multiple the travel and is already setup to work with that cable.