achteras volvo 240 overbrenging 3.31 of 3.15

[Rob]

Iets off-topic, maar ook niet helemaal: waar kun je aflezen welke as het is? Is er ergens een type-plaatje?

It's on the driver side axle tube on the back side. Often it is covered in undercoat and not visible. Brake cleaner will remove the undercoating without removing the printing.
Alternately, you can do it the old way. Jack the car up on jack stands and mark the drive shaft and inside of a rear tire. Count how many revolutions of the drive shaft it takes to make the tire make one revolution.