This is my circuit, output driver seems OK with it. The 5K is a pot, you control the RPMs that way. You could get slick and get a microprocessor to vary the speed up and down, that would be cool. The tach does make an unnerving buzzing noise, though. You can't hear it over the engine when you're driving, but on a coffee table it will be pronounced.
I'd give it 2 minutes until the novelty wears off!
