guys...i was telling my friend about my weekend expoits of keeping up with a brand new C320 on the NJ Parkway exceeding speeds of over 120...now he was like...you prob weren't doing 120...the speedo stops being accurate over 90...he said i was going like 100 even though it said much higher...now i know this was unsafe and foolish and i got a real scare when i had crazy brake fade for the tolls so i learned my lesson cause i nearly peed myself when i nearly didn't stop...but still i want to credit myself for going that fast...any thoughts??

Your friend is right, and wrong.... There is always a small error % on the speed it displays whether your going 20mph or 120mph; the only difference is the error reading is just amplified as you go faster... For instance all bmw's from the factory come with a 10% error on their speedo's, so when your going 30mph, your actually going 27mph ( a 3mph error) but if your going 150mph, then your actually only going 135mph (15 mph error).... get how it works???

Although your friend was kinda talking out of his *** about it saying 120 and you only going 100, the error % on maxima's is very small, you were probably going around 117mph (pure estimate)... but being 20mph off at 120 would mean a error % of like 17% and that's just not happening.....

Just so you know if you ever get bigger or smaller tires you should also have your speedo adjusted, it's also a good excuse if you get pulled over for speeding because each additional inch adds an additional error % to the output reading, except if you went from 16' to 18's it would say you were going 50mph and you'd actually be doing around 57mph (pure estimate)....

Also if you ever have to stop for an emergency downshift if you can & pump the emergency brake, it isn't good for your car but its better than slamming into whatever is in front of it..

-Nate

Sure. You can lie. It's safer than really driving 120 mph.

The speedo seems to be off by about 2 MPH at 130 so te percentage error is small. BTW, NEVER,EVER get yourself into a situation where you have to use the brakes in ANY situation when you r vehicle is going over 80 MPH, not only will you warp rotors quickly, but you can actually loose control of the car, the shaking is that violent.

I agree with medicsonic on the second point but not the first. Of course all cars are different even without mods, with mods they are way different. My 99 SE with stock wheels and tires is off roughly -6% using a radar/speed sign as the test. You know, those sigs that say speed enforcement ahead and then there is a board with digital speed readout about 100 feet ahead.

Where would you get it adjusted or how does it get adjusted?

SuDZ

wait so if you slow down your speed when traveling at 80 miles/hour it could warp your rotors, cause where i am the speed limit is 65 and people in the left lane cruise at about 80. So somtimes i will need to break suddonly if a car is slowing down in front of me. Also how fast would i really be doing when it says 80 and i have 18's with 40's for tires?

Ok, I'm talking full on panic stop, not tapping the brakes. Doing it JUST ONCE will warp them from over 100, trust me. And as for how far off your speedo would be? http://www.miata.net/garage/tirecalc.html

Damn Medic, you have speedy fingers. :) Here's the verbose version.
What Medic meant was, uneven heat warps rotors and jamming on the brakes at high speed will heat up your rotors unevenly, especially if you keep your foot on the brake pedal after slowing down. Of course, if you're going that fast and you need to brake, just do it. It's the safest thing to do.

Regarding your second question, you need to know what size wheels and tires were originally equipped on your car. If you know that, go to http://www.miata.net/garage/tirecalc.html and see how much you're currently off by.