At 01:05 AM 4/15/98 -0500, you wrote:
>Michael Z-Sykes wrote:
>>
>> I read somewhere (minitruckin' I believe) that when a speedo is off, it is
>> off my a percentage of the speed. Hence, if you are 2mph off at 20, then
>> you'll be 6mph off at 60. I would go by that,
>
>I believe that is correct also, it only makes sense. But mine is 3 to 5
>off at highway speeds. At 5mph it's probably dead on...
>
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WARNING: The following post started out quite simple but sort of mutated
into a horrible Frankenstein sort of creation. It is more likely than
not extremely boring, but if anyone is uncertain about this speedometer
error stuff, it might help to sort it out in your mind; assuming you
can stay awake. Ummmm, BWAAAHAHAHAHAHAHAA!!! :-)
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Yep, that makes sense since the speedometer error is a function of the
RPM of the wheel. The thing that really affects speedometer error is
the circumference of the tire. (Which we determine from the height times
Pi) What we are actually measuring is the distance that the tire travels
in one revolution.
The speedometer uses the tire's RPM to calculate the speed; 100
rpm is 60mph, 200 rpm is 120mph, etc. (That's not to scale; I'm just
trying to get a point across.) :-)
Put simply, consider a tire that has a circumference of 10 inches,
which we replace with a tire that has a circumference of 11 inches.
The speedometer doesn't know that the tire has been replaced; it is
still calibrated to a tire that travels 10 inches per revolution.
AT speedo thinks a 10" tire is going 11" tire is actually going
1 rpm 10 inches/min 11 inches/min
10 rpm 100 inches/min 110 inches/min
100 rpm 1000 inches/min 1100 inches/min
1000 rpm 10000 inches/min 11000 inches/min
So, as the tire spins faster, the error becomes greater.
AT speedo reads actual speed speedo error
1 rpm 10 inches/min 11 inches/min 1 inch/min low
10 rpm 100 inches/min 110 inches/min 10 inches/min low
100 rpm 1000 inches/min 1100 inches/min 100 inches/min low
1000 rpm 10000 inches/min 11000 inches/min 1000 inches/min low
If we had an 11 inch tire that was decreased to 10 inches, the same
thing would happen except the speedo would read high. In the case above,
the speedo is off by 90.9%. You can figure out the actual speed you are
traveling by dividing what the speedo reads by a correction percentage.
That correction percentage can be determined by dividing the circumference
of the old tire by the circumference of the new tire. (In the case above
that is (10/11), which is 90.9%; 10/.909 = 11, 100/.909 = 110, etc.)
For a real-world example, say I want to find out the speed I am actually
travelling when the speedo reads 5mph, 20mph, 60mph, and 100mph for the
following tire swap:
Old tire: 215/75R15
New tire: 275/50R15
The circumference in inches of a tire XXX/YYRZZ can be calculated like:
((((XXX * (YY / 100)) / 25.4) * 2) + ZZ) * Pi
(XXX is width, YY is sidewall ratio, ZZ is inner diameter)
Old tire: ((((215 * (75 / 100)) / 25.4) * 2) + 15) * Pi = 87.01222172443"
New tire: ((((275 * (50 / 100)) / 25.4) * 2) + 15) * Pi = 81.13719609271"
So, the correction percentage would be:
(87.01222172443 / 81.13719609271) * 100 = 107.2408536585%
5 mph * 1.072408536585 = 5.3620426829 mph (0.3620426829 mph high)
20 mph * 1.072408536585 = 21.4481707317 mph (1.4481707317 mph high)
60 mph * 1.072408536585 = 64.3445121951 mph (4.3445121951 mph high)
100 mph * 1.072408536585 = 107.2408536585 mph (7.2408536585 mph high)
The faster you go, the greater the error, although the *percentage*
of error remains the same. (Keep in mind that the precise numbers above
will only work in a perfect world. The real world has lots of slop that
could skew the results one way or the other, but the basic premise is
still valid.)
Anyone still awake? ;-)
If I screwed something up, corrections are welcome. :-) You thought
that the the spark plug thread was bad? A little more of this, and you'll
be beggin' for plug tips. ;-)
Hmmm, this might make a decent WWW/CGI program...
-Jon-
.--- stei0302@cs.fredonia.edu -- http://www.cs.fredonia.edu/~stei0302/ ---.
| DoD# 1038, EAA# 518210, NMA# 117376, USUA# A46209, KotWitDoDFAQ, RP-SEL |
| '96 Dodge Dakota v8 SLT Club Cab, '96 Kolb FireFly 447 (#FF019) |
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I do not speak for the SUNY College at Fredonia; any opinions are my own.
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