Electronically limited top speed (BMW)

[Banchi105]

Found the article- interesting read:

Originally Posted by Car and Driver Speedometer Scandal! Can you trust your most frequently consulted gauge? BY FRANK MARKUS April 2002 Regular readers have probably noticed that when we describe a vehicle that really gets our juices flowing, we tend to hyperbolize about the accuracy and precision with which the steering wheel and pedals communicate exactly what is happening down where the rubber meets the road. It has recently come to our attention, however, that many of the cars we like best are surprisingly inaccurate about reporting the velocity with which the road is passing beneath the tires. Or, to put it another way, speedometers lie. Yes, ladies and germs, we are scooping 20/20 and 60 Minutes with this scandal: Speedometers Lie! Okay, "exaggerate" may state it more aptly, if less provocatively. When traveling at a true 70 mph, as indicated by our highly precise Datron optical fifth-wheel equipment, the average speedometer (based on more than 200 road-tested vehicles) reads 71.37 mph. Wait, wait! Before you roll your eyes and turn the page, let us dig just a bit deeper and reveal some dirt. Sorted by price, luxury cars are the least accurate, and cars costing less than $20,000 are the most accurate. By category, sports cars indicate higher speeds than sedans or trucks. Cars built in Europe exaggerate more than Japanese cars, which in turn fib more than North American ones. And by manufacturer, GM's domestic products are the most accurate, and BMW's are the least accurate by far. One other trend: Only 13 of our 200 test speedos registered below true 70 mph, and only three of those were below 69 mph, while 90 vehicles indicated higher than 71 mph. Are our cars trying to keep us out of traffic court? To understand, let's first study the speedometer. In the good old days, plastic gears in the transmission spun a cable that turned a magnet, which imparted a rotational force to a metal cup attached to the needle. A return spring countered this force. Worn gears, kinked or improperly lubed cables, tired springs, vibrations, and countless other variables could affect these mechanical units. But today, nearly all speedometers are controlled electronically. Typically, they are driven by either the vehicle's wheel-speed sensors or, more commonly, by a "variable reluctance magnetic sensor" reading the speed of the passing teeth on a gear in the transmission. The sine-wave signal generated is converted to speed by a computer, and a stepper motor moves the needle with digital accuracy. Variations in tire size and inflation levels are the sources of error these days. Normal wear and underinflation reduce the diameter of the tire, causing it to spin faster and produce an artificially high reading. From full tread depth to baldness, speeds can vary by up to about two percent, or 1.4 mph at 70 mph. Lowering tire pressure 5 psi, or carrying a heavy load on the drive axle, can result in about half that difference. Overinflation or oversize tires slow down the speedometer. All our speed measurements were made on cars with new stock tires correctly inflated, but one might expect a manufacturer to account for wear and to bias the speed a bit low; results suggest that not to be the case. So we sought out the rule book to find out just how much accuracy is mandated. In the U.S., manufacturers voluntarily follow the standard set by the Society of Automotive Engineers, J1226, which is pretty lax. To begin with, manufacturers are afforded the latitude to aim for within plus-or-minus two percent of absolute accuracy or to introduce bias to read high on a sliding scale of from minus-one to plus-three percent at low speeds to zero to plus-four percent above 55 mph. And those percentages are not of actual speed but rather a percentage of the total speed range indicated on the dial. So the four-percent allowable range on an 85-mph speedometer is 3.4 mph, and the acceptable range on a 150-mph speedometer is 6.0 mph. But wait, there's more. Driving in arctic or desert climates? You're allowed another plus-or-minus two percent near the extremes of 20-to-130-degrees Fahrenheit, and yet another plus-or-minus one percent if the gauge was ever exposed to minus-40 to plus-185 F. Alternator acting up? Take another plus-or-minus one percent if the operating voltage strays two volts above or below the normal rating. Tire error is excluded from the above, and odometer accuracy is more tightly controlled to plus-or-minus four percent of actual mileage. 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. Not even if somebody swaps a big set of 285/35R-18s for stock 255/45R-16s. There's your explanation of high-reading European speedometers, with the highest readings on Porsches and BMWs that are most likely to lure owners inclined to fool with tire sizes. Of course, only the speedometer must conform. Trip computers are free to report average speed honestly. Try setting your BMW or Porsche cruise control and then resetting the average-speed function. Unless you've screwed up the tires, the trip computer should show a nearly accurate reading. Even General Motors, whose domestic speedometers are the best, must skew its readings slightly high on vehicles exported to Europe. So there you have it: the raw, unvarnished truth about speedometers, laid bare without the underhanded aid of secret pyrotechnics. Readjust your comfortable indicated cruising speeds accordingly.

Heres a link to the actual results by manufacturer: http://www.caranddriver.com/article....rticle_id=1907

[RC45]

GM's domestic products are the most accurate, and BMW's are the least accurate by far

That's all I needed to read.. I know I have never had an M3 able to chase me past 150mph - at which time they thought they were going 170mph...

[No.1]

Well, the CSL was certainly good for 156mph in this guy's case - i wonder what spped the speedo showed as he was clocked going this fast?

:arrow: http://news.bbc.co.uk/1/hi/scotland/4488668.stm

[DeMoN]

so its a known issue about BMW speedo lying? Damn! He prolly saw 172mph as I did too. Don't se why the CSL should have a heigher top speed by a noticeable margine vs the original M3. Now on cornering, thats another story.

[ZfrkS62]

"We have to say that, in our view, it defies common sense to suppose that he was doing no more than just holding the telephone and not actually using it."

???

why else would he be holding it to his ear?????

[T-Bird]

I know that my T-bird had a rev and speed limiter in it to keep the driveshaft from turning into a banana and it was also limited so as not to be as fast as the Mustang.

[ae86_16v]

Originally Posted by DeMoN so its a known issue about BMW speedo lying? Damn! He prolly saw 172mph as I did too. Don't se why the CSL should have a heigher top speed by a noticeable margine vs the original M3. Now on cornering, thats another story.

Yeah, like I said in the other thread, most of us know that we are definitely going slower than indicated. The CSL probably because of aerodyanmics, lighter and a slight more HP engine.

Originally Posted by RC45 GM's domestic products are the most accurate, and BMW's are the least accurate by far That's all I needed to read.. I know I have never had an M3 able to chase me past 150mph - at which time they thought they were going 170mph...

Yeah 150MPH is pretty damn fast. Although the E46 should be able to hit it no problem.

[ZfrkS62]

Originally Posted by T-Bird I know that my T-bird had a rev and speed limiter in it to keep the driveshaft from turning into a banana and it was also limited so as not to be as fast as the Mustang.

so are you saying ford didn't know how to balance a driveshaft in the 80's?

[T-Bird]

this was from 94-97 and it wasn't that as much as they didn't want to spend the money I guess since the one from the Mark 8 could be put in it's place and the limiter removed so that wasn't an issue. I've got an all Aluminum one-piece shaft in my car from a Mark 8 courtesy of my dad before I had the car and that allowed him to have the limiter removed the rev limiter is still capped though since the stock converter balooned at high rpms.

[ZfrkS62]

interesting. those converters must be pretty small to no like high rpms.

[T-Bird]

it's actually a 10.5 inch if I recall but I plan on going with a custom 9 incher which is stronger and can take the revs my new engine needs.

[ZfrkS62]

damn, i forgot about the new engine, how's it working out for you so far?

[T-Bird]

the engine is great it runs good but there is a damn vacuum leak somewhere in the system and I haven't had alot of time to hunt it down but I did find a couple of them. so basically the thing doesn't like to start right away and the thing sputters at first when hitting the gas. I have a new trans on the way so that's next on the list since the one I have is starting to slip. I will say the thing get's up and goes now. I did find out the asshat who put on my valve covers didn't torque them down and I had to pull the motor mount and tip the engine to get to the bottom bolts that were spewing oil out all over my exhaust and the road behind me.

[ZfrkS62]

damn, that sux about the valve covers.

[T-Bird]

tell me about it the hardest part was getting the damn motor mount bolt back in