Bill said:
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
The correct speedo for 7.50x 16 tyres is the 1408 figure, however the
difference is slight, approx 2% fast which is well within the accuracy
limits of the system, tyre wear, pressure etc. I would not bother to
change it.
The number represents the number of revolutions per mile of the speedo
cable.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
Bill, is that quite correct?...I always assumed it represented the number of tire revs per mile...OK then, let's see now...if my semi-retired mind can correctly remember some simple math...
To figure the outside circumference of a tire, take the sidewall height and multiply by 2, (remember that the diameter is made up of 2 sidewalls, the one above the wheel, and the one below the wheel) and add the diameter of the wheel to get the true diameter. So for the 7.50x16 tire...we first need to know the aspect ratio to calculate the sidewall height...early numeric tire sizes had a 90% aspect ratio...so 7.50" x .9 = 6.75" sidewall height, thus...
oh, why not make this simple and just take the measurements from tire rack's web site at:
http://www.tirerack.com/tires/tiretech/lighttruck/truksize.htm
...which states:
31 1/2" diameter equates to tire sizes > P275/70R16 LT235/85R16 LT265/75R16 32X11.50R15LT 7.50R16
...so...
31.5" .../12" per ft = 2.62' diameter...circumference = pi x D ...3.14 x 2.62' = 8.22'(distance covered in 1 tire revolution)
...1408 x 8.22' = 11573.76'.../5280 ft per mile = 2.19 miles, so my assumption was indeed incorrect.
So it would appear that the speedo cable must turn 1 rev per every 2 revs of the wheel. Since the speedo drive energy is derived from friction, how is this ratio actually accomplished?
...comments, please...
JT/ric
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