wish i could do this

 

Over the past few days, I used 3 of my company $1,000 GPS units to verify the speedometer error in my 03 G35 5AT. At 40mph it's spot on, at 60mph it's off by ~1.5mph+ (ie read high), at 70mph it's off by 2.5mph, and at 90mph it's off by 3.5mph. This data was verifed by using three different GPS units on seperate occasions. I'm not certain of accuracy of the GPS speed calculation, but I know that the units get us to within inches of our survey points vs the standard bought GPS which gets you to within 8 to 30 feet. I would assume the GPS is more accurate than the car's system.

Using the data I gathered, I would assume that at speeds over 140mph, the speedometer is reading anywhere from 5 to 8mph higher than actual. This is not uncommon. The $110K Porsche 911TT indicates 203mph, but in Motor Trend tests, it's topspeed is around 195mph. Motorcycle speedometers are notorious for being wildly off at speeds above 100mph.

 

thanks Dave, what is the diameter of ur tire?

 

I've used various GPS products and software for tracking commercial vehicles for years... many give current speed, avg speed and all that (actually much, much more)... but I wouldn’t trust it to be anymore accurate than a speedometer... in fact, personally I think the speedo is more accurate than the best systems I've used...

The only way to get a true reading is laser or radar gun

 

Car and Driver's testing procedures state that they always use a Racelogic Vbox (GPS tracking) to do their testing.

 

^^^ oooo... in that case it must be accurate... we all know they are the say all end all...

 

Is GPS that bad? I would have guessed that the oem speedo is notoriously out of whack. And would only get worse once everyone puts on aftermarket wheels/tires. Even tires of the same rated sized might have an actual OD diff than oem.

 

its not necessarily "bad" - but I would never rely on it for absolute accuracy any more than I would rely on OEM speedo with OEM everything...

If someone wants to dispute speed accuracy IMO a radar or laser gun would be the method of choice... I’ve seen significant deviation from various GPS products (industrial and commercial products)...

Anyway - I'm not going to get into a GPS speed calculation value debate... I just wouldn’t consider its reading as the definitive.

 

I wasn't asking the absolute accuracy of GPS. Just how accurate it is relative to the oem speedo. Especially when most of us aren't on oem or oem sized tires anymore.

 

I don’t think either are an accurate measure of speed. They are good indicators... I would trust a speedo as much as GPS... the difference is GPS can be documented.

 

I would think the GPS would be more accurate especially if you're traveling at a steady rate of speed. Speedometers, even electronic ones like the G has, are notorious for being off at speeds above 100mph as proven by numerous mags when they do top speed supercar tests. With GPS, you're being tracked by at least 3 satelittes and sometimes as many as 9, all trianglating your postion on the earth. The $1,000 units we use are far more accurate than your store bought units and we have a couple $5,000 units which will get you to within tenths of an inch which seems nearly impossible, but they work. Those don't work in the car though

http://en.wikipedia.org/wiki/Speedometer

Speedometers are not totally accurate, and most speedometers have tolerances of some 10% plus or minus due to wear on tires as it occurs. Modern speedometers are said to be accurate within 5% but as this is legislated accuracy, this may not be entirely correct. This can make it difficult to accurately stay on the speed limits imposed; most countries allow for this known variance when using RADAR to measure speed, although levels of some 3 km/h or 3% are also used in areas of tough enforcement. This causes many arguments due to motorists complaining that they were not doing the speed as reported. Revenue[3] is being increasingly blamed for these stricter measures. There are strict United Nations standards in place but it seems not being enforced leaving this matter in limbo for many countries. Excessive speedometer error after manufacture can come from several causes but most commonly is due to nonstandard tire diameter

Using the 3% to 5% rule, that means at an indicated 150mph, you're most likely doing between 143-145mph. On shorter tires like many run (.5" shorter commonly), then you're talking about even more variance.

 

I have hit a little over 130 and if felt like there was more to go. Ive always wanted to go like 160+.. such a rush

 

Dave and Jeff... you are both splitting hairs between speedo and GPS - when all my initial post said was GPS isn’t entirely accurate either... if you want a measure of true actual speed, I wouldn’t use either – they are both good “indicators”... speedo is fine to get an idea.

Dave, I don’t need to be schooled on GPS systems, I've been using them in the transportation industry for many years...

There are more accurate measures of vehicle speed.

 

I just did some research on the internet and you'll see that most GPS companies are claiming 0.1 to 0.5mph accuracy at speed all the way to 999mph. Like someone pointed out earlier, the mags have been using GPS to time thier cars so the systems have to be pretty dang accurate. I know you work in the transporation industry, but have guys actually proven the GPS units aren't accurate in tracking speed? The only way to do it would be with radar/laser. Since the mags and GPS industry claim exceptional accuracy, I would assume they're claims are not outragous.

 

In my experience there have been minor to sometimes moderate inaccuracies with GPS... however, they are very valuable for many reasons (besides speed)… The manufacturers are marketing their products, of course they will say its nearly perfect... magazines are gawd.

End of debate.

Its quite futile actually, because no one in this thread is going to have a GPS system with anywhere near the ability that we are talking about anyway...

So again - for the point that this thread began... the speedo is as good of an “indicator” as anything else... for “accuracy”, any one of us can gain access to a laser or radar gun.

Plain and simple. We can stop beating it to death now.