I use a bike computer on my HyperBike, and liked it so much, I now have one on the EE cart. It took me a bit to figure out how to calibrate, so I went back to my bicycle guy and he explained it to me, then gave me a pinger for my pneumatic tyres, which also have a different calibration.
I LOVE mine. It gives me all the information that Leia's GPS gives her, which has REALLY helped getting PJ up to speed, and lets me see when we are moving at a pace he will need. It also helps me to accurately work on aerobic conditioning - pick a pace and maintain over a particular distance. It also shows my ride time and distance, has a clock ( I am always late), and I can take the "head" off so I don't lose it, like the first one I had. There is also an overall odometer.
If you are a member of ADS, they have a drive time patch program, you log in the hours driven, and receive patches for various milestones. Kind of a nice, non-competitive incentive to get out there! The bike computer makes it easier to keep track.
They are highly variable in price. The first one I had was about $24 and I stole it from my bicycle. It's in the bottom of someone's water hazard. Pick your placement carefully. My new one - well relatively new, it has nearly 200 miles on it - was $50 on sale. As they are designed for bicycles, they are water proof and fairly jar proof. On the wire wheels, you should check the set screw on a regular basis - like every outing. On my HyperBike, I used liquid nails to ensure the little magnet doesn't wander off.
You don't have to use wireless, which makes them more costly.
You can also use a car GPS, IF it has the hiker feature and you have a place to mount it. With the HyperBike, I'd have to velcro it to PJ's butt, but on an EE cart it would work. If it doesn't have the hiker feature, the horses don't move fast enough, I am told by a Friesian driver.
If someone tells me how to do it, I will post a photo. I do horses' legs, not so great with tech.