I did it the old way. Just jacked up the rear-end, turned the tire one
revolution and counted how many time the driveshaft turns. It's that
simple. (The driveshaft turned right about 3.6 - 3.7 times per one tire
rotation.)
The physical gear ratio is 4.10:1 but that's with the stock tires
(245/70-15) not 32" tires.
- Bernd
-----Original Message-----
From: owner-dakota-truck@buffnet4.buffnet.net
[mailto:owner-dakota-truck@buffnet4.buffnet.net]On Behalf Of Bob Tom
Sent: Thursday, August 12, 1999 8:38 AM
To: dakota-truck@buffnet.net
Subject: Re: DML: RE: Ring Gear changes: 3.55 vs. 3.90
>> [snip] My closest estimation is that i'm running 3.60's with
>>the 32" tires and 4.10 gears. (I actually jacked up the rear end and
>>counted the driveshaft revolutions compared to one tire revolution.)
>>- Bernd
----------------------------------------------------------------------------
-
If you have a gear ratio of 4.10:1, jack up the rear end, put on a 24"
diameter
tire or a 30" diameter tire (or even a bare rim), mark the rim/tire
so you know when you have rotated it one complete revolution and count the
number of revolutions of the driveshaft, it will always be 4.1 revolutions.
What has happened when a different tire size is used is that the number of
tire revolutions per mile has been changed and this, in turn, is what
changes
the car's cruise rpm. Example: a tire with a true diameter of 26" has a
circumference
of about 81.68"; a tire 30" tall has a circumference of 94.25". That means
each time the 30" tire completes one revolution it will move the truck about
12.5" farther than the one revolution of the 26" tire. Therefore, the
taller
tire requires less input rpm (engine speed) to travel the same distance.
Conversely, shorter tires require more engine speed per mph. That's why
shorter tires seem to act like lower (higher numerically) axle gears, and
taller
tires seem to act like higher (lower numerically) gears.
The best way to know the true circumference of your tires is to mark the
sidewall
of a mounted tire, push the truck forward one revolution of the tire, then
measure the distance travelled.
The formula for calculating cruise rpm after a gear ratio or tire size swap
is:
rpm = (mph x trans. ratio x axle ratio x 336)/ tire diameter
2 things to keep in mind when using this formula. First, your true rpm will
almost always be higher with an auto. without a torque converter thanks to
converter slippage (the exact %age of real-world increase depends on the
road
speed, rpm and type of converter...figure at least a 10% increase). Second,
never rely on sidewall designation or manufacturer's specs when it comes to
tire diameter...get the tire, mount it, properly inflate it for your truck's
weight, and measure yourself.
There are 2 problems with the phrase, "effective gear ratio". First,
effective
gear ratio that it implies that a ratio has been changed but the diameter
of a tire
(tire size) has no effect on axle ratio. Second, effective gear ratio is
only
relevant if the original tire size is stated (as you have done).
The above, sorry for the length, is only one aspect (why does tire height
affect
cruise rpm) in understanding all about axle rears.
Hope this helps.
Bob
Burlington, Ontario
'97 CC Sport, 5.2L, 3.55, auto., 4x2, flame red
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