1997 nissan altima. tranny

I have a 1997 nissan altima with 80000 miles on it. Whenever I have drove it since I got it the rpm's would stay a little higher. For example im going down the expressway at 70 mph and 3500 rpms, or i would have to go to 4000 rpm's of a start to get speed. randomly on my way home from work the rpm's go down to 2500 at 70. I never touched the shifter,or the overdrive. Now whenever I drive it, I need much less rpm's to get up to speed....any ideas what could be wrong?

other info
i bought it from a older lady, she didn't drive it much.
Oil changed every 3000
Tranny fluid never changed (first assumption)
.
Thanks
:iceslolan

Auto tranny fluid should be changed about every 20k miles. With the amount of miles on the vehicle, I would recommend a complete transmission flush. I'm no transmission expert, but from what you described with the RPMS at 3500 going 70 MPH, it sounds like your torque converter (TC) wasn't locking up. At this speed, 2500 RPM should be closer to normal once you have TC lock-up. FYI, your TC doesn't lock-up until your engine reaches a certain minimum operating temperature (probably around 159-166 degrees F); so it's possible that when driving in very cold conditions, your engine takes much longer to reach this temperature which is why your TC might take a bit longer to reach lock-up. Without TC lock-up, your RPMs will be about 200-300 revs higher.

Very helpful. Are all cars like this?

Sorry I mis-spoke. I have a 2000 Altima with a 4.09 axle ratio, and auto transmission with 4th gear at a 0.69 ratio. My top gear final ratio is 4.09x0.69=2.82. At 70mph near steady-state with TC lock-up, I'm running at nearly 3000 RPM.

On your vehicle, you have a 3.65 axle ratio, with a 4th gear at 0.69. Your final gear ratio is 3.65x0.69=2.51. This means that it takes your engine less revolutions (i.e., RPMs) compared with my vehicle to achieve the same speed. This also equates to an 11% reduction in RPMs at any given speed in 4th gear, which we can use to extrapolate the expected RPMs you should see at 70 mph, assuming steady-state with TC lock-up, which comes to approximately 2670 RPMs. Even without TC lock-up, you shouldn't have seen 3500 RPM at 70 mph while in 4th gear, unless your engine was under heavy load (i.e., not a steady state), which most likely meant you were running in 3rd gear, rather than 4th gear; this could easily happen on very cold and windy days where you are fighting colder (i.e., denser) air combined with a strong headwind; climbing a moderately steep hill could also have this effect.

Very helpful. Are all cars like this?

Most cars built since the mid-1990's with electronically controlled automatic transmissions have the TC lock-up feature. This is one of the most significant improvements to automatic transmissions which resulted in better fuel economy at highway speeds.

And for each car that has the TC lock-up feature, it requires a warm engine temperature (which is also necessary to warm up the tranny fluid) before the transmission can lock-up. This is all electronically controlled, and is dependent upon transmission fluid temperature (which is also a function of coolant temperature), engine load, and a minimum RPM (usually need at least 2000 RPM before lock-up can occur). Also, TC won't lock-up until you are in 4th gear.