My '86 245 has a 3.31 Dana diff. Excellent fuel economy and decent acceleration, plus the 3.31:1 ratio means that I can drive around town in 2nd gear without over-reving the engine.

My owner's manual lists three possible ratios for 240's with the M46..3.31, 3.55 and 3.73. Since all of the n/a stick 240s that I've seen have the 3.31, I'm assuming that the 3.55 and 3.73 ratios were available on turbos only. I'd be willing to sacrifice a little fuel economy for more power off the line and have been thinking about swapping in the "guts" from a 3.55 or 3.73 diff.

Questions:

-Is it possible to just swap the gears, or do I have to do the whole axle?

-How much would fuel economy suffer with, say, a 3.73? Would I be constantly "rowing" through the gears in city traffic?

And, finally, which year turbos had the 3.55 and which had the 3.73?

Which ratio is more desirable?

Thanks guys!

-Jim