speedo accuracy
 

I recently did a calibration of my speedo against both my GPS and the 3 mile speedo accuracy check on the 395 near Bishop. I was very suprised to find it undereads.

At 50MPH indicated the actual speed was 52MPH
At 65MPH indicated the actual speed was 68MPH
At 70MPH indicated the actual speed was 73-74 MPH
At 100MPH (private road!) indicated the actual speed was 104MPH

The 3 mile course (steady 65MPH GPS) measured exactly 3.000 Miles on the GPS and 2.8 on the Odometer.

This is the first car I have ever had that undereads. This may be worth keeping in mind when near speed traps.

Of course every car is probably different. Awesome car.

This seems about right from what my wife said our 2000 ES does. She has been noticing from those unmanned radar stations that tell you what your speed is. I wasn't so sure at first as I have not really driven the car much, but if others think this is happening as well, it seems right. I will have to time the car and also drive with our gps as well. Thanks for the heads up.

i kind of tested mine once when i was on a road trip. with 3 other cars in my group we all set our cruise. while there mph ranged from 75-77 mine was set right at 70! what the heck!!?? that much difference between my car and 3 others?

A variance of 2 miles an hour most likely won't get you pulled over.

that sucks.

I got pulled over for speeding and I had my cruise set to 10 over the limit. the ticket said I was going 20 over. (yes I'm fighting it). The cop said he was pacing me at that speed. B.S. I saw him pacing me I had the cruise set to 110km/h the whole time.

I wonder if I could use this as a way out. I guess I would need independant testing done to my car though.

...unless you are already over the speed limit and think that you're already in the safety zone. In Central FL they sometimes use an airplane to catch speeders and there was an article in the newspaper about it. They admitted that they look for cars going 78 mph or higher. They don't have the manpower to enforce below that amount. If you're doing 75 and your speedo's off by a few mph???, you might get an unwelcome surprise.

dependign on the wheels that are on the car my speedo reads about 2 mph (winter tires) and 3 mph (summer tires) on the high side. I've checked it against a measured mile and with my GPS with the same results each time. I guess that this way when I set it to 10 over the limit, the car is really only 8 and it keeps me out of trouble :)

Of course since I have the GPS with me all the time I usually end up close to the 10 over anyway.

GPS
 

Hey,
As an aviator, one who relys on GPS for my JOB, I can assure you that your spedo is MORE ACCURATE. Why?? GPS uses a minimum of 3 satilites to triagulate your position on earth. There is no GPS system out there available to the general public that will get it "perfect". The SAT's talk to each other to narrow down the results, and your reciever mearly translates what the satilites are telling it. My aircraft uses GPS and ground RADAR to get my altitude fixes, and the RADAR is always closer to the actual barometrical altimeter. Aslo, figure this, your spedo is directly linked to the wheels. Just a thaught....

BruinsMan,

yes, your speedo is directly linked to the wheels... but, as you change tire sizes minor variance will occur and that adds up pretty quick. For a P5 assuming a 195/50-16 tire to be a 23.7in diameter when new and a change of .5" in the diameter of the tire (I considered a .25" for tread wear to be an extreme case) at 3000RPM will change from 68.1MPH to 66.7MPH (all based on FAQ gear rations).

In addition, there is a good deal fo variance from vendor to vendor for a specific tire size.

I guess that in the end there really is not a totally accurate way to indicate speed on most cars... I guess that if you are 1 or 2MPH off that would nit picking as well :)

In my case, trial and error with a measured mile, a stop watch, some radios, and a bunch of guys with nothing better to do on a Saturday afternoon (pretty sad) I found that the GPS was more accurate (over a 20 or so second interval) than my speedometer.

In all cases your mileage may vary :D