Back to the original post, don't pay attention to those little road-side speed readers. For one, if you are on a multiple lane street, it may get confused. For two, I've found them extremely inaccurate.

I live 120 miles from St. Louis, MO. At a point coming and going, I have two roadside 'your speed is' signs. Even when I'm the only car on the road, it will say my speed is 10mph different in less than 5 seconds. This has happened with my Explorer and my FEH (and often both readings are off.) I have a car GPS unit that is very close to my car's mph, often within 5 miles.

You have to give every meter a room for error, just like any other sensor. I think a +/- of 5mph is a good error range. (Hey, the home blood sugar monitors have an error rate of 20!)

 

Walt: I think 2 MPH error is pretty significant given today's instruments.

GreenGiant & Nitramjr: I understand how changes in circumference alter speed but it would have to alter the odometer also - and that doesn't appear to be happening. Tire pressure in my FEH has changed moreso than tirewear in terms of outside diameter but I don't believe either one has a significant enough effect on speed to make a noticeable change to it. In the winter the pressure was under 36 psi and the radar measured speed is the same compared with now where I have it over 40 psi.

Yaris: I like your explanation, for reasons based on wheelwell room no one would increase their rim size much at all, especially on the 4WD version; but you could go a little smaller in tire sidewall or rim size which would increase actual speed measured as one rotation of the axle (or a gear ratio from there).

Thanks everyone for the input! I think you are all on to something and you're right, there will be a day soon when velocity will be measured through mini-radar, GPS which my car already has, or something similar and not the current method which is obviously subject to some variability.

 

Radar readerboards operate on K-Band radar beams and the doppler effect. Blood sugar monitors operate completely differently. I'm a traffic engineer and I've used radar on the job to do a speed study. They are very accurate! I will concede that they change as your speed changes so the actual speed will 'vary' but if you proceed past them maintaining a constant speed you will get an accurate reading. I will also concede that they occasionally need to be calibrated with a tuning fork but I've never known ours to need adjustment. As a separate check, I'll compare mileposts and maintain 60 MPH to see if it takes exactly one minute.

On another note, we have a Distance Measuring Device in our work car. Once it's calibrated it measures distances to the nearest foot! It's still pretty accurate in spite of tire pressure changes and tire wear over time. In fact, it may only be off by a few feet over 30,000'.

 

Correct - tire pressure will have no impact on your speedometer - the tire is still the same distance around unless it is nearly flat and you run on the rim.

 

Assuming you haven't changed the diametr of your overall wheel/tire packages, you can correct this. My speedo was off in my 02 Escape. Remove the gauge bezel and then the 4 bolts that attach the gauge cluuster to the vehicle. You can then pop off the clear plastic gauge cover. I removed the speedo needle, found flat level turnpike and set the cruise contol at 60MH per my GPS. I then pressed the speedo needle back on in the correct position and it was accuarte w/i 1/2 mile at all speeds.