Fuel economy is very speed dependent and steady speed cruising <60mph will get you a noticeable improvement. The only 'downside' is you'll also notice the impact of using A/C. On the 2WD 1.2, I see a reduction of ~ 10mpg if I turn on the A/C when cruising at 50mph. The slower you cruise, the worse the effect; partly because you need less fuel to move the car, and partly because your journey takes longer at the slower speed (A/C uses fuel by the hour, not by the mile).
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