S320CDI: most fuel efficient cruising speed?

[ItalianTuneUp]

On long motorway drives I usually use cruise control, and I've been wondering what the magic speed is that gives the highest MPG.

I'm especially interested in replies from people with the W220 model of this car, which uses the straight six engine, iirc.

Try not to bring special premium-priced diesel into the argument, as it seems these give slightly better figures at a price.

I generally use 'supermarket' diesel...i.e. cheap diesel. Occasionally I treat it to a premium-priced diesel to clean it out.

 

[Borys]

My old w220 cdi 52plate, I loved it
Did a trip to Eastern Europe I got nearly 47mpg
Best car I ever owned when it comes to economy in this class-stunning

 

[ItalianTuneUp]

I agree...wonderful, comfortable car, and fuel economy can be not too bad if you're careful.

What kind of speed did you drive at to get almost 47 MPG? And was that with cruise control on or off?

Economy or premium diesel, tyres?

 

[V12]

65mph for me if I'm not in a hurry. Always seems to get amazing mpg for me!

 

[renault12ts]

At 50mph you should get over 50 mpg...but do you really want to do that?

 

[ItalianTuneUp]

65mph for me if I'm not in a hurry. Always seems to get amazing mpg for me!

What mpg do you get at 65?

 

[ItalianTuneUp]

At 50mph you should get over 50 mpg...but do you really want to do that?

Probably not, unless time is not an issue

I almost got 50mpg once although I didn't manage it again, so probably had the wind behind me on that day lol

 

[V12]

What mpg do you get at 65?

Depends on lots of factors, but in the summer I always see over 50mpg on almost every motorway trip.

 

[V12]

Simon, the last time Pawel had your car in, I noted that yours has a DPF, that would account for the difference in mpg between our cars.
Oh, and I always wear my ultra light driving shoes...

 

[ItalianTuneUp]

Simon, the last time Pawel had your car in, I noted that yours has a DPF, that would account for the difference in mpg between our cars.
Oh, and I always wear my ultra light driving shoes...

Ah Ricky, it's you, I did wonder if it was.

DPF= diesel particulate filter?

How does that affect economy?

The shoes must make all the difference...I did wonder what your trick for high mpg was ;-)

 

[ItalianTuneUp]

I think Pawel must groan every time he sees me. But I do like something that runs nicely.

 

[IanS320]

I'm a bit disappointed by economy at the minute. I've squeezed up to about 37mpg at motorway speeds (which doesn't seem affected by a few additional mph) but when the car sits in crawl traffic the average is about 23mpg. I was surprised by this...car effectively at idle....does going into neutral help? Does blue motion make a huge difference?

 

[ItalianTuneUp]

I think 37 is probably about right if you're doing about 80. Lower speeds should see the mpg raise by 5 to 10 mpg on the motorway.

23 sounds bad...about what you'd get around town without the lightest of right foot. How long did it take to drop from 37 to 23 mpg ?

 

[V12]

Simon, i'm sure Pawel loves you as long as you keep paying the bills.
Us S Class drivers keep him in business!
DPF is a topic for you to google. The decrease in mpg can reportedly be anywhere from 5 to 20% from what I read on t'internet!
Depending on the state of the filter it can restrict exhaust gas flow. Active regeneration can also affect the economy dramatically.
dieselnet.com has some good info with decent sources.

I'm a bit disappointed by economy at the minute. I've squeezed up to about 37mpg at motorway speeds (which doesn't seem affected by a few additional mph) but when the car sits in crawl traffic the average is about 23mpg. I was surprised by this...car effectively at idle....does going into neutral help? Does blue motion make a huge difference?

To start with you have the V6 engine which is known to be less fuel efficient than it's predecessor.
There are many other factors to consider, such as :
-Wheels / tyres
-Suspension / alignment
-Thermostat
-Driving route / time / traffic
-Ambient air temp.
-Perhaps most importantly of all...the driver.

 

[ItalianTuneUp]

And don't forget, Ricky, the company that one keeps whilst driving (choice of radio station etc)

 

[ItalianTuneUp]

Simon, i'm sure Pawel loves you as long as you keep paying the bills.
Us S Class drivers keep him in business!
DPF is a topic for you to google. The decrease in mpg can reportedly be anywhere from 5 to 20% from what I read on t'internet!
Depending on the state of the filter it can restrict exhaust gas flow. Active regeneration can also affect the economy dramatically.
dieselnet.com has some good info with decent sources.

Yes, you're right

I'll have to read up on DPF, sounds interesting. Does keep the pollution smell from entering the cabin though.

I think when I nearly got 50mpg, I had air con/DPF turned off, DRL lights off, and the wind was behind me, and it was downhill, and it was a full moon.

 

[V12]

You're confusing the DPF with the active charcoal cabin filter.
The DPFs only purpose is to reduce emissions and keep Brussels happy.

 

[ItalianTuneUp]

Ah, my mistake. So I suppose that at some point the dpf needs cleaning out at vast expense? And if done, one gains a small rise in mpg?

 

[IanS320]

23 sounds bad...about what you'd get around town without the lightest of right foot. How long did it take to drop from 37 to 23 mpg ?

I know...I thought so too. With a bit of resetting of the trip I find I do very low 20s in a crawl which can quickly mess up the overall figure when the journey is 10 miles of crawl, 10 quick motorway.

I suppose the car is telling me something very accurately that I used to just assume was different....I thought cars used little fuel at idle.

 

[Godfrey]

Best mpg when you are stuck in traffic doing below 60, but cruising uninterrupted at 80 achieves maybe 35mpg on a good run for the V6 320cdi