Effects of ambient air temperature on fuel economy

With the UK and some parts of Europe heading into record breaking temperatures we were recently asked what effect ambient air temperature has on fuel economy?

Cold temperatures

Winter & cold temperatures have a significant effect on an engines fuel economy. For a conventional petrol- or diesel-powered vehicle, fuel economy at -7°c in city driving is about 15% lower than at 25°c. The effect of temperature on electrified vehicles is even more pronounced. Hybrid vehicles, which rely on a larger battery, can experience a decrease in fuel economy of around 30% while all-electric vehicles can experience a decrease in fuel economy of nearly 40% in cold temperatures. Tests were conducted in a controlled laboratory setting.¹

Hot temperatures

Hot weather can increase fuel economy. As an engine warms up to its efficient temperature quicker and summer grades of fuels can have slightly more energy release properties along with warmer air causing reduced aerodynamic drag over cold air.

But keeping passengers comfortable in hot weather by rolling down the windows or using the air conditioning (AC) will dramatically reduce fuel economy beyond any potential savings gained from the warmer temperatures. Driving with your windows down can also reduce fuel economy. Open windows increase aerodynamic drag (wind resistance). AC use will reduce a vehicle’s fuel economy by more than 25%.²