Does anyone know why sometimes the RPM needed to be higher to go on the same speed?

[liveBLUE]

i am wondering why sometimes I need 2000rpm to go on 80km/hr and then sometimes i need 2500rpm, which is 500rpm more, to go on 80km/hr. Is it because of the temperature? or something else? Does anyone notice the muffler is kind of noisy, and if you turn off your music, you can hear your back is vibrating sometimes?

[stuts]

Hey liveblue if your problem with the RPM is happening when you first start your car in the morning or after long periods of sitting than yes it is the Temperature if it is happening after the temp thing goes off maybe get it checked out or someone else in this forum maybe able to tell you something else the other day it happened to me where my rpm went way up because I for some stupid reason forgot the car was still cold as other people in this forum have said the car needs time to warm up so take it easy till the temp gage goes off drive like grandma I know I do all the time

[liveBLUE]

The cool light is off. I know our car has to warm up first or it will take a while to change to the 4th gear. Every morning, when I drive my car, i can go on 80km/hr with only 2000rpm. When the car has stopped, the rpm stays at around 500rpm. In the afternoon, my car always go up to 2500rpm for 80km/hr, and when it stops, rpm stays at around 1000rpm. Sometimes the rpm just run a bit higher. Not sure why. I hope someone know about car tech can tell me or if there's some info i can read to know why.