Scruit

I just got a Garmin eMap GPS unit for my wife who's going to be doing a lot of long-distance driving for work.

I took it out in the car yesterday and noticed the Max's speedo disagreed with the GPS... At Max40 it showed GPS38. Max50=GPS47.5. Max60=GPS57. Max70=GPS66 Max80=GPS75

So, I've been driving around boasting about passing speedtraps at 75mph and not being pulled, but in fact I was actually doing 70 in a 65.

Should I trust the GPS over the Max speedo? I have factory size tires & factory wheels.

Damn, I guess I should be driving everywhere at 80mph then...

Max4Speed

iTrader: (20)

GPS is wrong

Sprint

iTrader: (10)

maybe your tires are wrong size?

Newman

iTrader: (3)

i'd trust the GPS. the speedo isnt perfect but i never thought itwould be off by that much at such normal speeds. HOPEFULLY (no offense to you), but HOPEFULLY its just something wrong with your speedo specifically.

dm7297

iTrader: (1)

gps doesnt lie there within a mph

LeeIt2Me

Shouldn't trust the GPS, especially for consumer used ones. If it's not military grade, I wouldn't turst it. Consumer GPS are made with offset, so you don't really have a exact reading,unlike the military ones does. My pretty penny in this..

Max4Speed

iTrader: (20)

Exacty

dm7297

iTrader: (1)

id trust it more then the spedo in the car though

Sonic

And a very pretty penny it was.

Washington DC Maxima

Yep.

Max4Speed

iTrader: (20)

Thank You!

Badaxxima

Test your speedo. Mark off a 1/4 or 1/8mi., set your cruise at like 40mph and time it. 1/4 mile at 40mph should take 22.5 seconds. Then you'll know. Remember:
Distance = Rate x Time
My folks use a Garmin GPSMap in their plane and it's pretty accurate, albeit with about a 1-2 sec. delay.

Max4Speed

iTrader: (20)

BUT REMEMBER he was talking about his car? j/k may be thats the case.

JeepRage

iTrader: (1)

trust the GPS... its right. i've been selling them for 4 years, and they are very accurate for speed. differential or not. my speedometer is way off too... i have the factory SE 15's and tires, its off about 5mph by 75 easily... "your speed" signs are off too... you know, the radar signs on the side of the road. trust the GPS, nissan is wrong

Bags

iTrader: (3)

I would think it would depend on the delay.. military applications are updated on different rates... so if the delay is 3-4 seconds if he was doing 60 mph for 10 seconds should'nt the speed be correct after the delay and then the correction?

I thought gps units were updated at a rate somewhere in the .010 sec range. But I do not own one , so I am actually asking.

Max4Speed

iTrader: (20)

Asking or WHOREING?

Badaxxima

That'd be your FIFTH post on this thread, who's whoring?

for sHo

FLY by a cop going at a constant speed and get pulled over. Get ticket with your exact clocked speed. See if it's the same as the speed you were going at according to your speedometer. Then you'll find out how accurate/inaccurate your speedometer is. Plead not guilty, go to court, tell judge the whole story, print out this thread and show him. In most cases he'll let you get away with it, and even if he doesn't, hey it's worth it.

Max4Speed

iTrader: (20)

Bags

iTrader: (3)

glad someone else said it... OSS- i am truly asking... i only know mil. spec ones.. so i was seriously asking.

Max4Speed

iTrader: (20)

Oh ok dude.. good luck hopefuly someone will explain it.

Badaxxima

That's what I'm here for.
BTW, police radar guns aren't like ATC radar systems, if the target is more than 20 degrees outside of parallel, they're less accurate than either GPS or Speedos.

Stereodude

False! The SA (Selective Availability) system which made consumer grade GPS inaccurate was turned off May 01, 2000. It has not been turned back on in since. As a result a Garmin GPS unit will read just as accurated as a military GPS unit. Your speed as measured from a GPS unit is very accurate. It can pinpoint your position with 19' with a decent signal. For changes in position it is even more accurate than 19 feet. Due to the way GPS measures any two points could be off by 19' from actual position. However, they will be off the same amount and in the same direction, so the computed distance between the point it still right. Take a look at the diagram below.

A1 and B1 are you actual position. A2 and B2 are were the GPS reports your position. Now the GPS can correctly measure the time you took to get from A1 to B1 regardless of if A2 and B2 are in the right location. The distance between A2 and B2 is the same as the distance between A1 and B1, so the speed measurement is exact even though the points might be off slightly.

Stereodude

sinewave

Good idea, except I would suggest that you get a much larger sample; the larger the sample, the less error. Drive your car on the Interstate at 60mph for 10 miles and time how long it takes with a stop watch. Use the mile markers on the highway, not your odometer.

Bags

iTrader: (3)

here's your profile.. http://forums.maxima.org/member.php?...fo&userid=9636


so who's whoring now?

Badaxxima

I just like joking with people, look at my Sig.; besides, I do make valid posts sometimes.

Badaxxima

Oh and sorry if I come off like an A**. I usually do. I don't know many people who are REALLY into cars like you guys so I don't really have anyone to converse and debate with about autos (cars not trannies). Maxima.org; what a great thing.

ericdwong

http://forums.maxima.org/showthread.php?threadid=93872

aadro

I have to respectfully disagree.
(1) The offset was caused by the Department of Defense (DoD) introducing an error in the GPS signals from the satellites. The GPS receivers themselves had no offset built into them.
(2) The offset was of the order of +/- 350 feet/second and would not cause such a big difference.
(3) President Clinton signed an order removing the DoD offset, so that the military and consumer precision were the same. [HOWEVER, I DO NOT KNOW WHETHER THEY RE-INTRODUCED THE OFFSET OR EVEN INCREASED IT AFTER SEPTEMBER 11.]
(4) I have found Garmin GPS systems to be EXTREMELY accurate.

Summary:
Except for #3 UPPER CASE caviat, I would suspect something wrong with the speedo and have it checked.

Hope that helped!