For a long while now I've been having horrible gas mileage (sometimes 11 - 13mpg city) but on select days, the car gets normal gas mileage. These normal days happen about once a month or so.


Anyway, the car for a very long time has had a slight whine (cant tell what its coming from but I can only assume gearbox) when rolling at low speeds. Also, the gas mileage is excellent on the highway. 65mph is still around 25mpg, but the needle leans more toward 40 above 75mph.

How can I diagnose the torque converter? What is the lockup speed of the torque converter?

I was also told that this may be the catalytic converter being clogged, is there a way to diagnose that? (aside from removing the o2 sensor and going for a drive)

Thanks a bunch!