Given your garaged car on the forum (91 Tempo is pre OBDII), you probably didn't have a scanguage to help pinpoint if the throttle response was non-linear in one or more spots. My Jetta's coolant temperature guage is like that, and it's maddening. The 10 year old TDI I'm driving has always had a drive-by-wire throttle, and there isn't the jerkiness or the level of hesitation you describe in my throttle response.
The issues you're having, that sounds like much more confined to the particular make/model you rented. Seemingly, they didn't pay much mind to the programming for the throttle response, which makes sense considering the Aveo will likely never be considered a "sporty" car that would warrant that attention.
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Current mod: Skidplate/Undertray for my MkIV Jetta. Next mod: CAD drawing for skidplate so other Jetta/Golf drivers can make one too!
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