I'm new around here, so sorry if this one has already been posted somewhere. I installed a Trail Tech TTO digital tach, and it's great for what I need.
Being digital, it's a little laggy, so it's not a great choice if your trying to reduce your lap times in the S40 while on the track.

Given the cruiser/chopper vibe of this bike I don't need constant, perfect, real-time info on rpms. I mostly just like to have some idea as to what the bike sounds like at various rpms so I have some clue if I'm shifting at optimal points.
It's also nice to be able to see rpms on the highway-- for both curiosity and to be able to confirm that I didn't forget to shift to the top gear! Finally, it's nice to be able to see the idle rpms.
Given what it is, I don't think it's the cheapest thing in the world at $39.95 at Amazon (I'd say a good price would be about $20), but it's not too expensive and installation is a breeze. It's battery operated and uses a wire wrapped around the plug wire to pick up the signal. There's no backlight, but, as I said, I only use it as an occasional guide, so not having it at night is not a problem for me.
I installed it behind the handlebars, using wire ties to secure it to the brake line guide. The first picture shows a close up of the installation, the second shows it as I see it while riding. Unlike the speedometer, the tach is in my line of sight as I ride, so I usually use the rpms to monitor my speed on the highway.
So, if you're looking for a relatively cheap, easy way to see your rpms on an occasional basis, I can recommend this option.
