78 280Z.  Factory 4 speed.  Car has been on jack stands in my garage for the last year while I've been restoring the underside.  Pretty much have finished up the rear end, but my wife really wants me to get it out of the garage for a while this summer before I can finish up the front end.  The restoration plan includes swapping out the OE 3.545 diff/4 speed tranny with a 3.90 diff/5 speed tranny from an 83 ZX.  I've already installed the 3.90 diff.  Would really like to delay installing the replacement tranny for a few months.  My question is will the car be drivable with the 3.90 diff and the OE 4 speed tranny?  I realize this combo won't be optimum, but by "drivable" I simply mean acceptable for a short term until I can get it back up on the jack stands after summer.  I'd only be driving the car easy and on nice days....60 mph max.  Thanks in advance for your advice.

Some people swap in the 3.9 just for better pickup in 1st gear.  4th gear is 1:1 in both transmissions.  If the engine is running well you probably won't mind the high revs.  I've had my ZX 5 speed with 3.9 on the local freeways at ~60 mph in 4th and it's not bad.  Probably won't bother you.  You might find yourself starting out in 2nd gear though, with the 3.321 1st gear in the 4 speed. It will be like having a 3 speed.

My calculation is that in fourth gear and 4500 RPM you should get 84 MPH. That is dashed on the stock tire diameter. 

Ps did you install the white colored speedometer gear?

 

I do have the correct (white) speedo gear to install in the replacement ZX 5 speed tranny, but the original gear is still in the OE tranny.  Are you saying to go ahead and swap in the white gear in the OE tranny so the speedometer will be accurate?

 

It will be drivable, but you will shift up to second a lot early than you are use to. Hill starts are going to be a lot easier;) If it is temperary I wouldn't worry about it much.

You did grt the speedo gear when you picked up the tranny diff combo? You need the white one with 19 teeth. See attachment.

The guys in the 620 trucks don't know any better. Thay have the earlier pre 1977 280Z 4 speed with the 3.592:1 first gear and a 4.375 diff. So a 3.321:1 with a 3.9 diff is tall but not the worst combination you could come up with.

There are people that use the close ZX transmission with the R180 diff and 4.11 gead setup. Thats close to what you will have in first gear.

My quick calulations with 14" wheel and tyres with a diameter of 620mm (my situation) you will have a speed 31.62km/hr @ 3500 rpm compared to the original setup 34.79km/hr @ 3500 rpm. You will loose about 10% on speed.

Thats basd on the final ratio of 11.77 for the 3.545 diff and 12.95 for the 3.90 diff, and the distance of 1.95mtr travelled per revolution.

 

Edited May 25, 20195 yr by EuroDat

I run the BWT -5 with 3.90 gears. 1st gear is mostly useless . In 4th it’s not bad, but I have a cammed motor. My motor just starts to smooth out at 3500+ rpm. I’m just starting to get in my power band. Like to have taller 1st and 2nd gears ,  but makes for a very quick launch. 

Thanks so much gentlemen...I do have the white/19 tooth speedo gear (had not thought about needing to install that with the OE tranny/3.90 diff combo).  Tire/wheel combo is aftermarket, but same overall diameter as OE.  Glad to find out that the Z will at least be drivable until I can get it back up on the jack stands in a few months.  I appreciate all the comments.

 

Just take al9ng your smart phone. Tune it to “Waze.com”. It will give you your GPS speed.