You can buy dedicated units, a few manufacturers make them. HKS to name one.
Apexi do an RSM (Rev/Speed Meter) which is a little display unit that doubles as a speed cut defender.
New these units are quite pricey, but second hand they'll be around $100USD.
Sorry for being ignorant but does this raise the speed limiter or what? i just upgrades the tires on my tundra and want to raise the limiter to match the speed rating. thanks