RE: Engine RPM at highway speed

[Richard Kinsley <r.kinsley@xxxxxxxxxxxxxxxx>]

Brian Schlump wrote:
> 
> I'm puzzled by the difference in engine RPM with my 2 cars, that each 
> have a 3.23 Sure Grip.  I usually drive at 62 mph on the 2 lane roads in 
> 
> our area.  My '64 Belvedere has a 426 Max Wedge with a 4 speed 
> transmission and P245/60/15  rear tires.  It tachs at 2,200 rpm at 62 
> mph.  I recently had a '68 Charger restored and it has a 440 engine with 
> 
> a 727 big block transmission and F70/15 tires.  It tachs at 3,300 rpm at 
> 
> 62 mph.  I've checked the rpm on both cars with a second tach and I've 
> had my wife follow me in her new car to verify that I'm doing 62 in each 
> 
> one.  The 727 was rebuilt as part of the restoration.  It shifts through 
> 
> all gears and downshifts into 2nd when I hit passing gear on the 
> highway.  The fluid hasn't discolored in 600 miles, and doesn't smell 
> burnt.  Today the restoration shop installed a new converter, but that 
> had no impact on the rpm at highway speed.   Does anyone else have a 727 
> 
> with a 3.23 rear end combo?  If so, what engine rpm do you run at 
> highway speeds?  Any other ideas?
> Brian
> 
> '64 Belvedere
> Woodstock, IL.
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The final ratio in your 4 speed is 1:1 I'm sure and the 727 should be 
direct drive also at 1:1 so the tire size should be the factor since the 
rear-ends are the same. I wonder if you have something other than 3:23 
in one of them?

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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