diff swap
 

I need to replace the differential on my 81 w123 240d manual trans. I have one from a 300td that I think will fit but I'm wondering how the different gearing will affect driving. How can I tell what the gearing of the 300td diff is? It seems like first gear of the 240d trans is pretty low Any advice?

The ratio is stamped on the lower driver side of the rear face. You have a 3.69, a 300TD will have 3.46 if non-turbo and 3.07 if turbo. The lower the number the slower the acceleration. Economy is not significantly affected by the ratio with diesels, its not really worth swapping since you don't have the power to go past 85mph anyways.