Effect of drag coefficient on consumption of ICE vehicles
September 2025
This one started as an itch of wanting to get a general idea of the effect drag coefficient has on fuel consumption, more specifically, on fuel consumption of vehicles with an internal combustion engine (ICE). But a bit of research already surfaced the realization that this will probably end up as a futile endeavor or at least an activity that wouldn't yield any indicative results. As it turns out, the drag coefficient (cd or cx) does not play a major role in efforts to achieve substantial improvements of mileage for a vehicle with an ICE. At least not as big a role as it would with EVs. The efficiency of ICEs is so low that the engine's efficiency gets to be the most important factor affecting the vehicle's mileage, other efforts are relegated to playing second fiddles. In an ICE, some 60% of energy available in the fuel is wasted in the form of heat and so any gain achieved by an improvement of a vehicle's cd (or of any other vector impacting the overall consumption, like the rolling resistance of tires) gets to be immediately multiplied by 0.4 at best and with that sort of neutralized. EV motors, in comparison, convert stored energy to work with an efficiency of some 0.9 or 0.95 I think, and so with EVs a much bigger part of an improvement of cd gets to be translated into a gain in vehicle's fuel efficiency.
But just for the heck of it, a nice candidate to try to quantify the effect of cd improvement on consumption of an ICE vehicle is the Volkswagen's Passat, because its 2023 model moves through air quite more stealthily compared to its predecessor, with a 0.25 value vs the 0.31 of the previous model.* In the early nineties, when the sleek and attractively styled Opel/Vauxhall Calibra hit the lights, I remember the automotive news articles never failed to mention its excellent cd (my memory serves me the value of 0.31, while internet sources say 0.26 to 0.29, but I guess the internet's right) and what an achievement that was. So some 35 years ago it took a coupe's silhouette to dip under 0.3 while today even boxier shapes are able to improve easily on that number. Not to mention the relentless onslaught of ever more streamlined EVs in recent years, with Kia's EV4 achieving 0.23, Hyundai's Ioniq 6 boasting 0.21, Tesla S and Mercedes EQE slightly improving even on that and, finally, Lucid company with their Air model dipping below 0.2. Ain't designers and engineers super cool?
But back to the Volkswagen vehicle. Since the newer model is quite more aerodynamically efficient than the previous, an observed reduction of fuel consumption in the newer model could, all other things being equal (which they probably aren't), perhaps be explained by the decrease of aerodynamic drag. Let's see some numbers. Since the newer model is available only as a station wagon (estate) I'll be comparing those. The VWs, at least in Europe, are (or, due to dieselgate, were) mostly beloved for their diesels. The VW diesel mainstay is the 150 hp engine (both in the 2010-2022 and the 2023+ model) so let's see the numbers for station wagon Passats with that engine (data gathered as visitor of a popular mileage tracking site). The 2010 model gets an average of 5.8 l/100km, while the newer one is satisfied with 5.53 liters. A similar improvement of almost 5% is observed also with vehicles powered by the 150hp gasoline engine, with 6.57l gasoline for the newer vs 6.88l per 100km for the older model. But of course, even when this improvement has been noted, nothing is known of the reasons for it and it could just as well be that other factors are at play, like drivetrain improvements or anything. Particularly because for the vehicles equipped with a stronger diesel engine (190hp) the numbers tell of a 5% INCREASE in consumption. Who knows, maybe the new engine is particularly fiery and owners like to vent it on the Nurburgring every other week. So, it is not sure that the sort of 5% improvement in mileage (in 2 out 3 engine variants) is solely down to a more streamlined design. But if it were, I guess that would be quite a result, worthy of copying more broadly in the automotive world.
Regardless of where improvements came from, cars' mileage has improved a lot during the time I happen to be interested in cars. When I was a kid, the Zastava car my father and many of our fellow citizens drove (the model 101 with a small Fiat derived 1100 or 1300ccm engine), had a word-of-mouth consumption of 10l/100km. And those cars went nowhere. Achieving 110km/h on the motorway elicited enthusiasm like launch day on Cape Canaveral. Nowadays family cars are much bigger, heavier and have the benefit of two- to threefold the amount of power at tap, so they won't make any fuss when pushed past 160km/h and yet can easily be satisfied with an overall 6l/100km gasoline on the tankstelle (electrified models, i.e. hybrids, requiring even less).
Well, it seems I didn't learn much regarding the query of the initial itch, but managed to observe that engineers, designers and all other sorts of petrolheads/chargeheads are the real deal and that their efforts work miracles in progressing us to a more efficient future. I truly love their work ... Keep it up gals and guys, may results be with you!
* Source (Accessed Sept 2025)
Ivo Makuc, 2025
byguesswork@gmail.com