Theoretically, if you change your final gear set to a larger ratio, and got a modifry yellow jacket or yellow box, you should change your oil more often, right? How many of you actually do?

 

With a correctly installed and set Yellow Jacket / Box the milage displayed is the milage driven.
That is why one would use a Yellow Jacket / Box, to correct the fault "generated" by a different (non OEM) final drive ratio.
Not just for the speedometer, also for the odometer.
They use the same signal.

For the diff most (and me too) would recommend following the severe maintenance schedule.
There is not a lot of oil in the diff and it is working hard to serv you.

 

I think the point r_duff is making is that to drive the same miles requires more revolutions of the propshaft and hence more work of the crown and pinion.

 

yes, that was my point

 

Just what I need right now - another variable to consider.

Wait - that's a constant now.

 

In which case isn't the yellow box irrelevant? The engine will turn more RPMs regardless of whether or not the yellow box is present. The yellow box only exists to give correct mileage/speed readouts.

And to provide input on the oil question. I say no, here's why. While the engine might be turning more revolutions there is more to consider here than just the straight speed of the motor. The power delivery with aftermarket gears is altered in such a way that the engine is being lugged less due to it's now default operating range at higher RPMs. So while your engine might be turning more revs, they're "easier" revs due to the new way to delivering the power via the different gearset in the rear end. If it were my car, I'd follow the same oil change intervals in my engine as when the rear end gearing was stock.

I would however, swap out the differential fluid more frequently as it will no doubt see more stress. I think it was Xviper that gave the best example. I believe he said something to the effect of, the higher you modify the gearset in the differential, the more the stress point is moved AWAY from the engine. So your driveline stress is now going to be more focused on your diff.

 

Which is the answer to r_duff's question.