RPM at highway speeds in 4th gear?

[BTF/PTM]

Just curious. What RPM is a 240z normally at cruising at 65ish on the highway in 4th gear? My z has an R200 in it with a standard 4-speed and I suspect it's a 3.90:1 as I'm screaming along at about 3,800rpm in 4th with the speedo indicating about 85mph just keeping up with traffic.

[Arne]

With stock size tires, a 4 speed 240Z with the stock 3.36:1 ratio should be turning about 2850 RPM or so.

Tire size can have a significant effect as well. What size tires do you have on the rear?

[BTF/PTM]

205/60R14, probably an inch or two shorter than stock tires.

[Arne]

Yup, that'll make it worse. Since we don't know how fast everyone else is driving, it's hard to reverse calculate the ratio. But I'd guess that a 3.9 is probably close. Not the best for use with a four speed, that's certain.

[BTF/PTM]

Agreed, I have a 5-speed on standby but I'm working on getting more of the exhaust fume issue taken care of before I get side-tracked.

I should probably put fresh fluid in the R200 anyway, so I can get the ratio then.

[zztom]

On my drive back from the East Coast Z Nationals at Carlisle PA. I was cruising for long stretches at 2500rpm and doing 70 mph. Those new rotors in front took ALL the shimmy and shake out of the steering. It was a very smooth 6 hr drive.

[Zforce]

I have 3:70 gears in mine buy never got the proper speedo drive gear so it reads about 10 mph faster that actual. If i'm indicating 80 my rpms are about 3900 or 4000, if I step on it and let it rev to 5000 it reads 105.

[BTF/PTM]

I don't think mine even gets to 105 when I stand on it and let it pull.

As a related side note, is it possible to pull the diff cover and replace its gasket with the car intact? Mine has a pretty good soggy spot that I should fix. I can look at the numbers on the gears that way, too. I can use the method of turning the axles and counting pinion rotations, but it'll be hard to tell between a 3.70 and a 3.90 just by eyeballing.

[Inf]

... is it possible to pull the diff cover and replace its gasket with the car intact? Mine has a pretty good soggy spot that I should fix. ...

Also wanting to fix this, my differential has a pretty persistent wet-road-grime look to the underside where the cover meets the body. Hoping to hear any feedback people have

[d240zx2]

My tach is getting goofy. When I first owned the car prior to 1977 and with a 4 speed, 3500rpm = 70 MPH. 3000 = 60. Before I got the 5-speed and R-200 3.90, it was 4000 = 70. Age on electronics ain't pretty. Thinking of getting a refreshed one from Arizona Z....

[jmortensen]

Also wanting to fix this, my differential has a pretty persistent wet-road-grime look to the underside where the cover meets the body. Hoping to hear any feedback people have

You can't get the cover off with the diff in the car, but try tightening the cover bolts as they can loosen over time.

[BTF/PTM]

^ thanks! On the same note, is filling the diff as simple as pulling the upper plug and using a manual pump to fill it until the fluid starts to dribble out the hole like with most Japanese gearboxes?