If you really want to know how fast you're going without resorting to a radar gun, try buying a GPS (Global Positioning System) unit. It will tell you exactly how fast you are going every second and it's pretty accurate. It's good for up to 1000mph so it even works on airplanes. Also gives you altitude.
That's how I figured out that my speedo is actually off by about 5mph on the low side! All this time I thought I was doing 65 I was actually doing 70.
I typically just use mind for hiking. Basic units are between $100-$150 which is all you'll need to determine your current speed.
