That was me who stated "Here we go again". Yup, totally correct. In my
case, the ACTUAL gear ratio is 4.10:1. What I get to the pavement (the
EFFECTIVE) is obviously different due to the oversize tires.
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.
Here's the specifics for my application:
Stock Tire Size: 28.5" (245/70-15)
BFG Replacement: 32" (285/75-15)
Now if the effective ratio with the stock tires is 4.10:1 then, doing the
math with the RPM and MPH readings at 65mph (2500 RPM in 4th gear), the
effective ratio with the 32" tires is: 3.66 (Pretty darn close to what I
measured on the first try).
* You don't use 5th gear since it is an overdrive...you want to always
check with a 1-1 gear....just an FYI *
- Bernd
At 09:59 AM 08/12/1999 -0400, you wrote:
>Somebody said it best - HERE WE GO AGAIN.... The actual gear ratio has
>nothing to do with the size of tires!!!!!!! The actual gear ratio is
>controlled 100% by the gears which are installed in the rearend. There is a
>phenomena, which many have called the "effective" gear ratio, which is
>affected by the tire size. This phenomena can be expressed as an effective
>gear ratio, considering the actual gear ratio and tire size, but it must be
>measured relative to some standard value. (A good standard would be the
>as-delivered condition of the truck.)
>
>I have actually made an Excel spreadsheet to study different options and
>combinations of ACTUAL gear ratios and tire sizes, which will calculate an
>EFFECTIVE gear ratio, as compared to the stock setup on the truck. As a
>learning exercise, I would encourage others to do the same, in order to
>better understand the relationships between tires, gears, and rears.
>
>As a side note, if you are going to do the tire rotation/count the driveshaft
>rotations exercise, it is much more accurate to rotate the tire multiple
>times (10 is a good number), count the driveshaft rotations, and then divide
>by the number of tire rotations. This removes much of the error associated
>with only rotating the tire one time.
>
>Steve Menendez- P.E.
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