I'm not sure what you meant by not having overdrive. The stadard 4 speed automatic transmission for the 4 cylinder RAV4 has overdrive. But the condition you are seeing sounds like an overdrive issue or lack of overdrive. The days that you get the tach up to 3500RPMs - what happens when you back off the gas a bit - does the RPMs drop? How often you vary highway speed - or do you lock it in a set speed? Are you running any extra accessories during the different times (defrost, A/C, powering other devices, etc.) ? With 600 miles - the car should be a good way into being broken in and the ECM should have learned a baseline for the driving conditions - but it may be just a case of an over-sensitive kickdown to the transmission (sometimes by pressing the gas pedal a little more will cause the transmission to kick out of overdrive or lockup mode in response to the pedal). Might also be a feature of drive by wire in the new RAV4.
Thanks for the response. Should have said "no ECT" button instead of overdrive. On the 3500rpm days, when I back off the gas, the readings reduce to around 3000, just like on the 2500rpm days, when I back off the readings reduce to around 2000! I generally vary speed on the highway, and no, there are no extra accessories running during the different times. Also tried to "kick out" the transmission to shift into a higher gear by depressing the gas pedal.
What is "drive by wire" and "lockup mode"? I also reported this to the dealer, but I'm not optimistic they'll come up with a ready answer. If I drive it every day, it's fine! Just on the day after it sits, it shows 1000rpm's more than it should....