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I'm surprised at the improvement in fuel econ in the CX-5 compared to CX-3. We routinely get better mileage in the CX-5 by around 4-5 MPG with almost identical commutes for work. With our particular cars, the CX-5 is clearly the bigger car with bigger engine but more efficient drivetrain. The bigger engine means you don't need to get into the throttle so heavily for similar acceleration as the CX-3, so the bigger engine is less frequently running in an inefficient operating range. This was a surprising revelation for me. Totally unexpected for me that the CX-5 would give superior MPG over the CX-3.
FWD vs AWD may have something to do with the variance between the two?
 

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Certainly, the FWD is a benefit to fuel econ compare to AWD, if all else is equal. AWD adds extra friction and inertia to overcome, putting added load on the driveline which means more fuel consumption. The CX-5 adds an extra 1/2 liter of displacement, something like 700 lbs to the curb weight, and more frontal area to create added aerodynamic drag. Frontal area isn't the whole story when it comes to drag, though, and it's entirely possible that the CX-5 actually could have a lower overall drag coefficient. I'd be surprised if that was the case, but it's possible. I haven't compared the gearing in the two transmissions. I thought they were the same 6-speed auto, but maybe the ratios are different for each application?
 

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Weight between the two models is about 450lbs. They use the same transmission but possible the ratios are different. Also possible you're into the throttle a tad more on the CX3 due to it being a sportier drive.
 

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Just looked up curb weight specs and it was 2952 lbs for CX-3 Touring AWD with 2.0L petrol engine with automatic trans while there CX-5 Sport FWD with 2.5L petrol engine with automatic trans weighs in at 3433 lbs.

So, by the numbers it's a 16% increase in weight.

2.0L engine in CX-3 makes 146 bhp and the 2.5L in CX-5 makes 184 bhp. 26% increase in power.

Power to weight ratios figure in at:
CX-3 = 0.049 bhp/lb
CX-5 = 0.054 bhp/lb

Things will likely skew more in favor of the CX-5 if you used horsepower measured at the wheels due to the efficiency losses in the AWD on my CX-3.

This wasn't entirely unexpected, but I assume aero drag would be greater on CX-5 which would negate any possible power-to-weight advantages.

The remaining possibilities I can think of are driving styles between my wife and I, possible gear ratio differences, and different tire diameters (effectively a gear ratio difference). Getting around in city traffic around here, I often find myself activating the kick-down switch on the gas pedal in order get my little sled moving.

I do sometimes tend to drive a bit more aggressively than my wife does, but I still see nearly the same improvement in fuel econ when I drive the CX-5. I find that I don't feel it as necessary to use the kick-down switch as often with the bigger engine which is often sufficient to get the acceleration I'm looking for.
 
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