I've got a 2.4L N/A Volvo S40 2008 with a 5 speed AT transmission.
I've noticed a weird behavior: under normal acceleration (e.g. from a stoplight) the transmission very quickly changes into 4th (at about 40-50 km/h) and the engine is at around 2.5-3k RPM. When reaching ~ 70km/h the RPMs drop to about 2k (still in 4th).
If I put the gear into manual mode (when in ~50km/h) and downshift into 3rd gear, I actually get a reduction in RPM, from ~3k in 4th to ~2k in 3rd.
The obvious explanation is that when the car is in 4th, the torque converter 'slips' (i.e. not locked), and when the car is in 3rd, the torque converted locks. If the torque converted would have locked in 4th, it would have locked in ~1k RPMs, which is too low for acceleration.
So, I understand why the torque converted isn't locked in 4th but it does in 3rd.
What I don't understand is why the transmission is going into 4th so quickly.
Because the RPMs are higher and the torque converter is 'slipping' - the fuel consumption is higher (some of the power goes to friction in the torque converter's fluid).
Because I'm in a higher gear, I'm also getting less torque at the wheels, so I'm also accelerating slower.
It seems to me that upshifting at this point is a mistake made by whatever computer controls the AT, and the torque converter is compensating for it. It also seems to me that manually downshifting (or holding it in 3rd) allows for quicker acceleration and lower fuel consumption.
Is it normal for the gearbox to upshift so soon (4th in ~50km/h)?
If so, why? It seems to me that upshifting so soon is fuel inefficient and slower