If nothing else is changing it should be a simple ratio (pardon the pun) of the old and new gear ratios.
For example, if you are going from stock 4.10 gears to 4.88 gears the ratio would be:
4.10 / 4.88 = 0.84 or 84%
This means that at any given speed you will be going 84% of the speed shown on the speedometer.
If the speedometer reads 60 mph then you're really going 60 x 0.84 = 50.4 mph.
Note: Higher number gears will make you go slower than what the speedometer shows because the drivetrain is turning more times per revolution of the tire. Larger tires will make you go faster than what the speedometer shows because the drivetrain is going farther (i.e. circumference of the tire) for each revolution of the drivetrain. This is why higher number gears make a great combination with larger tires.
The ratio calculation will also work with tires, but you have to calculate the circumference (3.14 x Diameter = circumference) and compare those numbers.
To answer the other part of your question, most people use 4th gear in calculators because they are almost always 1:1.
Hope this helps...
