JLai

Its funny how top speed run discussions are allowed.

I've heard it was electronically governed at 155, but I cannot verify first hand.

GiGGaplease

agree, why are we talking about 155, not like anyone is going to go out tonight and try to hit 155..

cskubis

You are welcome to your opinions but GPS measures speed by doppler shift, not by exact position. That is why GPS manufacturers can quote accuracy of velocity measurements like .1 knots (<.1 MPH). I would take anything that eliminates sources of error, hence speed gun or GPS.

This link gives some more info: http://www.caranddriver.com/features...dal_feature__1

bboysteele

I am still not 100% sure about your theory on GPS. There is still errors involved in that along with a speed gun. Speed guns I would agree would probably have the least amount of error but they still do. Speed guns are required to be re-calibrated to give accurate readings due to them losing accuracy over time (not sure if this is for both radar and laser but I think it is). GPS uses 3 satellites to calculate position or is it 5? I think it is at least three. Still they are not 100% accurate. I remember doing speed calculations in my PreCal or Calculus class and we had this same discussion but we were able to get the calculations more accurate by adding corrections to the math but that was a long time ago. I remember there were errors in the speed but not to the extent you have put. The article is good, I remember reading that, but my only complaint would be that it is 6 years old and who knows what has changed since then on how manufactures calculate speed. My bet would be that it has gotten better but I would also say there would be errors like you say but not as great of an error. Good find on that link!

rrrevin

Two links to check out.. the first deals with the speed rating of tires.
PLEASE PLEASE PLEASE everyone.... for your safety.. check out the speed rating of your tires BEFORE you attempt to see how fast your car can go. You don't want to be hitting 150+ mph only to have your tires blow out from underneath you.. very messy.

Please refer to the following link:
http://www.tirerack.com/tires/tirete....jsp?techid=35
Scroll down to the Speed Rating section.

The second link deals with a good way to gauge your speed and your speedometer.
http://www.tirerack.com/tires/tirete...currentpage=35

Black Betty

iTrader: (60)

Thanks for the link, very interesting and educational reading. I will check the accuracy at different speeds using the average speed function while cruising at different set speeds and see how they compare.

aznutx

i try

DrJones

iTrader: (1)

I haven't been brave enough to get her over 125ish. Plus HPD can be pretty unforgiving.

Black Betty

iTrader: (60)

No HPD on the track. If you're doing that fast on the roads they should put you in jail. If they can catch you!

JLai

Only if you're talking about road coursing. Our cars won't even be seeing 105 mph traps in the 1/4...Not stock, anyways.

BiskitG37

When Motortrend reviewed the new G37 last summer - they got a top speed of 146.

Sirhc7897

That would be what my non-manly version tops out at...

Seems strange to me that the Auto version would cut out lower than the 5sp version....The gearing is certainly there to do it? It is certainly stable at speed though...My Lightning used to get a little floaty over 145...

A one time measurement of speed using GPS could have the potential to be highly inaccurate as GPS units have a spherical position tolerance (IE when your handheld unit says position quality 15' your position is somewhere within a 15' radius sphere...) You may not actually be out by 15' but the unti can guarantee that the position it is displaying for you is within that tolerance of your actual position...You can jump from one side of this sphere to the other every time it re-measures (usually 1-5 second intervals) so you can see where a one time reading could be highly inaccurate and give a massive spike of peak speed. Incidentally you will never see a low spike as it never gets recorded in peak speeds (the mistake far too many people make when using handheld GPS for top speed is to look at the peak which can record a spike)...

However a GPS unit used to average a speed (not just using individual observations) is highly accurate (and this is the display you see when it is reading out in the "speed" setting rather than the "peak speed" setting...

I am a surveyor by trade and use the stuff every day...We use survey grade equipment (errors measured in mm's vs the feet of recreational units) and still see spikes in speed measurements...

But they are correct...your speedo will always read higher than actual speed...You will usually see it be fairly accurate up though posted speed limits (70mph) but the error will grow substantially after that...

djkymar

iTrader: (5)

When i was cruising around at night on dry clear road i hit 151 then i had to slow down due to the turn but the car feels so smoth at that speed. Also i notice that you cant set your cruise control if you going more then 95 it wont let you.

bboysteele

^^yes cruise control has a limit.

ucla bruin

iTrader: (7)

So I guess Black Betty's avatar is a shot of "his friend's" speedo at 149mph.....

Not sure if this is relevant but GPS does take 3 satellites to triangulate a position. There are errors associated with that calculation: user range error and a type of dilution of precision (often times GDOP, geometric dilution of precision). There could be geographic errors as well, for example, if your car is in a deep canyon, but we don't really run into that scenario often. So in terms of positioning for civilian usage, there is some degree of error. I'm not familiar with how the GPS receivers calculate for ground speed but if it relies on the satellites constellation, then it's definitely not perfect.
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