My speedometer is off, how do I figure out what my actual speed is?

Question

There was one of those highway department signs that said "your speed is". I had my cruise set at 55 and when I went through, the sign said 51 mph.

I thought I would just have to subtract 4 miles from whatever speed my speedometer said, but my husband said that wouldn't be right. He said that as my speed increased, so would the difference in the mph. So how do I figure out (without being stopped by a patrolman) just how fast I'm traveling?

Answer 1

Radars aren't accurate. Speedometers aren't accurate. Fortunately, clocks are accurate :) and freeways have mileposts, and those are accurate too.

You've never noticed mileposts but they're there - either in the median or on the edge of the road. Tiny signs with numbers, and no other purpose. Sometimes they show even miles, sometimes tenths of miles. You'll see they're there like clockwork, their numbers incrementing consistently. Sometimes they're off on the side because an exit is in the way.

Use a clock with seconds to time how long it takes to go between them. Hold your speed absolutely the same, so you know what speed you're checking on your speedo. Don't do 1 mile, do 3-4 miles and then take an average.

Then just do the math. How many seconds did you do per mile? That's X seconds per mile. There are 3600 seconds per hour, so:

3600 seconds per hour
divided by
your measurement of seconds per mile
=
your actual speed miles per hour.

Your husband is right, the error may vary by speed (so less error at slower speeds, more error at higher speeds.) . No way to be sure, except to measure it.

It's also possible the radar was wrong, and your speedo was right.

The usual reason for wrong speedo's is changing wheels and tires. If the diameter of the tire changes, that'll do it!

Answer 2

I don't see where 4MPH is a big deal but use the millage signs along the highway they are usually 1 mile apart (Some big cities have them 1/2 mile apart) Time your self at 60 MPH it should take you 1 minute

Answer 3

all speedos on all factory cars can be off at times. the best way to find out how fast you are going is driving next to somebody at 10,20,40,60,etc mph and have them tell you when they are at a steady rate of speed. then note your engine rpm and the gear you are in. a better way would be to ask a friend with a speed gun to test you since those are usually more accurate.

Answer 4

the cheap way is to get on a street with mile markers.Take your truck as much as 60 mph on the speedometer and clock the time over one mile with a watch or provide up watch.fifty one sec = 70,fifty two sec = sixty 9.2,fifty 4 sec = sixty six.6,55 sec = sixty 5,fifty six sec = sixty 4.2,fifty 8 sec = sixty two,60 sec = 60, sixty one sec = fifty 9.that ought to cover any blunders.The speedo would be a similar quantity of mph off in any respect speeds.

Answer 5

do u have a R P M gauge in dash look at that instad 4 speed at 1000 rpm is about 15to 20miles a hourand so on

Answer 6

I'm not convinced the "your speed is" sign was more accurate than your speedometer.

Answer 7

Just travel along with the flow of traffic, other than that use your best judgement.

Answer 8

How bout you drive next to someone you know, who is driving right next to you and see what they are driving at

Answer 9

have it recalibrated.. it should cost less than $50