In a message dated 1/5/2009 5:26:13 PM Central Standard Time, DLylis@aol.com
writes:
> So. . .If I am tooting along in fourth gear at 3000 rpm with a 3.70 diff I
>
> should be going 60 mph +/-. However I have a 4.10 diff which is a 10%
> reduction so I am actually going 54 mph. Then. . .I flip on the Jtype OD
> and take
> it back up to 3000 rpm and I am now going 72 mph. OR. . .I maintain 54 mph
>
> and the rpms drop to 2400. Yes?
>
Doing the math, with a 4.10:1 diff, you will be running 54 MPH @ 3000 RPM (as
you stated) and flipping on the O/D will drop your RPM's to 2400, also as you
stated. But bringing the engine speed back up to 3000 will bring the car's
velocity to only 68 MPH.
Figure it this way: with a 3.70:1 diff and overdrive, 3000 RPM will give you
75 MPH. 75 MPH *(3.7/4.1) = 67.6 MPH. Just like 60 MPH *(3.7/4.1) = 54 MPH
without overdrive.
Also consider each 2 MPH tick on the tack as roughly equal to 4 MPH in direct
and 5 MPH in overdrive. With these two benchmarks and the increments, a
little mental math will give you your speed reasonably closely.
BTW, my calculations for the J-type tells me 3000 RPM in overdrive is closer
to 76 MPH, but who's counting?
Cheers,
Dave
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