A brighter speedometer? LED display?

These days, I think the speedometer is actually a readout from the car's computer, and the sender talks to the computer. So it may be harder to find that signal and tap into it without disturbing the car.

I'd start by getting a maintainance manual for the car -- which is a good thing to have anyway, if you plan to make any mods to the vehicle -- and look though its diagrams to find out where this signal comes from and where it goes to. Then get a meter onto it to find out what it's doing, and set up some form of high-impedence (to avoid disturbing the computer) tap. Run the output of that to a frequency meter with appropriately calibrated reference clock.

Remember, if you're going to mount this on the dashboard: Do NOT block visibility (the police get significantly upset with that), and make sure there's a dimmer control on the LED's brightness so this doesn't ruin your night vision (red LEDs may be a good choice, for just that reason).

Or... There are probably aftermarket secondary speedometers commercially available, from catalog houses such as Whitney. That would be already built, and come with installation instructions; all you'd have to do would be wire it, mount it, and possibly calibrate it. Heck, there used to be "car computers" which used this signal and a few others to dynamically calculate MPG and the like, long before cars had their own computers smart enough to report that sort of information; those too might still be available. One piece of advice when dealing with these catalogs: Always buy something specifically intended for your vehicle rather than the generic, and never buy the absolute cheapest one in the catalog because it's going to be awful.