Re: speed vs RPM-Spiders

[Simon Favre <favres@xxxxxxxxxxx>]

Alan Lambert wrote:

>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.

Whew!  I used an old C program that's been floating around a while.
It's called "tiresize."  Here's what it gives for your tire:

% tiresize 205/60/14
Specification  Sidewall  Radius  Diameter  Circumference  Revs/Mile  Difference
205/60-14         4.8in  11.8in    23.7in         74.4in        852        0.0%

At 60 MPH, the 852 revs/mile = wheel RPM.  Multiply by 4.5556 and .79,
and you get 3066 RPM at 60 MPH.  Munge that around, and you get 19.57
MPH per 1000 RPM.  Pretty close.

The nice thing about tiresize is that I can feed it a series of sizes
for comparison:

% tiresize 175/70-14 185/65-14 205/60-14
Specification  Sidewall  Radius  Diameter  Circumference  Revs/Mile  Difference
175/70-14         4.8in  11.8in    23.6in         74.3in        853        0.0%
185/65-14         4.7in  11.7in    23.5in         73.7in        859       -0.7%
205/60-14         4.8in  11.8in    23.7in         74.4in        852        0.2%

Having gone through ALL that, my old rule of thumb is, take the number
off the tach (RPM/100). In 3rd, that's the same as MPH. In 4th, you
add 50%, in 5th, you double it to get MPH.

Yes, the faster traffic on I-280 is going 80+. ;=)

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