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

This site may be helpful:

http://www.premier1.net/~john242ti/engtran.html

My first choice would be a 3.54, followed by a 3.73, but anything would be better than the 3.31!

Someone recently pointed out to me something that I think I may have been missing for years. When we write that the Volvo rear end is a 3.73 or a 3.91 on the 240 Turbos, do we mean the rear end gears or the final reduction ratio after the OD is applied? In other words, do those rear end numbers apply to the direct drive (4th in M46 and M47, and 3rd in AW71) or to the OD?

Philip Bradley

Philip,

It's standard USA hot rod speak. In other words 3.73:1 is the actual numerical reduction ratio of the rear end (differential gears - ring and pinion gears) itself. Doesn't have anything to do with the tranny.......

Far as I know 240 turdbo's only had:

3.73 with M46

3.91 with AW71

Since I have a swapped in M46, I have a 3.91 rear. Not a really bad combo. The ducker gets off the line in a hurry and yes proper rowing is required, but the right gear is there for the situation. I get 24-25 mpg, not so bad for a slightly modified 245Ti with M46 and driver that has Led Zep on all the time and can't hear the engine.

Some of the very early 240s had 3.91s and even 4.10s.

Yes, the guts can be swapped, but you need to seek the services of a qualified 4WD shop, or another place that has the same kind of expertise - it's not a shadetree job.

The easiest way to do it yourself is to swap the whole axle. Just undo the rear brake lines, the driveshaft, the trailing arm and torque rod bolts, and drop it out - put the new one in.

The subject of where to find which ratio is very cloudy, as often there were differences between similar cars even in the same model year. A 3.31 will only be found on a manual tranny car. Autos, both Turbo and NA, usually have 3.73s. The 3.91 was used on most of the old 3spd (no OD) autos, as well as most of the 242 GTs. The 3.54 is most commonly found in manual Turbos, and some V6s.

Switching gearing shouldn't take much off your range-per-tank, unless you do mostly highway driving. You won't necessarily be shifting more, but sooner - and you may find you like cruising around town in third instead of second. The biggest issue is the first gear ratio. Earlier cars had a slightly taller ratio that may work better overall, but I don't know of anyone who's actually done this kind of swap. You just have to decide if you can live with that really short first gear.

Paul Grimshaw's "Volvo Performance Handbook" recommends the 3.54 ratio/manual tranny as just about ideal for a NA 240, especially with a performance cam. I imagine if you do mostly in-town driving, and stick with the M-cam, a 3.73 would be quite liveable.