# Speedmeter

Is there any way to quickly tell if your speedometer is accurate? … or there anyway to calibrate your speedo? It seems as if a lot of cars are passing me on the highway.

Good ol’ stopwatch or second person watching the second hand on their watch.
Drive a segment of straight highway with mile markers, at 60 mph showing. Time the mile markers. 60 mph will be one mile in one minute (60 sec.).

I agree, but you need to do it a few times because the mile markers themselves aren’t necessarily placed at exactly the right point. I’ve noticed some states have the “check stations” where the markers are correctly spaced, but otherwise you need to average a few.

Another way to use the highway markers is to drive 10 miles based on the markers and check your trip odometer at the beginning and end of the 10 miles and calculate the difference between the markers and the odo.

For instance, if the marker distance was 10.0 miles and the trip odo was 11.0 miles, the trip odo and speedometer are 10% higher than your actual speed, i.e. thespeedometer would read 55 mph when the car was traveling at 50 mph. My cars and trucks tend to be around 2 or 3% off.

When I’m on the highway doing 65-68 mph in a 65 mph speed zone, most cars are flying by me.

Ed B.

Borrow a GPS unit. Most units will have a page that displays the instantaneous speed. Stick it up on the dash where you can glance at it and compare it with the speedometer. I have done that with my car and am amazed at how consistently high the speedometer is compared to the GPS reading.

Much like the mile markers, you have to average the samples for a GPS reading. GPS units are only accurate to about a dozen feet at best, but can be even less so depending on reception conditions. This makes the instant speed measurements not especially accurate, because at any given second the distance between the two points that it took you X amount of time to travel may be totally wrong.

The best way to accurately detect speedometer error with a GPS unit is to reset the odometer on your car and on the unit and then go on a long highway trip and compare the two figures you end up with. Because the errors in the GPS unit are more or less random over time it will average out. However, do note that this won’t work in city driving because when you’re stopped, the receiver will interpret the changes in reading while you’re stationary as movement.

Our Garmin 260W reads a consistent 2 mph faster at 60 mph than our car’s speedometer, always! Whether the GPS is accurate or not, I have not yet determined but it is very consistent.

I don’t trust highway mile markers; have found some that seem to be placed in error.