You know how all the cars and bikes you buy in the game have a base speed? Does anyone know what speed the taxi is? I am assuming some cars are not worth buying because they may be slower than the taxi. I am not sure at what point it's worth it to buy a car in the game.

The taxi and police cruiser are both repainted Yomoshoto Evasion cars, so unless they modified the speed of the taxi to be a slower version, it should be the same speed as that model of car, so any car slower than that isn't worth it. You can test it by having two sims, one with a car and the other without, travel to and from the same destination at the same time (pause and queue the actions then unpause) and see who gets there first. I assume the taxi has the same speed though.

The taxi has a speed of 30 meters per second (one sim minute).
A car called CarHatchback, presumably the cheap hatchback car, has speed 37.
There are 2 cars called CarUsed, these have speeds of 32 and 33.
So I think that all of the cheap cars are faster than the taxi.

Where did you find the speed of the taxi? I thought I'd be able to buy a taxi in debug, but I didn't see it. I know you can see the speeds depicted as meters per second using AwesomeMod. I just couldn't find the taxi!

If what you're saying is correct (which will assume it is) than any care is worth getting.

The speeds are in the XML's (GameplayData.package) on the vehicles. An XML named Taxi has the speed for this vehicle.