Speed vs RPMs

[LEN_LEEB@xxxxxxxxxxxxxxxxxxxxxxx]

     Jaap asks about the speed vs rpm equation:
     
     >speed in mph = (rpm*60)*3.1417*(14/12 + 
     >(20.5*2*0.6/2.54)/12)/5280)/4.5556/.079
     >
     >in words, speed in mph equals tach rpm times 60 minutes/hour times PI 
     >times the rolling diameter ( which is 14inches divided by 12 plus 
     >(205mmdivided by 10) times the aspect ratio divided by centimeters 
     per inch divided by inches per foot divided by feet per mile divided 
     by the rear end ratio divided by the transmission ratio.
     > But is it realistic to assume the rolling diameter is not affected 
     by compression of the tyre where it meets the road surface?
      
     I agree! When I figure my speeds in gear, I use the manufacturer's 
     numbers for revs per mile. This is obtainable in the mfr's brochure.
     For instance, the original tires on my Spider, 185-70x14, were listed 
     by Pirelli as 851 revs per mile. This is a much easier number to use 
     than tire diameter plus width times aspect ratio, less some constant 
     (variable?) to account for the tire "squish" under load.
     For those who don't want to do the math, here's my numbers:
     
     1st gear ......  5   mph/1000 rpm
     2nd gear ......  8.5  "    "   "
     3rd gear ......  13   "    "   "
     4th gear ......  17.5 "    "   "
     5th gear ......  22   "    "   "
     
     The above is based on gear ratios of 3.32, 1.99, 1.35, 1.0 and 0.79 in 
     1st thru 5th, with a final drive ratio of 4.1, and the above-mentioned 
     tire size. I use the chart to get a quick check on speed with only a 
     glance at the tach.
     Len

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