Some houserules for GURPS. They were written about 15 years ago for 3rd Edition (based on the Vehicles 1st edition). There's a 4th Edition take on GURPS Metric here - which is more general purpose.
For a science fiction campaign I was running, I needed to design some spaceships. However, neither GURPS Space nor GURPS Vehicles were quite right. The latter had the right level of complexity, but it didn't use the metric system. If I'm designing spacecraft, I don't want to work with miles per hour.
So I converted GURPS Vehicles to the metric system, and modified the technology along the way (I wanted a Hard SF campaign, so there are no reactionless drives until very high TLs).