Speedometer/odometer

ford
focus

#1

I have noticed when I pass one of those police radar units, that the speedometer on the car registers about 4 miles per hour higher than the radar. Does that mean that the odometer on the car is miscalculating also? The car is a 2003 Ford Focus with over 150,000 miles on it.


#2

There’s a margin of error built into the speedometer and the radar gun. The two may be off enough for there to be up to a 5mph discrepancy.


#3

Odometers by law have to be very accurate. And they are set to match the tire size. Taxis need to have them custom calibrated as well.

Speedometers, on the other hand, are usually set to be a bit optimistic to save you from speeding tickets. Years ago they were really off; a Ford V8 from the 40s and 50s had a top speed of 88 mph, while the speedometer showed 100 or more when going flat out. The guys who bought Fords in those days had bragging rights!


#4

Since I have GPS in all my cars, I can get a very accurate reading of my speed. I’ve found that the radar trailers are off more often than not. It’s surprising to me that photo radar tickets are even legal as they are often terribly inaccurate. Despite public perception, when a radar is pointed in your car’s direction, it’s not necessarily measuring your car’s speed. It’s returning the fastest speed it sees. There have been cases, for instance, where cops running radar have been surprised to see a car clocked at north of 200mph only to realize they were actually getting the readings from a low-flying plane. There’ve been other cases where stationary objects like trees have been clocked at illegal speeds.

In short: If you want an accurate check of how fast your car is going, get a GPS.


#5

Sorry, but radars do not report the speed of the fastest thing they see. Speed radars read the target with the largest radar return. For instance a Semi truck will have a larger radar signature than a Miata and will be more likely to be locked onto by the radar.

I don’t always trust a GPS either, for distance logging I’ve had it show a 4 mile discrepancy between the legs of a 2-way (35 miles each way) trip using the exact same route. Sure I’ll give you that I might make more left turns in one direction but not 4 miles worth.


#6

Sorry, but radars do not report the speed of the fastest thing they see. Speed radars read the target with the largest radar return. True

I don’t always trust a GPS either, for distance logging I’ve had it show a 4 mile discrepancy between the legs of a 2-way (35 miles each way) trip using the exact same route

The better
ewer GPS units are very accurate. The older ones did not sample often enough and failed to factor in differences of altitude.