Idantal95

Hello,

I just got a 2020 GLC 300 4Matic. I went on a 300 mile trip on comfort mode and cruise control at 75mph and only got 22mpg.

I was wondering if anyone has advice to maximize my fuel efficiency. I know I should’ve been on ECO mode. Is it better to drive manually (without cruise control) in order to benefit from the cylinder deactivation? It doesn’t seem like cylinder deactivation kicks in when cruise control is active.

Appreciate the advice in advance.

Thank you,
Idan Tal

no_mulligan

Go to a MB dealership. Get the vehicle checked out.

superswiss

What kind of fuel economy were you expecting? 22-23mpg is the average for these boxy things according to fuelly. The speed is part of your problem. Drag grows exponentially with speed. Going 75 mph instead of 65 mph increases drag by ~30% while speed is only 15% faster. Going 75 mph instead of 55 mph has 85% more drag, so almost double the wind resistance going only 20 mph faster and SUVs are not exactly aerodynamic. Slow down if you want better fuel economy.

https://www.fuelly.com/car/mercedes-benz/glc300

Teckno

How many miles are on the speedo ? Has the car been serviced annually to MB specs ?
A few things to do before paying the Dealer to run a set of test >
firstly prior advice here re speed is excellent.
1. Check tire pressures as per fuel filler door for front and rear, at load and ‘high speed’. These are cold psi, not psi after you have driven to fuel station to add air. You could have a garage ‘empty’ the air in your tires and refill with Nitrogen, larger air particle, less heat build up and less loss of air between checks, usually used in race track, but having 2.5 Ton rearing down the highway at 75mph will add heat and stress to the rubber.
2. Increase PSI to about 4PSI higher to give better handling, better braking response and better mileage, some slight decrease in comfort, but worth it.
3. Add an after market bottle of fuel tank and fuel injector cleaner, try a heavy duty one from a reputable shop. May have to repeat with new bottle after 3 or 4 full normal refills. Also what grade of fuel ?, try non eco fuel, use highest Octane, better engine response and fuel usually had additives in fuel to clean your system.
4 after your next oil change, add a bottle of friction modifier, like a ceramic or teflon coating in the engine with all moving parts, helps reduce friction and heat, engine functions better.
5. Certainly try Eco setting next time, then run manual changes, BUT Adaptive cruise control is worth it, less fatigue, less speeding fines and a better experience.
Let us know how you go.

Rmlasalle

I routinely get 30MPG on long trips averaging 70MPH and using cruise control in my2018 GLC 4matic.

eddieo45

while the particulars are rife with differences this reminds me a lot of my CLK experience. 22mpg was about all I ever got in my CLK350, and 75mph highway drives were the bulk of my miles. When I had to unexpectedly replace that car due to an engine issue I reluctantly bought a 550, the only CLK available to me at the time. That engine handled 75mph so effortlessly I routinely got 28-29mpg and sometimes got 30mpg or more. It's why I chose an ML400 over a 350. My wife gets about 22mpg in the GLC on average with mixed city/highway driving in ECO.

Idantal95

that’s what I thought I should get. Do you use eco mode?

edpare

2020 GLC300 RWD CPO, 31k miles since purchase, 30.6 mpg lifetime average. 50% I road @75mph, Driver Assist on always, ECO mode always, Auto start/stop always, adaptive cruise control when not gliding, system disengaged when downhill is steep enough to "Glide". Michelin Defenders @ 34psi. Name brand premium gas always. M-B service: two A's, one B. Best mileage above 8,000 feet in Colorado.

superswiss

I find that hard to believe. MBUSA only has the GLC coupe left on their website, but the official highway mpg is 26. Must be going downhill a lot. RWD will have slightly better mpg.

Yeah, the dirty secret of these small displacement forced induction engines is that they have to work harder at those speeds. On a long highway cruise going 75-80mph I easily do around 24 mpg with my C63S. The engine practically idles at that speed in 9th gear. Compare that to 12 mpg around town. It's how you drive relative to the car's power envelope when it comes to mpg. Top Gear did a funny episode once where they drove a Prius around a track, followed by an M3. The Prius was going as fast as it could and the M3 was basically casually following. At the end they calculated the mpg, and the M3 had better mpg than the Prius.

mikapen

​​​​​​That's almost our experience.
​​​​​​In order to "average" 70 requires mostly 80+ driving, so our trips "average" 60mph / 30 - 32mpg. It still involves some aggressive driving and a radar detector.
Aero does impact the mpg, since averaging 50mpg gets us 32ish.

Interestingly, our '20 GLE450 3.0 six got +2 highway and -2 city.

I believe I get better mileage with My Foot than with Distronic because I can anticipate things like traffic and hills. But I like my Distronic.

Peszter

I just did a 200 mile trip last weekend. 2020 GLC300 4matic 27k miles. Pulled 29.4 mpg in the rain and using ac on roads that varied in speed limit from 45 to 70, with me observing legal limit and using cruise control where practical. I only fill up with 93 octane fuel from TopTier stations.

62bassman

Idan Tal,
My 2018 glc300 4 matic, 40 lbs tire pressure , always gets around, and above 30 mpg on nonstop highway trips driving at 65-75 mph, around 22 in city and averaged 22 mpg over the last 40K, I always stay in Comfort mode above 60mph. Unless you are driving long uphill pulls and/or driving aggressively you should be getting better than 22.
you must still be under warranty. Have your dealer test drive the vehicle.

DanD.

I usually drive 10mph above speed limit and living at PNW we have a lot of hills
Getting 24mpg