for those that dont have gps (or are concerned as to its reliability)...
do a speedo test next time you are on a hwy and see the 5km sign things.... or use google earth to accurately plot a 5km section using easily identifiable landmarks like overpasses or off ramps. either way, once you have your NICE AND FLAT 5km 'test track' (to avoid cruise control dropping speed), you'll need 2 things.... a stop watch and a passenger. google earth is a terrific resource and is more than accurate enough for this exercise.
1) before your start line, get up to 110km/h (or whatever the speed limit is) and set the cruise control. take careful note of what the digital speedo says, and also what the actual speedo says.
2) get the passenger to hold a stop watch and also make sure they are prepared with a fixed point on the car that they can refer to when looking from their seated position and the starting point outside. what I mean is in order to be accurate, their eyes and seating postion must be the same from start to finish... head resting against head rest, and their eyes fixed so they are gazing past a fixed point (like the side mirror or edge of the front windscreen) so when they pass the start and end points, that point passes this fixed point on the car.
3) as soon as the passenger sees the start point on the fixed point of the car, start the stop watch. if you want to test your trip meter at the same time- do so. dont try and reset on the fly however, just note the exact reading on it when passenger starts watch.
4) as soon as the passenger see the end point on the fixed point of the car, stop the stop watch (and if you did so to start, note exact reading on trip trip meter to see if it actually recorded 5km exactly).
5) provided your passenger was quick on the trigger and kept their reference points accurately (and your 5km distance was accurate), you'll have an exact time for 5km travelled.
Car travels 5km in "x" minutes and seconds.
You need "x" to be exactly 1 hour, to give you km per hour.
1 hour is 60min, or 3600 seconds.
If it took you 3 minutes exactly, that's 180 seconds.
3600/180= 20
5km x 20 = gives you your speed, in this case 100km/h ACTUAL SPEED.
Here's some working examples:
3min and 3.25 seconds recorded on the stop watch = 183.25 seconds. 3600/183.25= 19.645293 x 5km = 98.23km/h (compare this to what the digital and dial speedo was telling you during the test, and see which- if any, was accurate)
2min and 57.75 seconds recorded on the stop watch = 177.75 seconds. 3600/177.25= 20.253164 x 5km = 101.27km/h (compare this to what the digital and dial speedo was telling you during the test, and see which- if any, was accurate)
The main things are a) your 5km distance is accurate (using your trip meter is no good because that's what you are testing and suspect to be inaccurate), and b) your passenger is spot on with their 3 line of reference points (their eyes, the fixed point on the car, and the start/end points) hence why you cant do it yourself- you need to concentrate on the road).
But the above method will most definitely give you an accurate speed measurement (and also if your odometer and trip meter is reading fast or slow), which you can then compare to what both speedo's (and if available your GPS) is telling you.
