My Speedo and GPS were dead on...I changed from 205X45R17 to 215X40R17 and the Speedo reads 2 mph fast, exactly where it should be.

 

At what speed?? With a different zie tire, you will get a % difference in the speed reading, not an exact speed difference.

 

Because conversely, someone so equipped may NOT stop at red lights that he knew did not have a camera, foolishly risking the lives of other drivers for his self centered 'me first, red lights are for other people' attitude.

I drive as fast as any, but a red light is a red light. I was almost hit by a ^%*^@& SUV that blew through a red light. There's no scenario where one runs a red light FOR safety. It's for inattentiveness, or self importance. Usually the latter.
Your scenario where someone would be blocking an intersection if they DIDN'T run the red makes no sense. If you're behind the red light, you're not in the intersection.
I loathe speed cameras, but I'm perfectly fine with red light cameras.

Oh, and my MCS speedo is within 1mph according to my GPS.

 

Cheap MINI speedos My toyota is dead on ....

 

One thing to remember is that the tire diameter will vary as much as 1/2" from full tread (10/32) to worn out (2/32) - that in itself is a ~2% difference.

And Chows, your Toy may be dead on, but that mirrors it's soul!

 

Nope, its those super accurate Japanese parts

BTW, why is every so hung up on GPS since its not accurate anyone. You got to know it at least 10 - 15' off on location (and the unit is designed to lock on to the closest road).

What makes anyone think the speed part of it is dead on like Toyotas speedos?

http://www.gpsinformation.net/main/gpsspeed.htm
http://www.ncbi.nlm.nih.gov/sites/en...&dopt=Abstract

 

Well the city can always put in decoy cameras at most intersections. Just a device that will emit the same signal that the detector detects.

 

I don't know - I think being within 0.9 MPH greater than 65% of the time is pretty darn good, and that's without differential corrections.

Sure, it may not be the 0.1 MPH that Garmin et al advertise, but 0.5 MPH is a reasonable expectation, and in any case, that's tighter than the resolution of the speedometer.

 

Ever since I got new tires last year, my speedo has been much more accurate, based on what the Garmin GPS says. And it got even better once I had the tires rebalanced.