I noticed today that my Z is running slightly higher RPM's. It used to run at around 2800-2850 rpm's at 75 mph and now its running closer to 2900-2950. Any ideas as to whats up with that?

 

There is more gas in the tank than the original test.
Tire pressure is less than the original test.
Weather conditions are different: temperature, humidity, barometric pressure.
You put on 100 pounds.

 

More wind?

 

The relationship between engine speed (RPM) and vehicle speed (MPH) is determined by the tranny/differential gear ratios, and cannot change.

If you actually note a difference, then the problem has to be either in the tach system or the speedometer system.

bill

 

1/4 of those could lead to the answer.

like the other guy said, its a mechanical link. Adding wieght or fuel would only slow acceleration, but at a given speed/gear the rpm will be the same.

such a small change could be caused my a low tire pressure or bad signal.

 

I must have been thinking ¼ mile ET.

 

If this is the problem, how would I be able to get it check out? Without going to the stealership that is....

 

On an automatic, a slipping torque converter can do this.

 

Wow I wouldn’t even notice this.

 

Yeah I do have an AT. Should I get it checked out, or is this nothing to worry about for now?

 

Bigger rims?

 

Automatic?...*sigh*

 

You're forgetting that ratio is directly affected by tire diameter, which can certainly change.

 

The TRUE speed will vary with tire diameter, but the measured speed is based on the rotations of the wheel. Since the ECU doesn't know what size tires you have, it always displays the same speed for the same number of revolutions during the same time period.

 

The ratio between actual speed and RPM will change with tire diameter.

But the ratio between indicated speed and RPM is fixed.

You could put on bigger tires, smaller tires, or jack it up take the tires off, and the speed reading will not be affected.

bill

 

Nope same size rims and tires since Ive had the car. It just started doing this about a week ago.

Just something to randomly throw out there, I put that fuel system cleaner in my car, then exactly one tank later my girl puts another one in without my knowledge, how would this affect the car?

 

You probably need to have it driven by a Nissan tech with a scan tool to see if the torque converter is locking up when it should. With the same size tires and wheels, automatic, you should have the same speed @ rpm in gear as every other Z out there with the same tranny and final drive ratio. Confirm this number for sure to verify you actually have a problem, and if you do, torque converter issues, or leaky injectors are the only things I can think of that might cause that, unless its tachometer or speedo error. A scan tool should be able to verify this as well, by checking actual RPM and MPH against what the tach is showing. If not that, then for tranny issues; How many miles on the car? What does the transmission fluid look / smell like, and has it ever been changed?

 

I bought it with about 55,000 miles on it, its now at 74,000. I have not yet looked at the tranny fluid, and I have not changed it. I was planning on changing it this weekend after I get payed because Im going to get an oil change as well.

Should I take it into a dealership to change the tranny fluid?

 

Ooops.

 

Therefore, something has changed.