For the first few years your new Ford came from the factory with a speedometer.  But production of the cars became  more and more efficient,  and they were turned out faster and faster. The speedometer manufacturers couldn't keep up, so Ford quit installing speedometers. You could still buy an aftermarket speedometer for your Ford, but a lot of folks didn't. So today many Model T's have no speedometer.

If you want to install one, there are three options. You can buy an original Model T era speedometer, and it will be fine. Its drawback is the price, usually several hundred dollars. Another choice is some sort of GPS device. This has the advantages of being easy to use and less expensive. Its drawback is that it depends on being able to pick up a satellite signal. The third choice is a bicycle speedometer. Its advantages are price (it's least costly of the three options) and accuracy. Its disadvantages are delicate wires and an odometer that goes back to zero when you install a new battery.

This page shows installation of a bike speedometer. I have read that some of these go haywire due to interference from nearby ignition coils. That has absolutely NOT been a problem with this speedometer. It's a Cat Eye Velo 5, and I paid less than $25 for it. In fact I've  bought it twice. One is on my 1915 runabout and the other is on my 1923 touring. Here's how both are installed:

In cars with a horn button on the steering column, that makes a very handy mounting location. The wire comes up through the tube along with the horn wire, and the computer is mounted over the button with a couple of plastic zip ties.

My 1915 has no horn button, so I made this substitute from a piece of 3/4" conduit. The small item to the right is a bracket for attaching the magnet to a wheel.

This shows the bracket clamped on the steering column with the speedometer computer zip tied to it.

This is the bracket for mounting the sensor. It's just a little piece of pipe welded to a washer.

Here's the bracket attached by the spindle nut. The sensor is mounted on it with Gorilla tape. The magnet is attached to the wheel by a hub nut. Note that the magnet mounting bracket is different from the one in the earlier picture. There have been some changes over the years as I figured out what didn't work very well and what did. This photo also shows the Achilles' heel of this speedometer. The delicate wires are easily damaged by road hazards like gravel kicked up by the wheels. Here you also see that extra wire needs to be spliced in. The wire as it comes with the speedometer is fine for a bike, but isn't long enough for car use. A few feet of speaker wire will solve that.  I cover the splices with shrink tubing, which has been cut off here to show the splices and the broken wire.

My solution to the delicate wire problem is to run it through some 1/4" rubber hose. So far that has worked. I regularly drive on gravel roads, and since adding the hose I've had no more broken wires.

I've attached the hose to the front spring with a zip tie, which you can see next to the shackle. Some people mount the sensor and magnet in the rear of the car instead of using a front wheel. I don't see any reason why that wouldn't work, but I haven't tried it.