ORIGINAL: DaveG55

Least expensive way to measure land speed for anything is with a stop watch and pencil and paper. pretty easy too. It is easiest to do with a friend but not impossible alone.

For ground vehicles:
Put two lines on the ground 132' (44 yds) apart. Do this in an area large enough that you can be running at speed when you cross the first line.
Activate the stop watch when the vehicle crosses the first line and stop it when it crosses the second line.
Take the time on the watch (I'll use 2.5 seconds as an example) and multiply it by 40 - 2.5 x 40 = 100 (this number is how many seconds it would take to go 1 mile at that speed)
Now divide 3600 (seconds in an hour) by the number above (ex = 100) -3600 / 100 = 36 MPH

For land speed (not air speed) of a plane just use longer distances and change the multiplier. 176 yds (528') multiply by 10 or 88 yds (264') multiply by 20. The rest is the same.

Of course the accuracy you get will depend on the reaction time of the person using the stop watch but you will get a decent estimate of the speed w/o having to strap anything to the vehicle or buying any equipment.