Oh man...now we're getting into it. ;)
With a stock/factory tire/rim...that would be correct. Anytime you change
the tire size, you're also changing the effective ratio to the
pavement...not in the differential. When you turn the tire, you don't
check it from the rim...check it from the top/outer tread on the
tire. (This give the same effect as measuring it in your second example -
physically moving the vehicle down the road and marking the changes in
distance vs. tire size.)
I'm not going to get into details...it's coming from experience in a shop
and from what i've physically seen happen. Does it make sense...not until
you actually try it. (And I totally agree with your statement: "but when
the industry refers to higher gears and, yet numerically, the number is
lower, it is against normal comprehension and is complex.")
Later,
- Bernd
At 11:29 AM 08/12/1999 +0000, you wrote:
>At 09:38 AM 8/12/99 -0500, you wrote:
> > [snip]
> >I've done the tire/rotation check more times than I care to count (from
> >past shops and over the years) so I know what i'm doing. You don't have to
> >turn it 10 times...three or four is more than sufficient. But what the
> >heck, it all averages out the same.
> > [snip]
>
>Unless I am missing something in the interpretation, I will have to
>respectfully disagree that jacking up the rear end, rotating the tire
>(one which is a different diameter than stock) one complete revolution will
>result in a change in the no. of revolutions of the driveshaft.
>
>Visualize, if you will, for a minute. Gear ratio is 4.10:1 (for example
>only).
>Put a straight line mark on outer edge of the stock tires sidewall. Place
>a straight
>ruler (whatever) on the marker and through the center of the axle shaft,
>extend this marker
>to the outer edge of the rim and draw another marker.
>
>Rotate the tire one complete revolution. The driveshaft will have turned
>the 4.10 revolutions. Note also that marker on the rim will have also have
>made
>one complete revolution, no more and no less. Therefore, a bare rim (i.e.
>tire diameter of zero) with one complete turn will also give the same 4.10
>revolutions of the driveshaft.
>
>Now, imagine a larger (or smaller) diameter tire and put a marker on its
>sidewall
>in the same way as above. How many times will the driveshaft rotate? 4.10
>rotations.
>
>The only way to see a difference is to have the truck on the ground, put a
>marker
>on the ground where the marker on the stock tire is, push the truck so that
>the tire
>rotates one complete rotation, put another marker on the ground where the
>tire's
>marker now is and measure. Now, repeat but with the tire swap and count
>the number
>of driveshaft rotations. This is the 'effective gear ratio'.
>
>Apologies to all concerned if this can be proven incorrect. Again, sorry
>for the
>length, but when the industry refers to higher gears and, yet numberically,
>the number
>is lower, it is against normal comprehension and is complex.
>
>Bob
>Burlington, Ontario
>
>'97 CC Sport, 5.2L, 3.55, auto., 4x2, flame red
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