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Car Forum / BMW Cars / April 2004Factory specification on speedometer
How accurate is the speedometer supposed to be? i.e. what is the factory tolerance? +/- 2 miles per hour, or 2% or what? I drive my typical speed and get passed like I'm standing still. My 1998 740IL is off by 5 miles an hour accross the board. BMW tells me this is normal. > How accurate is the speedometer supposed to be? i.e. what is the factory > tolerance? +/- 2 miles per hour, or 2% or what? I drive my typical speed > and get passed like I'm standing still. This was mentioned in another thread some time in the past few days. The factory spec is that the speedometer can show up to actual speed + 10% +2.4 mph. Check http://ackthud.com/shawnfogg/pics/speed.jpg > How accurate is the speedometer supposed to be? i.e. what is the factory > tolerance? +/- 2 miles per hour, or 2% or what? I drive my typical speed > and get passed like I'm standing still. Thanks, The Dealer said +/- 10% but this makes more sense. I wonder why? My Ford is right on the money checked both by timing mile posts at 60 mph and via GPS. Court > This was mentioned in another thread some time in the past few days. The > factory spec is that the speedometer can show up to actual speed + 10% [quoted text clipped - 3 lines] > > tolerance? +/- 2 miles per hour, or 2% or what? I drive my typical speed > > and get passed like I'm standing still. For the speedo the be spot on is very unusual. Most places it is not permitted for the speedo to read too low, so the manufacturers adjust them to read a little bit on the high side. Does your Ford have the original size tires? If the tires are larger than stock (or over inflated) the speedo will read lower. > Thanks, > [quoted text clipped - 3 lines] > > Court My Ford, an Expedition, has original size tires. Given an earlier post, +10% +2.4mph would at 60 on the speedometer actually be 57.6 to 52.36mph. The +10% means they expect that tires will reduce in diameter by 10% over their life. Assuming a tire is 24" in diameter that would equal 2.4". I doubt that the average tread depth on a car like the Z3 is 1.2". More like 0.3 to 0.5". I think this just allows them to put a wider range of tire sizes on the car without making special adjustments to the speedometer. Just so I know how fast I'm really going, I'm going to take my portable GPS and take a run down the highway. > For the speedo the be spot on is very unusual. Most places it is not > permitted for the speedo to read too low, so the manufacturers adjust > them to read a little bit on the high side. Does your Ford have the > original size tires? If the tires are larger than stock (or over > inflated) the speedo will read lower. > My Ford, an Expedition, has original size tires. Given an earlier post, > +10% +2.4mph would at 60 on the speedometer actually be 57.6 to 52.36mph. [quoted text clipped - 3 lines] > 0.3 to 0.5". I think this just allows them to put a wider range of tire > sizes on the car without making special adjustments to the speedometer. The speedo is not set to +10% +2.4 mph from the factory, those numbers are for the maximum allowable error. Most people find that their speedometers are quite a bit more accurate than that. > The speedo is not set to +10% +2.4 mph from the factory, those numbers > are for the maximum allowable error. Tell me why in this day and age there should need to be this sort of tolerance? Even in the days of mechanical devices - the old eddy current types - many makers could easily better this. BMW - and some other makers - use this 'legal' tolerance to *deliberately* make their speedos over read. You can draw your own conclusions as to why.  Signature*Learn from your parents' mistakes - use birth control. Dave Plowman dave.sound@argonet.co.uk London SW 12 RIP Acorn > The Dealer said +/- 10% but this makes more sense. Don't think any speedo is designed to under read - it would break C&U regs in many countries. Over reading by up to 10% is a common reg - but a nonsense in these days of pulse counting devices. A cheap DVM will better 1% when measuring frequency, so apart from tyre wear there's no reason why a speedo shouldn't be at least as good. So saying, the clock on my E39 is rubbish as well, losing about 3 minutes per week. Signature*If horrific means to make horrible, does terrific mean to make terrible? Dave Plowman dave.sound@argonet.co.uk London SW 12 RIP Acorn >> The Dealer said +/- 10% but this makes more sense. > [quoted text clipped - 4 lines] >a speedo shouldn't be at least as good. So saying, the clock on my E39 is >rubbish as well, losing about 3 minutes per week. I love that word - rubbish. In my experience, the speedo is accurate within 2 mph at 85mph. If I set the Cruise so the needle is dead nuts on 85, then reset the Speed button on the OBC, the average speed the OBC displays settles in after a few seconds to within 2mph of the indicated speed. The indicated speed is always higher than the speed shown on the OBC. > How accurate is the speedometer supposed to be? i.e. what is the factory > tolerance? +/- 2 miles per hour, or 2% or what? I drive my typical speed > and get passed like I'm standing still. |
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