Matt Beazer <teseract@moparhowto.com> wrote:
> Well, at a "True" 60mph the revs are at 2000rpm on the dot. I turned
> the overdrive off to be sure, and the revs jumped to ~2800.
> I came home and measured the tires as accurately as I could on my dirt
> driveway. The result is my mostly worn out (at the wear bars in the
> center) 31x10.50x15 tires on too narrow (6") rims are ~30" in rolling
> diameter. This makes sense considering all the tread is worn off.
> I get to climb under the truck on Friday and change the pitman arm and
> lube everything with Amsoil HD Synthetic grease. We'll see what the
> tags say on the diffs.
I'm guessing 3.91. You're real close, but something is still
missing... According to my calculations + assumptions, your true
speed at 2000rpm should be right around 66mph, assuming a true tire
height of 30", 3.91 rear and I assumed an overdrive ratio of .69:1.
With those same specs but changing the rear to 3.55, your speed at
2000rpm would be just under 73mph, which isn't even in the ballpark,
and you'd be trucking along at about 80 with 3.23s. (4.10s woulds put
you at about 63mph and 4.56 at 57mph)
I must be missing something though, because when I plug in the data
for your 2800rpm at 60mph out of overdrive (1:1), your speed should
actually have been just under 64mph to get 2800. ~2600 would be
closer to 60mph in 3rd with 30" tires and a 3.91. Its possible I
guessed your overdrive ratio wrong, but even so I would have expected
it to be closer than it is coming out.
Here's the formula in case you want to play with it:
http://faq.dakota-truck.net/Transmission_and_Gears/speed_from_engine_rpm.html
Another thing to keep in mind is that your tach may not be
accurate. An RPM of just over 1800 at 60mph would be about right for
your truck in overdrive, assuming a 3.91 gear. It might come down to
that, although there's probably no way to check that without using a
multimeter with a tach function or an aftermarket tach. I'm starting
to wonder if that might be in play here. It just occured to me that
both tach numbers you gave are almost exactly 200rpm above what the
calculations say it "should" be reading. Hmmm... ?
--
-Jon-
.- Jon Steiger -- jon@dakota-truck.net or jon@jonsteiger.com -.
| '96 Kolb Firefly, '96 Suzuki Intruder, Miscellaneous Mopars |
`-------------------------------- http://www.jonsteiger.com --'
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