Does driving my diesel car engine with highest gear but lowest motor speed damage it?

Question

I have a Volkswagen Passat B8 Variant car with 1.6 TDI 120hp diesel engine car from 2016. Manual transmission. Diesel and gasoline prices are at record highs currently in my country of Austria in Europe (2 EUR per liter of diesel which translates to roughly 9 USD per gallon, the "normal" price is usually around 1.0 to 1.2 EUR per liter) so I have been looking for way to save on fuel while driving.

I noticed the following

• My diesel engine does not turn off if I simply put in any gear and let it drive without pressing gas pedal even once; the car moves as it should without a single push on the gas pedal, even if I stand still and want to start moving it in first gear
• If I do that in the highest gear (6th gear), I reach a speed of around 60 km/h (37 mph) which is exactly the max. speed limit in my hometown on main roads coincidentally
• Unsurprisingly, the engine is using far less diesel when driving like that - I can bring down my average consumption down to 3.8 l/100km (61.9 mpg)
• When driving at any gear without pressing gas pedal the engine is a little bit louder compared to driving while pressing the gas pedal a little bit but the difference in noise is not that much; also no warning indicator or anything lights up - in general there seem to be no issue that is obvious to me

My question is - does driving like that damage the engine or maybe the transmission or anything? Is it recommended to drive like that as long as the sound does not bother me?

Answer 1

What you are describing can cause some issues:

• Clutch wear: Getting a car moving from a standstill on too high a gear is hard on the clutch and will wear it out much faster due to slipping. You should get started on first and work your way up the gears so you don't burn your clutch
• Lugging: diesels can run at much lower RPMs than gasoline engines, but there's a limit to everything. Lugging an engine is where your RPMs are too low for the load on it, this can cause overheating, pre-ignition and other issues which can cause expensive problems long-term. Plus, it's counter-productive as running at too low an RPM actually reduces your fuel efficiency. Basically, if your acceleration is very slow or you lose speed going up a hill you are in too low a gear. Also, if you have a lot of vibration your RPM is too low.

What you want to do is get up to speed using the gears as you ordinarily would, then switch to a gear that puts your engine at the optimal RPM for efficiency. For Audi group diesels this is somewhere around 1700rpm.