How much abuse can the Yaris take?

[Altitude]

supmet - I never used the word "can't" in anything I stated. I rarely speak in absolutes.

There is a proper way to use any mechanical device and improper use typically leads to early failure. If you know what you are doing you likely won't harm the transmission, but using it like a manual all the time just increases the chances you'll break something or cause it to wear out prematurely. But hey, it's your money and your ride.

Marfulion - the best thing you could do is drive the car like it was your own. If you treat your own cars like shit then knock yourself out and don't worry about it. Though I do think you have some obligation to let the owner know of your lack of consideration for their property.