RE: Engine RPM at highway speed

[Jim Jablonski <jim@xxxxxxxxxxxxx>]

Rich,

I believe there is an error in your formula below to determine MPH ... 
one needs to divide by 168 not multiply.  In other words:

((RPM x Radius)/Axle Ratio)/168

as opposed to

((RPM x Radius)/Axle Ratio)x168

Example:

((500 x 14.75)/4.10)/168 = 107MPH

as opposed to 

((500 x 14.75)/4.10)x168 = 3,021,951MPH

It's also important to note that the radius is measured in inches.

Not trying to show you up ... just thought I would point it out for the 
crew.

Regards,

Jim Jablonski
Royal Oak, Michigan
'64 Fury

> 
> 
> The formula for gear ratio and speed/rpm relations is:
> 
> 1) rpm times w, divided by r times 168 equals mph 
> 
> 2) mph times r, divided by w, then times 168 equals rpm
> 
> w = radius of tire
> r = rear axle ratio (actually final drive ratio)
> mph = miles per hour
> rpm = (aw come on you know)
> 
> In my younger years I made myriads of charts like this so I'd know how 
> fast I could go, theoretically. They work out quite close.
> I hope you can interpret the formulas written out this way. Of course if 
> 
> you know one you can swap to all the other combos.
> Rich Kinsley  '64 Polara 4 dr 318poly w/goodies


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