As a comparison, My dad's frontera (2.2 dti) would around town in traffic, go as low as 14MPG and up to 40MPG on a run.
The ZT's are a big car and around town you can't I don't think expect realistically any more than about 30MPG, even on a run, you're talking 48-50MPG unless you're at 60 in which case you may get more.
Although not a small job - a thermostat would be something to look at getting done.
Jake
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