So i've read other forums posted way back and it seems that the R180 differential is the best to use with an '81 5spd tranny. What are other options, since the R180 is offset and would require mounting mods.

 

Other options? Like what other diffs can you use??? uh... the R200. And it's not the "R180" that is "better," it's the gear ratio.

 

180 or 200 just refers to the size of the gears. (width, or durability) If you have an R180 in your car now then the R180 swap is easier. As for using with a certain transmission, then whoever told you to use the R180 probably also included gear ratio information. Maybe 3.9 ? or 3.9:1

 

ALright well i calculated this, if i take the R200 (3.365) out of a '79 zx 4spd/5spd and used it to replace my stock '81 zx R200 (3.9) would that improve gas mileage?

 

It would a lil... but then the car would be slower. You can't have both.

PS... if you want super good gas mileage, buy a 4cylinder Honduh.

 

What are you trying to accomplish with your swap?

 

The 1979 4-speed 280ZX 2+2 has an R180 3.36
The R200 3.36 is found in the 1974 260Z 2+2 with a 4-speed I believe.

either way, those are almost impossible to find in that gear ratio. (3.36)
3.54 is your best bet with the tall 5th gear 1981-83 non turbo 5-speed.

23 city, 32mpg highway with that gearing if your car has a strong engine with good torque and a proper fuel mixture. Thats what I got with my 280ZX 2+2, T5, NA engine, and 3.54
I averaged about 25mpg with easy driving on my daily commute. Better than my 4-cyl Frontier