Speedometer or GPS
 

What is more accurate when flying down the highway? Speedometer or the GPS? There is a consistent difference of 4 mph between the 2. The GPS registers slower than the truck.

GPS should be more accurate. It's VERY common for analog Speedos to be off, usually reading high. And when they are off, it's by a percentage, so it may be off by 4 mph at 60, but then would only be off by 2 mph at 30.

GPS Is going to be the most accurate. Most of my cars speedos are 2-5 Mph Off.

I used my GPS to determine how off each car is. Funny thing is my old Dodge truck that uses a cable is dead on.

If you are running non stock tires it can throw the spedo off I think. GPS should be more accurate.

Bernie P

That is true, a taller tire will make a slower speedometer

GPS will be more accurate, And many times, it is the Speedo guage that is wrong. I had the speed sensor on my BMW from the ODB-II port, and it matched the GPS. The speedo on the dash read high.

Remember that your GPS receiver is also averaging quite a bit. To get a good reading, find a straight stretch of highway and hold constant speed for ~1/2 mile.

With smaller tires your indicated speed be higher than your actual speed.

just standard size tires

Also borrow a second GPS and compare all three over a few runs.

If I am really concerned I take two GPS units and take a drive with them. Consistantly the GPS units are nuts on while the speedo is the one that is off...

GPS is the defacto standard of speed measurement. Unless you are rapidly changing your speed, it is dead on.

Went a through a speed trap that shows your speed. My GPS read 112kmh, so did the speed trap. My speedo show 98kmh. So I guess if you use the cars speedo, 9/10 times you won't get nabbed speeding.

I changed my speedo to a kms one and it reads about 10% too fast. Whereas the old one, a mph one was correct. I think Fishcop is using my old one on his car as he was driving off his rev counter.

We have friendly speed traps (ones that show you how fast you are going without a big ugly cop writing you a ticket) so I know exactly how the 911 and the Corolla read. Interestingly both are wrong by the same percentage.