So either my measurements I took in the dark on my dirt driveway are off (no
big surprise) or my tach is off 150-200rpm (also no big surprise) or a
combination of the two. I dropped the formula into Excel and I agree with you,
I think it's safe to say I have a 3.91 rear end, since I doubt anyone would go
through the trouble to swap the front and rear ring and pinion for $250+ each
plus labor to put in 4.11's. Which is fine with me, the 3.91 with the external
tranny cooler, power steering cooler and V8 means if I ever feel like buying a
hitch hauling something heavy I have the perfect setup to do it without the
fuel economy the 4.11's would give me. I'm much happier than I'd be with the
3.23's considering if I want fuel economy I can hop in my Neon. :)
Thanks for your help,
MattB
In article <gu9dde$7rr$1@bent.twistedbits.net>, jon@dakota-truck.net writes:
>
>
> 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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