in oklahoma the weather is treacherous right now, the streets are completely covered in snow and slush....so tonight on my way home in my five speed, on a large four lane road, with noone else around, utilizing the "go straight even when braking traction nature" of the limited slip, i punched it in fourth gear to get them spinning, then slammed into fifth and kept it going, all the way to redline, all the way to 153 mph tire speed.......exhilirating, then promptly put it into third, not having veered to the right or left at all, and continued on my journey......i wouldnt recommend doing this, probably wasnt very smart, but it was worth it...have fun guys, be safe

 

So you were actually going 153, just spinning tires?

 

yes, my actual speed was about 40 mph probably, but my tires were going 153, bumping the rev limiter, only for a sec, dont want to damage my engine.(even given the stout history of the vq30de)

 

Not bad, but most likely with the car REALLY moving at that speed, you would have only been able to get it upto the 140-145 range TOPS

 

oh thats disappointing.....drag limited i guess?? well itll be a long time before i ever get to really try it out....have to have a safe place to try that speed for real

 

It is also true that at those speeds, the speedo can be a bit 'anxious'. One of the old(4-5 years ago) org discussions was about the inaccuracy of many speedos at that speed. I've had my speedo pegged at 140(on the autobahn), but was probably going no fast er than 125.

Zam

 

Oh really? Man how did I hit 150 once...hmm

 

Unless you clocked that with radar, it is probably innacurate.

Zam

 

Yeah, I was passing you at 143 when you hit 150.

 

Especially when I flew by the both of ya'll at 120.

 

I'ld say that speedo = 140 to actual = 125 is quite a big discrepancy. Are you sure that the differences are this BIG? If it is I'ld say it's time for Nissan to go back to speedometer making school?

 

At high speeds the inaccuracy get bigger

 

Hmmmm...must run in the family!!!



ADMAN

 

Ouch! Now I have to buy a manual to prove that I'm cool. It's true, c'mon. Someone please believe me!

 

who did that to your civic? just point him out man, we'll take him out back

 

I read in a review of the 2001 SE 5-speeds that top speed was drag limited. From what I've heard over the various discussions here it would appear that the autos have the speed limiter. All I know is I can't go over 132.5 mph. From my understanding, at that speed, the ECU cuts off fuel flow to the cylinders.

As far as speedometers being off at higher speeds. I've been able to use police radar on the last 4 vehicles I've owned. Each and every one has been 1-2 mph fast at city speeds (0-40 mph), 2-3 mph off at highway speeds (40-70 mph), and I've only measured up to 80 mph and it was 3-4 mph fast. I would suspect at 140 mph on your average speedometer you would acutally only be going 134-135 mph.

Tony

 

Depending on what combination you have, the GXEs with security and convenience(4th gen) weren't limited. GXEs without were stuck at 112mph. Not sure where that theory rests with the 5th gens.

I think C&D did a test on the accuracy of speedos at higher speeds. Most cars aren't very accurate at anything above 100mph. I wonder how many cars on the road have ever seen those speeds anyhow...

Zam

 

Yeah...it's pretty funny when you're racing an oldschool vehicle and you watch them shut down because of the limiters. Just be careful, I had that happen to me running up the backend of a 5th Gen G?E on the freeway. It was almost like he slammed on the brakes...but, without the brake lights!


ADMAN

 

Three letters: GPS I have a GPS handheld that measures instantaneous speed very accurately (.1MPH). I don't have my Max yet (it's on order), but the speedo in my Lumina is dead on the money all the way up to about 80MPH. I've not had occasion to push it further when I had the GPS unit in the car.

Comparing the speedo to radar is like comparing two unknows and trying to make any conclusions about either. Radar is usually only good to about plus or minus 5MPH. GPS is the real way to measure your speed.

Stereodude

 

Radar is usually only good to about plus or minus 5MPH. GPS is the real way to measure your speed.


Police RADAR is much more accurate than GPS. GPS uses satelites in orbit many miles away from you. The rotation of the earth, the direction you are travelling, road grade, etc., all have negative effects on GPS. Police RADAR uses radio frequencies and bounces them off a source (your car) and gets direct readings as a result of the returned frequency vs. the sent frequency. Police RADAR is even more accurate than your speedometer.

Tony

 

Obviously you don't understand how GPS or radar works. GPS is a triangulated signal based off of timecode. All the GPS satellite are in dead sync and broadcast the exact same signal. The GPS unit is not the same distance from all of them, so there is a delay in the signals it gets from some of the satellites. It knows where the satellites are and can determine how far it is from them. It can determine my position within 17 feet anywhere on the earth with GPS. It can measure how far I traveled even more accurately than 17 feet.

Radar on the other hand is a joke compared to GPS. It measures the red shift (doppler effect) of a radio signal that was reflected from the car. This is a far less accurate process. The fact that a radar gun is closer to the car is irrelevant. If radar is so accurate why do most states limit the police to plus or minus 7MPH? If a cop clocks you at 6MPH over the limit he can't use the radar as evidence in court against you because the radar is not certified for that level of accuracy. The other detail that you are missing is that radar only measure speeds parallel to it's own axis, so unless the person with the timing gun is dead ahead of the car direction of tracel, or dead behind its direction of travel the radar will only measure a percentage of the cars real speed.

Stereodude

 

I will stipulate that is sounds like you know much more about GPS than I do. However, I was already aware of the fact that different satellites are used to measure a vehicle's speed AND direction AND location. However, police RADAR is used only to measure speed, not direction or location. Its responsibilites are much less and therefore concentrates its technology one only one aspect. Vehicle speed.

Normally I wouldn't risk arguing the whole RADAR thing with how and risk being modetated. However, I don't want everyone reading this forum to get the wrong idea about police RADAR.

I don't know what you're talking about when you say "red shift", but you are correct that RADAR uses Doppler. The doppler effect states that the frequency of an object in motion is relative to the postion of the source and its listener. RADAR puts out a radio frequency (in the gigahertz, or billionth of a hertz) that is reflected off of a moving vehicle. The frequecny that is returned is different than the one received. It is either higher (with an approaching vehicle) or lower (with a receeding vehicle) and this is called the Doppler shift. The difference in frequency is then calculated by the RADAR and produces a vehicle speed. It is EXTREMELY accurate.

Most modern pieces of RADAR equipment truncates the speed it registers. If you are travelling at 48.8 mph, then the RADAR will register 48 mph. It will not say you are travelling at 49 mph until you are actually doing 49 mph.

You are correct that RADAR needs to be on-axis. To say that it needs to be "parallel" as you put it is the right idea, but not technically correct. RADAR measures speed, not direction. So the most accurate speed reading is when the source vehicle is moving directly towards the RADAR. If you are at an angle, the indicated speed on the RADAR will be lower than the actual vehicle speed. Stationary RADAR will always be in the favor of us, not the policeman's RADAR.

Your last question is WAY off base. You are suggesting that the "state" sets policies that cops can't pull people over for doing less than 6 over. Additionally, you state they can't use that RADAR reading in court because the RADAR is not accurate enough. You couldn't be more wrong.

Individual departments set policies for how fast cops can pull people over. State law determines (and Federal law) how "accurate" police RADAR is. Most State and Federal laws have taken judicial notice that a propertly constructed and operated RADAR is a accurate way to measure vehicle speed. The number one factor in RADAR inaccuracies is operator error. The real reason a lot (but not all) departments don't go pulling people over for doing 2-3 over is because they don't want to make "criminals" out of the majority of the public, who on average, do around 5 over the speed limit wherever they go (myself included).

If you have any more questions about police RADAR, please ask, but don't be giving out false information.

Tony

 

talking about safty.... 150+?

 

i wasnt actually doing 150 buddy, did you not deduce that?? should i use smaller words?

 

GPS is still more accurate than radar. When you have a 12 channel GPS reciever with 7-12 satellites it is incredibly accurate.

Red shift is the dopler effect. You are correct that the frequencies are very high. The waves that reflect back from the car are at a very slightly lower (or higher) frequency (as you said). The difference in frequency between the original wave and the wave that is reflected back from a car traveling along at 50MPH isn't very large. Actually its near microscopic. This is why radar guns require constant calibration. I'd rather not get out my engineering and physics textbooks to compute the frequency different of a Ka band wave after hitting a car going 50MPH.

It doesn't matter how many digits after the decimal place you have precision is nothing without accuracy. I can measure your speed to 58.021MPH, but unless I get the same # repeatedly those digits are useless

But thanks alot for repeating most of what I said. Especially about the whole angle thing. The vector of the radar gun and the vector of the cars motion have to be parallel as I said, but in order for that to happen you have to be either in the way, or in the cars wake. That's exactly what I said.

So the only thing we differ on is the accuracy of the RADAR gun. I've not spread any miss information as you're suggesting. Regardless of why the cops don't ticket you for going 6 over the fact is they don't. If they're forbidden by law because lawmakers think it's ****, or if they're forbidden because the RADAR isn't accurate enough who cares.

GPS is still more accurate than radar. You even said that radar only measures speed in one direction and errors in the drivers favor. GPS can measure your velocity (a 3d vector). But people complain that their speedo's read higher than radar and conclude that the speedo is wrong. If the radar is at a 5 degree angle by the time you're at 150 it's off by 13MPH. I don't see how you can legitimately claim that's more accurate than GPS. GPS actually will get more accurate (as a percentage of speed) the faster you go.

Stereodude

 

"The difference in frequency between the original wave and the wave that is reflected back from a car traveling along at 50MPH isn't very large. Actually its near microscopic."

The doppler shift (difference between transmitted and received frequencies) for K band RADAR is 72.023 Hz. So for every one mph that a vehicle is travelling, the doppler shift is 72.023 Hz. The operating frequency for K band RADAR is 24,150,000,000 Hz (yes, 24 billion). So, for a car travelling at the RADAR at 55 mph, the returned frequency would be 24,150,003,961 Hz. So, yes, the difference is very small, but regardless of this small number, the RADAR is still capable of calculating it just the same.

"This is why radar guns require constant calibration."

No, RADAR guns require PERIODIC (once or twice a year) calibration to ensure that they are working properly. It has nothing to do with the small shift in frequencies as you suggest. In addition to this, officers are required to check the RADAR for functionability and accuracy with tuning forks and internal RADAR checks daily. In reality, the vast majority of RADARS either work or they don't. The daily function checks and annual calibrations are required by law to ensure, on a mechanical basis, that they are working as they should.

"I'd rather not get out my engineering and physics textbooks to compute the frequency different of a Ka band wave after hitting a car going 50MPH."

I just did for you, using K band RADAR. Actually, Ka band RADAR uses a variety of frequencies, anywhere from 33.4 Ghz to 36 Ghz. So, for any given Ka band RADAR in use, the operating frequency can be anywhere in between these frequencies. At, let's say, 33.4 Ghz, the doppler shift for one mph would be 99.6 Hz. So a car travelling at the RADAR at 70 mph would get a returned frequency of 33,400,006,972 Hz.

"But thanks alot for repeating most of what I said. Especially about the whole angle thing. The vector of the radar gun and the vector of the cars motion have to be parallel as I said, but in order for that to happen you have to be either in the way, or in the cars wake. That's exactly what I said."

Actually, using the word "parallel" is incorrect. If a cop is sitting on the side of the road facing south, and I am travelling towards him northbound, the closer I get the the more erroneous reading he will get with the RADAR. This is because the angle of approach in relationship to the RADAR. Even if I am moving exactly parallel to him, the angle may still be off. So, if I am at an angle of 10 degrees, he may be getting a 30 mph reading when I am actually doing 33 mph. It will always be in favor of me. If I had my scientific calculator with me I would figure the exact numbers out for any given angle.

"I've not spread any miss information as you're suggesting."

Actually, you have. You have said that police RADAR is only accurate to within 5 mph. This is not true. It is extremely accurate. Much more so than your speedometer.

"I don't see how you can legitimately claim that's more accurate than GPS. GPS actually will get more accurate (as a percentage of speed) the faster you go."

RADAR will not get more or less accurate depending on speed. This is not the way it works. RADAR is just as accurate at 30 mph as it is at 100 mph.

I guess I will never be able to prove this to you. I am doing trying to prove my point. I have enjoyed telling my side of it and I truly respect your opinion as I hope you do mine. Thanks for your input on this and we probably should quit because no one cares about all this except for us!!

Respectfully,

Tony

 

According to the geometry of the situation, if the vector of the radar gun and the vector of the car's are parallel, but the car is not directly in front of or directly behind the radar gun the radar gun is not pointed at the car. So really what I said it correct. However I suppose I could have worded it better.

As for the 10 degree angle it will be off by .5MPH. The cosine of 10 degrees x the actual speed will give the speed it will read on the radar. That equals 29.5MPH. (I made a math error before when I said the radar would read 13mph off at 150 from 5 degrees. It will only read .6MPH off)

But this isn't getting anywhere like you said, so... I guess it's time to drop it.

Stereodude