The ratio never changes. This means that if I know that line pressure is 55 psi at idle, in a 700 R4, the the PR spring must be 5.78 lbs.
Pressure divided by Ratio = Force
So, 55 psi divided by 9.5 = 5.78 lbs.
Now, let's look at a math formula for shift speeds.
Suppose we had shift speeds of 15 mph and 20 mph, for the 12 & 23 shifts on a transmission. 20 mph may be too early for the 23 shift. If we adjust TV modulator, we will move both shifts. We don't want to do that because the 12 shift is fine, so let's work with just the 23 shift spring.
EXAMPLE: Original spring divided by Original MPH = Ratio
As, 4 lbs divided by 25mph = 0.2
Ratio x Desired MPH = New Spring
0.2 x 25 mph = 5 lb spring
A 5 lb spring will raise the shift on this transmission to 25 mph.
All you need to know is  Where is it shifting now (at MINIMUM throttle) and what does the spring weigh.
This formula will get you very close, but may be a "tad" off, because we are not accounting for TV pressure helping the spring. This is why you want to check it at minimum throttle, so TV has the least effect.
Finally, let's look at speedometer ratios.
Suppose we put an exchange transmission in a car, and now the speedometer is off, because the speedometer drive gear has a different tooth count. What do we have to do to the driven gear to correct it?
Let's say the old drive gear had 7 teeth and the old driven gear had 21 teeth. The exchange unit had 8 teeth on the drive gear.
Old Drive Gear divided by the New Drive Gear = Ratio
7 teeth divided by 8 teeth = 0.875
Old Driven Gear divided by Ratio = New Driven Gear
21 teeth divided by 0.875 = 24 teeth
A 24 tooth driven gear will correct the speedometer error.
Let's do one more speedometer change. This time the old drive is 9, and the new drive is 10. The old driven gear is still 21.
Old Drive Gear divided by New Drive Gear = Ratio
9 tooth divided by 10 tooth = 0.9
Old Driven Gear divided by Ratio = New Driven Gear
21 tooth divided by 0.9 = 23.33 teeth
We can't get a 23.3 tooth count so we round it off to 23 teeth. Now the speedometer will be close, but not exact, because we had to round off the number.
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Jeff  74 Cherokee S 401/400/QT  basically stock
