I was looking at this earlier, because I hit about 3500 rpm in 5th gear at about 83MPH and i was wondering according to my calculations i should be hitting about 83MPH at about 2600RPM if i switched the stock 3.9 with a 240's 3.364 differential, any thoughts?

http://www.geocities.com/z_design_studio/

End result i want to hit about 80-90 mph at below 2900rpm

 

My goal here is not to discourage you but my buddy drives his 1980 2+2 all day long in 5th gear at like 3,500+rpm on I95 in Ga and gets between 26 and 29 mpg. At 3,500 rpm the engine may sound like it is working hard but it is right in the pocket for good driving...you aren't hurting it by driving at 3,500 rpm...the redline is 6,400 rpm. I am not sure your gas mileage is going to be any better by putting in a taller gear and you will be slower off the line. But yes...you rpm's would be lower at the same speed with that taller gear. Just remember that if you drop the rpm's below the optimal power curve, which could happen with a taller (numerically lower) gear, you will make the engine work harder to maintain that same speed...so your mpg's might suffer slightly.

 

I think the original poster's quest is to keep the engine under the threshold at which the ecu decides to leave closed loop operation.

The sad fact is that the car was designed when the speed limit was 55 and that cruising over 65 with stock gearing will put the ecu into a mode at which it stops listening to the 02 sensor and instead runs rich.

 

The calculator you found is a good website.
Your are correct, but also note that a 3.54 gets you at 2800rpm at 80mph. Maybe slow down a little.

I'd go with a 3.54 personally. Good luck finding a 3.36

 

I could see it running rich at WOT when the car is going 0 - 45mph but over 65mph I am sure there is quite a bit more air being sucked in the engine to mix with that extra fuel in open loop mode. We can ask why the poster of this thread is looking to lower the rpm's though at those speeds. What are you hoping to gain?

 

Better gas mileage... https://www.zdriver.com/forums/280zx-s130-forums-77/cruising-23963/

 

I see...yeah, I don't read ALL the post but all this could have been ask in one thread But I do agree with Bleach with the 3.54:1 and the 81-83 5-speed, however a T-5 would be almost as good on gas as the non turbo 5-speed on the highway but would be better off the line in slower traffic. That would be my choice...performance and gas mileage.

 

anyone planning to drive 90mph for a long distance should be more concerned with survival than gas milage...

also, your gearing is so tall at that point that you may just find that the car runs out of power at 2900rpm. 95 might be your top speed.

 

These engines were designed to operate most efficiently at 3000 to 3500 rpm. That is the sweet spot. Any changes here, especially with taller final drive ratio could negatively affect your performance and fuel mileage and more importantly, as has been mentioned, bog the motor which is very hard on the internals.

You'll most likely see better gas mileage by making sure the car is properly tuned and running in tip top shap. Check, clean and seal your electrical connections, vacuum hoses, etc. and you'll be amazed at how well these cars can run bone stock. You should be able to get over 450 miles on a tank of gas if the car is properly tuned.

 

I do know that over 3500rpms on the turbo 280zx that the ecu no longer listens to the 02 sensor.

I confirmed this on our ecu simulators at standard motor products ecu division.

I haven't check the non-turbo ecus but I'll bet they are similar in that regard since the speed limit was substantially lower to the point that you had to be breaking the law speed wise to run that engine rpms.