Ford Fiesta Ecoboost Kia Rio SX deceleration graph 1

The graph above represents the deceleration rates of the Fiesta SFE EcoBoost and Rio SX from 45 to 30 mph in each car's top gear and the tallest non-overdrive gear. In the case of the Kia's six-speed transmission, fourth is technically the final non-OD gear at 1.04:1. But being a hair away from 1:1, the Rio was decelerated in third (1.29:1) as well. The speeds were chosen to emulate the roads around my home and the throttle-lift situation as I try to time the traffic lights without downshifting or braking, saving fuel. The benefit from a flatter curve is losing less momentum over distance, so there is more conserved speed as you get ready to accelerate right back up to 45 mph. As the curves demonstrate and the seat of my pants verifies, the Fiesta just wants to keep on rolling. Third-gear deceleration perceptibly feels similar to the Rio in its highest gear.

Naturally, the deceleration rates are affected by a bevy of vehicle variables, including aerodynamics (though the aero effect will be less pronounced at our selected speeds), the cars' frontal areas, rolling resistance (different types and sizes of tires; the Fiesta runs a 185-width opposed to the Rio's 205), and driveline friction can shift the curves in one direction or another. But the gas engines pumping with the throttle closed plays a major role in slowing down.

Ford Fiesta Ecoboost Kia Rio SX deceleration graph 2

The second graph adds the speed over time curves of the two cars' panic stopping from 60 mph, to highlight the power of good, old-fashioned friction brakes. Both didn't take more than 3 seconds to come to a full stop.

With an efficient turbocharged engine less prone to wasting momentum, I'd have to imagine the Rio SX's Real MPG of 30/40/34 mpg city/highway/combined would draw closer (maybe not dramatically so, but there's room for improvement) to the Fiesta SFE EcoBoost's 34/47/39 mpg. At the national regular-octane average price of $3.67 per gallon at time of writing via the U.S. Energy Information Administration, the Fiesta is in line to save nearly $1000 in fuel over 70,000 miles of driving. (Our spec cost of ownership mileage distance as determined by pricing partner IntelliChoice.)

In addition to the 1.0-liter engine, I'd even sub in the Fiesta SFE EcoBoost's ride, alleviating the Rio's occasionally thrashing motions from the rear end. A Rio SX with the Fiesta SFE EcoBoost's engine and ride -- now that would be a memorable subcompact I wouldn't mind driving everyday.

Engine rpm change, as eyeballed on the tachometer

Ford FiestaSFE EcoBoost

Third gear: 2950 to 1950 rpm (from 45 to 30 mph)

Fifth gear: 1700 to 1050 rpm

Kia RioSX

Third gear: 3050 to 2050 rpm

Fourth gear: 2500 to 1750 rpm

Sixth gear: 1950 to 1250 rpm