Speedometers

[atgcpaul]

Driving into work this morning got me wondering why my car's speedometer is divided by 10s or even 20s but nearly all the posted speed limits I encounter end in 5s. Not that I follow the speed limit very closely, but I wonder if more people would drive closer to the speed limit if speedometers were went from 0, 5, 15, 25, etc...

I currently have a 2003 Mazda Protege and 2010 Hyandai Santa Fe--both are marked off by 10s. I found some pics of our 2000 Nissan Sentra and that was marked off by 20s.

[cwsmith]

Doesn't your car have "tick marks" indications for the rest of the gauge? My 2006 Dodge Caravan has marks for every mile but numbers only on the 'tens" as you indicate. (I had to go out and look.) My 1995 Miata only has the numbers for every 20 miles, and tick marks for every five in between.

There's really only so much room on the face of a gauge, speedometer or whatever. I guess the designer has to pick the best combination of number size and space allowed.

I guess never paid much attention to the actual placement of numbers. (Like I said earlier, I had to go out and look.) I guess they are good enough on either car for me to answer my wife's, "How fast are you going?", with an answer or "73", which is pretty much my cruise speed in a 65 zoned interstate.

I imagine that digital speedometers give you the exact speed, but I don't really know for sure. The few times I've put the GPS in the car, I think it showed the exact number.

CWS

[leehljp]

While not directly related to your specific comment about built in speedometers, - I have a Garmin that gives a very accurate speed readout, and I use that as a reference rather than the '09 Camry's speedometer. LOML's car has a built in navigation, but built-in Navis dont usually display the speed because that would be redundant with the speedometer. LOML wanted a Garmin similar to mine that would pick up the posted speed limit and indicate if she were speeding or not. On long trips, she uses both the on-board and the dash mounted to give more information.

As a result, we both have become accustomed to digitized speed display - along with the posted speed indicator.

[JimD]

I don't know why speedometers have numbers at the 10s and tick marks for other points but that doesn't bug me. What bugs me is inaccuracy. My Suzuki Grand Vitara is fine but my bimmer is off by about 5%. It reads faster than actual mph. It has an analog display and I added a digital by making changes in the cars software. The analog cannot be changed to read actual mph but the digital can. So I use the digital or my GPS (also a Garmin). I made a little custom bracket to put the GPS beside the gauges and wired it in. It is there all the time. It also provides the Bluetooth hands free phone capability in that car. I need to update it's maps.

But anyway, markings are not as annoying as deliberately displaying an inaccurate speed. German engineering is nice, usually..... Kind of like the metric dimensions on Festools...

[Stytooner]

I don't recall which car it was, but one of my older cars had 55 highlighted in red.

[TB Roye]

The speedo in my Wife Mini Cooper is off by 3 reads fast. From what I understand is it is common among BMW/MIni's to have in accurated speedos. The Mini has that BIG center mounted speedo and a tach with a digital speedo in front of the driver, that is what I use. When traveling I have my Garmin GPS and that speedo is dead accurate. When I worked for the CHP we used to Dyno all the new cars and fill out a slip with the correct speed and were allowed +- 3mph. In the field we used a radar gun to calibrate every other month. An Office drive and ran the radar guy and wrote the speed down. Now if you want something that is a joke try a Mini fuel gauge once it reaches 1/2 tank have now Idea how much is left. 13 gallon tank and it is all over the map when filling it up.

Tom

[cwsmith]

Not to high-jack the thread, but my son had a mini... I wasn't at all impressed. Like you said, the fuel gauge was not at all accurate, and his speedometer was off by close to five miles at highway speed. It was an automatic and developed a buzzing in the transmission, which was one of the reasons he ended up dumping it. According to the local BMW dealer (who won't service Mini's) the transmission is not serviceable and he would have to buy a whole new one... ordered from Germany, as they didn't stock anything in the U.S., they said.

Then he slammed the trunk lid down on too much stuff in the trunk, when he was moving and broke one of the top hinges. They are apparently riveted in and though a new hinge was less than a $100, the cost of getting it fixed was closer to $2K.... as they had to remove the interior: seats, headliner, side panels, etc. Nice car to drive, but service seemed a bit unreasonable.

Back to speedometers, IIRC my first van, a 91' Plymouth Voyager had the '55' marked in red also.

I waiting for the day, when you buy a car and instead of the conventional instrumentation, you'll find an I-Pad plugged in. The driver will configure the instrumention to suit their own taste and it will include several choices for custom panels, including phone, GPS, etc.

A couple of weeks ago I was at the Doctor's office (he's a pilot) and there were several copies of AOPA magazine there. If I was interpreting the articles correctly it's not only feasible, but actually in use, I think they referred to it as a "glass cockpit". IIRC Burt Retan employs something similar in one or two of his designs. Not sure if it's actually an I-Pad, or something similar.

[LCHIEN]

Spedometers have accuracy problems. The problem is that they are dependent on wheel diameter and revolutions to determine the distance traveled.
the larger the diameter the farther it travels on one revolution. But they can only count revolutions and depend on assuming the tire diameter.

My tires are 225/55 R17.
http://tire-size-conversion.com/tire-size-calculator/
says that the diameter of the tire is 26.7", the circumference is 84 inches and turns 754.5 revolutions per mile.

However a tire has 10/32" to 11/32" of tread when new, you're supposed to replace it when down to 2/32 but some people drive til its bald.

So there's a difference of as much as 11/32" in radius and twice that in diameter when new vs old. That's a 2.5% difference in the diameter of the tire.
Furthermore there's the conundrum of inflation and load flatteneing the tire some, decreasing its effective radius. I don't have a figures on that but if its on the order of 1/2" due to load and inflation then variation the change can be another 3.7%

So I can account for a possible 6.2% error in spedometer readings just there alone and that assumes that the spedo is absolutely correct, I suspect they use a spedo with only approximate gearing to indicate correct speeds given the range of tire sizes and differentials available for vehicles. The same inaccuracies apply to your odometer.

I've had some cars that were right on and other cars which were 3 mph high at 60 mph. According to GPS and to time measurements on mile markers on the highway.

GPS will be much more accurate for instantaneous readings.

[x00018]

My Honda motorcycle and Subaru both showed a faster speed. I was told it was because speeding tickets are very expensive in Japan.

[JimD]

BMW owns the Mini brand so they've apparently used their same obnoxious speedometer design in the Mini. The inaccuracy of a BMW has nothing to do with tire size. My digital is within 1 mph of my GPS but my analog is off about 5% - 3 mph at highway speeds. So my car knows how fast it is going but tells me it is going faster.

There are stories that BMW does this because of laws in Germany that penalize the manufacturer if the speedometer reads less than actual speed. I don't buy that either. They know how to make it accurate, they choose to lie, possibly thinking they will cause drivers to go slower.

[leehljp]

I waiting for the day, when you buy a car and instead of the conventional instrumentation, you'll find an I-Pad plugged in. The driver will configure the instrumention to suit their own taste and it will include several choices for custom panels, including phone, GPS, etc.

A couple of weeks ago I was at the Doctor's office (he's a pilot) and there were several copies of AOPA magazine there. If I was interpreting the articles correctly it's not only feasible, but actually in use, I think they referred to it as a "glass cockpit". IIRC Burt Retan employs something similar in one or two of his designs. Not sure if it's actually an I-Pad, or something similar.

There are Apps that can be downloaded that do "some" instrumentation for you on the iPad and iPhone; Altitude, speed, distance traveled, time traveled and more.

I posted a link a few months ago that Ryobi had some instruments / tools that plugged into the iPhone - such as a scope. With that in mind, I am sure there are some plug-in instruments along with apps that plug into the cars computer and let you create your own customized instrumentation on iPads, iPhones and Android tablets/phones.

I have one App on my iPhone that gives me the speed (which reads identical to the Garmin) and it has an altimeter which is quite off by 30 - 50 feet in altitude where I live. It has an interesting feature in that can reverse the screen so that you can set it on the dash and it reflects off of the windshield at night - looking somewhat like helmet/visor displays from fighter jets. Quite cool. Look right through it. I only tried it once for a couple of minutes before LOML said I was too distracted!

[capncarl]

I don't really have a problem with my speedo being inaccurate, it's the idiot fools that choose to ignore the instrument and violate the speed limits excessively. The habitual speeders. I live in a rural farming area, all 2 lane blacktop, herds of deer, slightly hilly curve wooded roads that maintaining 55-6o is about all a sane person would do or risk a deer coming through the windshield. I've noticed that most of the violators are young men in pick up trucks. I'm sure they are late for an important meeting less they wouldn't be going that fast. My newest complaint is tow truck drivers, I can't recall ever seeing one obeying the speed limit. If it is a roll back its rolling on. I know they have great brakes and can stop safely if they suddenly come on someone slower on the road, yea, right. In the past year we've had a number of people killed in accidents where truckers are speeding and are not able to stop in time and are run over. Each accident causes an uproar for mandatory speed limiters on trucks but it seems to go nowhere. I don't care if it causes the truck company more money to obey the law, let them run over their own wives and kids first and then tell me about their not making money.
capncarl

[Whaler]

I just bought a new Subaru XV Crosstrek and instrument wise they got it right with both an analog and digital speedo. My 78 year old eyes have trouble with just a quick glance at an analog speedo but no problem with the digital, much safer for me.

[LCHIEN]

Dick, I'm really glad to see its reading zero at the time of the picture!

[Whaler]

Dick, I'm really glad to see its reading zero at the time of the picture!

I thought that might be the safest way to do it.