C300 petrol and GLC 300 diesel driven back-to-back

[BTB 500]

Our 2019 C300 Sport Estate (petrol) went in for a service on Monday, and I was given a 2020 GLC 300d AMG Line Coupe (diesel, obviously!) as a courtesy car. It was 61 miles each way, so I drove both cars 122 miles over exactly the same (mainly motorway) route there and back at the same speeds (as close as I could manage, using cruise control most of the time) which made for an interesting real-world comparison. Both cars had roughly the same mileage on them (just under 7k and just under 6k respectively).

Obviously they weren't identical vehicles (estate vs SUV coupe), but the fuel consumption was a surprise. I got 39.1 mpg in the petrol car and 40.4 in the diesel - around 3% better, but as petrol is currently 2.5% cheaper the fuel costs seemed to be basically the same. This was just using the on-board display in each car of course, so not a particularly rigorous test! The GLC felt a little bit more 'lively' (both cars with 9G gearbox in default comfort mode) as in needing less movement on the accelerator ... presumably it has more torque (and maybe more bhp - I've not checked the specs). Sound-wise there wasn't much in it as the 4-pot in the C300 isn't the most refined petrol engine.

Drifting away from the engines, the GLC was significantly harsher going over motorway surface bumps ... presumably due to the AMG Line suspension. I guess there's an upside in cornering but I didn't really put that to the test! It had pretty limited rearward visibility, and with tinted glass and a black interior (including black headlining) it was very dark inside. The boot seemed pretty tiny but of course you don't buy a coupe for load lugging. It had MBUX rather than the NTG 5.5 in our car - the touch screen was nice but the navigation display at junctions / roundabouts (using the front camera view with direction arrows overlaid) struck me as a bit distracting as you don't really want to be looking at the screen. I don't personally get the whole SUV Coupe thing but my 13 year old son thought it looked great - "like a BMW"

Overall it was interesting to drive the GLC Coupe but the C Class estate definitely suits us better, so I was pleased to get that back

 

[C240Sport97]

The harshness is caused by the rubber band 20" wheels.

Amazed that petrol is essentially as economical as diesel, and without the need for adblue

 

[BTB 500]

The harshness is caused by the rubber band 20" wheels.

Amazed that petrol is essentially as economical as diesel, and without the need for adblue

As an aside we were really impressed with the ride quality of the S205 - much more refined than our previous S203 (both Sport spec. with 17" wheels).

 

[C240Sport97]

As an aside we were really impressed with the ride quality of the S205 - much more refined than our previous S203 (both Sport spec. with 17" wheels).

That's partly because S205 can be had with 19" wheels

 

[Mactech]

It's always interesting to make comparisons and be able to feel first hand the differences between vehicles.
I have driven neither of these cars, but have experience in petrol C class and diesel SUV Mercedes.
Ride quality is always something of a compromise in SUV's as they are taller and require more support in both springs and damping due to the higher centre of gravity. Bigger wheels don't help in this area, but can be made to be acceptable if engineered correctly.
The taller SUV has also a larger frontal area. This will be proportional to the aerodynamic drag on the mostly motorway runs you did, and therefore the mpg.
If you add in the transmission losses of 4WD and the fact that diesels are more effected by cold ambient temperatures, then your observed mpg figures don't appear too surprising. I also don't understand the SUV coupe thing, but there are plenty about now, so somebody must.

I have both diesel and petrol vehicles which average over 50mpg, and also one of each which return in the 30's for many varied reasons.

PS It's the heaviest and lightest of the vehicles which are the most economical.......

 

[BTB 500]

The taller SUV has also a larger frontal area. This will be proportional to the aerodynamic drag on the mostly motorway runs you did, and therefore the mpg.
If you add in the transmission losses of 4WD and the fact that diesels are more effected by cold ambient temperatures, then your observed mpg figures don't appear too surprising.

The GLC must have a bit more frontal area, but probably less than you'd expect as some of that extra height is in the wheels & suspension (i.e. ground clearance) rather than the body. And against that the coupe profile at the rear must have an advantage over a more square-cut estate body in terms of aerodynamic drag. Drivetrain losses would definitely be a factor if the GLC runs in permanent 4WD (not sure on that, but it probably does) - I suspect that was the most significant thing as this was at the start of the week when it was pretty mild down here.

 

[190]

The GLC must have a bit more frontal area, but probably less than you'd expect as some of that extra height is in the wheels & suspension (i.e. ground clearance) rather than the body

One thing they did with the blue efficiency saloon models was to lower the ride height by 15mm below the standard car to reduce aerodynamic drag. If lowering by such a small amount makes a worthwhile difference then that implies that a raised ride height of several times that amount will significantly worsen it.

 

[BTB 500]

One thing they did with the blue efficiency saloon models was to lower the ride height by 15mm below the standard car to reduce aerodynamic drag. If lowering by such a small amount makes a worthwhile difference then that implies that a raised ride height of several times that amount will significantly worsen it.

Lowering a standard road car a small amount reduces the frontal area fractionally because less of the wheels are exposed. As part of an overall package aimed at improving efficiency it makes sense, but the impact of that alone would be pretty tiny. My point was just that height alone doesn't necessarily imply a lot of extra frontal area. A large SUV the same overall height & width as my Vito would have less frontal area (because the body isn't as tall, and the wheels/tyres making up the difference in height have less area).

A GLC Coupe will certainly have a bit more frontal area than an S205, as I said. However I suspect the more streamlined body shape at the rear would negate that in terms of overall aerodynamic drag.

 

[Mactech]

One thing they did with the blue efficiency saloon models was to lower the ride height by 15mm below the standard car to reduce aerodynamic drag. If lowering by such a small amount makes a worthwhile difference then that implies that a raised ride height of several times that amount will significantly worsen it.

Yes, the frontal area includes the area under the car. This is a big contribution to drag as in is very 'dirty' air on almost all road cars. The interaction of air, road and car at speed is not a pretty picture. Race cars have more aero dynamic surfaces on the underside than the topside. The opposite is true of road cars.
Don't be fooled by the fastback look, most are done by stylists not aerodynamasists and rarely actually any better.
But if you want really dreadful Cd figures, look no further than a man on a motorbike.....

 

[190]

But if you want really dreadful Cd figures, look no further than a man on a motorbike.....

I'm always amazed by the famous Vic Willougby experiment that showed what was possible even at 30 MPH. With 45lbs extra weight acceleration was still improved Wasn't much of a looker though!

 

[Mactech]

I'm always amazed by the famous Vic Willougby experiment that showed what was possible even at 30 MPH. With 45lbs extra weight acceleration was still improved Wasn't much of a looker though!

View attachment 107546View attachment 107547

Yes, it’s really not difficult to improve on the Cd of an unfaired motorcycle. With an upright seating position they are almost 1.2. That‘s 4 times that of a modern car. I estimate the frontal area of a car as being less than than 4 bikes...
A town hall is sleek by comparison, the only thing I can think of that’s worse is a parachute.....and that is by design!