Check this out:
https://www.fluxhybrids.com/
They actually claim a 144% improvement. This is based on the EPA combined estimate, and they test this using similar methods as the EPA. Their first test vehicle was a Ford Ranger that saw a 100% improvement.
The kit works by mounting to the back of the transmission. They told me that the car basically thinks that it is going downhill. I was told that they are working on improvements so that the driveshaft does not have to be shortened.