RE: Engine RPM at highway speed

[Jim Jablonski <jim@xxxxxxxxxxxxx>]

Oops.  Left a zero out of the RPM, but formula is still correct as I 
stated (see below).

Jim


Jim Jablonski wrote:
> 
> 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:
> 
> ((5000 x 14.75)/4.10)/168 = 107MPH
> 
> as opposed to 
> 
> ((5000 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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