This seems bizarre but I figured I'd ask if anyone else has seen this. Over the last several years I've noticed when going to car shows in my '60 that my speedometer reads about 4mph faster (than compared to my GPS indicated speed) in the mornings but around 4mph slower in the late afternoon return. This usually when I'm doing 55mph. ???? Note; the only difference I can think of it the temperature, cooler in the AM and warmer in he PM.

 

Does it ever read correctly ?

 

The only time I take a GPS with me is to car shows I've never been to and as such don't drive it mid day so don't know. What gets me is that at those common speeds (55) it consistently reeds fast in the mornings and slow in the mid afternoon.

 

I’d try a different gps.

 

What is the diameter of your tires compared to the OEM tires ?
I realize that the time of day does not enter into this , but may help to establish a common base line .

 

I would be thinking it is the position of the satellite in relationship to the earth. When you are directly under the satellite, perpendicular or at 90 degrees to it, the radio signal has less distance to travel and thus would be most accurate.
If the satellite is ahead of you or behind you, the radio waves have a farther distance to travel and thus may not be the same as when you are perpendicular to it.
This combined with the latest theory that the earth is spinning faster than it did last year throws the reference MPH off.
Just guesses, wait for the experts if there are any.

 

I don't think the GPS is an issue (although I have another to try) because when my wife used to follow me to car shows in her C8 (always good to have your antique followed by a contemporary car) she would say I was always speeding on the way home while I was trying to obey the limit.

 

Does this happen no matter where the shows are? I wonder if a significant hill could make a difference.

 

This seems to be the norm and why I noticed it.

 

If you're going up hill you're going closer to the satellites faster than if you are going down hill and away from them. Try a car show on flat ground and see what happens.

 

I live in Michigan, it don't get much flatter than that. Also, no change in speed up hill, down hill or in synchronous orbit.

 

Sounds like one of those problems with no solution

 

Your car is identifying as a 1958 Plymouth Fury.

This is just the start.

 

Christine???
Or maybe the car is tired in the afternoon???
Or maybe the GPS is from different satellites in the afternoon. I am not up on the space cadet stuff.
Or maybe you need to ask GOOGLE or just find something else of greater importance.

 

The US GPS system is composed of 30 some satellites which send radio signals that the GPS receiver uses to calculate the location, speed, and elevation of the receiver. Most receivers require the acquisition of at least 3 satellites to calculate an accurate position. The satellites are tracked by a radar system that calculates the satellites position and adjusts the signal from the satellite if the orbit changes. When GPS became commercially available we used it in land surveying some 40 years ago calculate a position to an accuracy of 0.02 feet, but this accuracy required that the receiver remain on station for an hour or more. Today the same accuracy can be obtained almost instantaneously when using the latest equipment. Even an inexpensive GPS receiver will give a speed inaccuracy of less that 1 mph, but it will show some small location change which results in a small speed error when actually standing still. I am speaking about a GPS receiver that is connected to a computer which shows the exact location readings, not something like a Garmin or your car's GPS which filters out the variations in readings before displaying them.

But back to the speedometer question. The C1 speedometers are calibrated using a cable speed of 1000 rpm, which should read 60 mph if the speedo is properly adjusted. If you have an electric drill of known maximum RPM you can connect it to the transmission end of the speedo cable and spin it up to maximum speed and check the accuracy and consistency of the readout. Personally I cannot believe that just a temperature change would cause an 8 mph swing in the reading.
Charles

 

> Personally I cannot believe that just a temperature change would cause an 8 mph swing in the reading.<

Agreed, that's why I was asking if anyone else has seen this.

 

It's easy to eliminate temperature. Drive it somewhere, turn around and come back without waiting for the ambient temperature to change. Easy to eliminate whether speed or speedometer is changing. Have someone follow you at exactly 55 and see what their car reads. After the show do the same thing and see if the speedometer is changing or if you're really running 8 mph difference. I don't think you'll find either temperature or speedometer problems, but at least you'll eliminate them in your mind. It's not something I would spend any money or time on anyway.

 

What is the GPS unit? Unless you have a high speed, high accuracy unit I would suspect that is the problem. I use a Dual XGPS 160 SkyPro with my track data logger. It locks to multiple GPS and GLONASS satellites and updates at 10 times per second for very accurate speed readings.

The speed function in Map/NAV GPS units, and even worse in smart phones, is based on a much slower update rate with a lot of filtering.

 

Why then does it seem like, when my wife follows me in her C8, does her speedometer jibe with the GPS?