Originally Posted by calpalmer' post='750091' date='Dec 21 2008, 06:29 PM

I don't really disagree with what you're saying.... but I think it's more than this (i.e., the American litigiousness...). If that's all it was, you'd see ALL manufacturers doing the same things with their speedo's, but most do not opt for the 2-3mph at 60. But several German and Scandanavian manufacturers do, and as I said earlier I have read somewhere that there is a regulatory reason for this.... Can't verify it, and can't really remember where I read this, but maybe someone from the continent will embellish.

Cal

Originally Posted by DD_545i' post='750031' date='Dec 21 2008, 04:26 PM

I can't believe there were so many posts in this thread without one person yet stating why the speedometers are **ALL** inaccurate.

ALL speedometers are inherently inaccurate. It's a very difficult task to display the correct speed at all speeds. You're either 100% accurate around low speeds like 30mph, or you get mid-range speeds like 90mph right, or you get the upper speeds like 150mph right. No speedometer has ever been fully accurate at all of them.

So, with speedometers being basically inaccurate to some extent, what's a manufacturer to do? They only have one option and that's to tune the speedometer so that it under-reads by a couple or three mph. The result of doing that is that you'll get a range of inaccuracy of between 0mph and perhaps as much as 5mph, but it will ALWAYS be under reading.

Imagine the situation where they weren't under-reading. You look at your speedometer and it's showing 55mph and you're in a 55mph zone, and you get a speeding ticket for doing 57mph. How quickly will the American lawyers be all over that and suing the car manufacturers? Oh you bet there'll be a lot of class action law suits, especially in America where two thirds of all the lawyers in the world live (to service 5% of the world's population).

Simple fact about it is, speedometers are inaccurate generally, and car manufacturers adjust them to under-read because nobody can sue them for displaying 55mph when you're actually driving only 52mph.

The person who said their speedometer was spot on accurate, try it at lower and higher speeds and you'll find it under reading. You must have found a lucky sweet spot where it matched the GPS speed. If BMW were to notice it, they'd adjust it to under read a bit more for you

Whenever I'm setting my cruise control on a stretch of road with a high risk of mobile police radar guns appearing, I always add an extra few km/h for speedometer inaccuracy.

Originally Posted by DD_545i' post='750031' date='Dec 21 2008, 04:26 PM

I can't believe there were so many posts in this thread without one person yet stating why the speedometers are **ALL** inaccurate.

ALL speedometers are inherently inaccurate. It's a very difficult task to display the correct speed at all speeds. You're either 100% accurate around low speeds like 30mph, or you get mid-range speeds like 90mph right, or you get the upper speeds like 150mph right. No speedometer has ever been fully accurate at all of them.

So, with speedometers being basically inaccurate to some extent, what's a manufacturer to do? They only have one option and that's to tune the speedometer so that it under-reads by a couple or three mph. The result of doing that is that you'll get a range of inaccuracy of between 0mph and perhaps as much as 5mph, but it will ALWAYS be under reading.

Imagine the situation where they weren't under-reading. You look at your speedometer and it's showing 55mph and you're in a 55mph zone, and you get a speeding ticket for doing 57mph. How quickly will the American lawyers be all over that and suing the car manufacturers? Oh you bet there'll be a lot of class action law suits, especially in America where two thirds of all the lawyers in the world live (to service 5% of the world's population).

Simple fact about it is, speedometers are inaccurate generally, and car manufacturers adjust them to under-read because nobody can sue them for displaying 55mph when you're actually driving only 52mph.

The person who said their speedometer was spot on accurate, try it at lower and higher speeds and you'll find it under reading. You must have found a lucky sweet spot where it matched the GPS speed. If BMW were to notice it, they'd adjust it to under read a bit more for you

Whenever I'm setting my cruise control on a stretch of road with a high risk of mobile police radar guns appearing, I always add an extra few km/h for speedometer inaccuracy.

Originally Posted by audit5050' post='750255' date='Dec 22 2008, 07:15 AM

It may be over 6 years old, but this article from Car and Driver lends some insight into speedo numbers.

http://www.caranddriver.com/reviews/hot_li...eature/(page)/1

Originally Posted by DD_545i' post='750271' date='Dec 21 2008, 10:52 PM

The key part from that article being:

"The European regulation, ECE-R 39, is more concise, stating essentially that the speed indicated must never be lower than the true speed or higher by more than one-tenth of true speed plus four kilometers per hour (79.5 mph at a true 70). Never low."

To meet this rule of never indicating lower than actual speed, and accounting for the inaccuracy of speedometers generally (even if they're a lot more accurate these days), you simply have to under-read.

Originally Posted by porsche911targa' post='750615' date='Dec 22 2008, 08:52 PM

I did the comparison of the odometer with the mileage on the GPS, this morning. I drove exactly 20 miles according to the GPS and the odometer was also a match at 20 miles even though my speed was off by three miles the entire way. I don't know how they do it