When traveling I use a portable GPS system. As part of the dispaly I get the current miles per hour. When I compare it to the car’s speedometer, the GPS reads a couple of miles per hour slower. (OK 2.3 mph difference at 60 MPH)
I’ve trived the system in three cars (1995, 1998, 2002) and get the same result. The GPS tells me I’m going slower then the car’s speedometer. Recently a group of us tried to follow some direcetions (“Go 3.7 miles and trun left.” etc) All of use complained that the distances were off.
Some years ago I read where a man driving a rental car determined that the car’s milege was off. Supposedly this was a built in error for “safety” However, the guy was being charged for miles not driven.
Ao how accurate are car speedometers? Do the police really give you a 5 MPH leeway because of it?
Car speedometers are effected by tire size and that is somewhat variable as they wear.
GPS calculates speed by taking 2 points and computing the distance/time between them. It always does a direct line while the car might not be going exactly straight.
This has been a topic on the geocaching.com message boards whic you can search on - conclusion we really don’t know but most settle on the gps (becasue it allows us to drive faster).
As for the leaway - it’s not really due to speedometer inaccuracys. They can pull you over for 1mph over and it has happened. It’s a game they play you have to pick 14,9,4 mph over the posted speed limit - depending on how the cop feels. It really sucks that’s they way it is but that’s how it is.
I have heard about rental car companies charging extra fees for renters who speed based upon GPS findings. When you turn your car in, BOOM!, you get hit with a fee from someone who says you went 80 MPH for 10 miles, for example.
Can or will the PO-lice ever use GPS to issue speeding tix?
Not anymore. While civilian GPS units may be built more with price in mind than with absolute accuracy, Selective Availability, the system of deliberate errors introduced into the civilian GPS signal, was turned off May 1, 2000. PRESIDENT ENDS SELECTIVE AVAILABILITY
As for speedometer error, the April 2002 issue of Car and Driver had a decent look at the subject…
They determined that some of the error was due to tire wear and inflation issues but a larger part of the error for many makes of autos was due to governmental regulations of speedometers. The rules vary from country to country of course, but in most cases the simple rule is that the speedo can never read lower than your actual speed. European regs, if I understand correctly, require that the auto manufacturer must build in enough tolerance to compensate for the owner changing wheel and tire size meaning that the error is particularly large for cars with more room for tire size changes.
From my motorcycling days, I remember that the bike reviews from on particular magazine listed the speedo error as part of the stats. Every bike’s speedo read high, usually by a fair amount. My 2000 Honda VFR was about seven percent high and my GPS, a bicycle computer and the bike review all agreed to some fraction of a percent.
Most of the GPS units I’ve played with figure speed once a second or so, more than often enough to minimize the effect of a curving course. Most speed error on a GPS comes from traveling up- or downhill. I believe that most/all consumer GPSs figure speed as a two dimensional problem since GPS units without a barometer have such an awful time with altitude.
I have a GPS receiver (Garmin eTrex Legend) in my '00 Chevy Blazer. I use the speedometer in the 20 Km/h (it’s lower limit) to 160 Km/h (my Blazer’s top indicated speed) range. The speedometer, in that range, ALWAYS reads a speed 5 Km/h higher than the GPS indicates.