optimum speed - fuel consumption

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Since we bought our 2.8 Peugeot diesel Autosleeper 3 years ago I have kept records of milage and fuel consumption (I have an iPhone app that makes it easy) and because I am professionally involved with testing engines I have tried to find the optimum cruising speed (ignoring headwinds) that gives reasonable journey times. I can't find the required data to calculate the road load equation for the van but I have done still air 'coast-downs' that help to confirm the rate of power increase with speed due to air resistance (you have to think of something on these long drives!)
The worst fuel consumption was recorded over single track Scottish roads, which make good sense and the best was coming down the A74 with a very strong cold following wind.
The problem is that optimum speed on the Mways is about <=60 miles an hour which is a fraction over the average truck governed speed. But once the brain is switched into that internal cruise control and I know that consumption is over 30mpg it can be a nice relaxing experience and I no longer worry about those rich and profligate Motorhome owners that howl past at 70+.
I admit to defeating my own policy by traveling with a half full water tank which increases the vehicle mass but its not too great an effect so long as there aren't too many junction and roundabouts on the route.
I would be interested to know what fuel consumption modern Euro5 compliant peugeot diesel Motorhomes get on average.
 
The max torque on the 2.8 engines is at 1800 rpm. This is not the best for mpg though.

I have the 2.8 JTD and drive at 2000 rpm which gives the best results (about 23.8 mpg on a reasonable mixed run, more on motorways). This equates to around 56 mph but my van has a 5 tonne MAM.

If you are looking for the best speed, be aware that your speedo will probably be reading high by about 4 or 5 mph at cruising speed. You can check this by using a satnav and compare its speed reading, which will be more accurate. Personally, I never bother with the speedo, I just watch the rev counter.
 
The best I've got from our 2.8JTD is 27mpg. When the trailer is on the back, it drops to 23mpg

I have found that the air filter is critical. The performance drops of at 5k to 6k of use

@ Maingate - I'm interested in the idea of a Tacho and I'll have to see about an add on
 
I think I have mentioned on a previous thread that on the old Murvi panel van 2.8 td we achieved 35mpg including the eberspacher heating system over 11 years & 115,000 miles.

So far the new Murvi panel van 2.2 td has achieved 31mpg including the webasto heating sysytem. Buuuttt as we have only done 35.000 miles so far I expect it to improve with age & mileage.

Bit like me !!!

Dezi :pc:
 
Our 2.8 TDI gives on average 25mpg at about 2000 to 2500 rpm. It is a bit of a brick at almost 3m high and a 4.5tonne limit.
 
***** said:
I thought a Tachometer was standard equipment?
It was on our 2004 Ducato!
You are not mixing Tachometer with Tachograph
Click to expand...

Perhaps I should have said "Rev Counter" and we don't have one on our 02 plate; by the sound of it, it might be useful
 
only bought Motorhome end of last summer (Fiat ducato Carioca 5, 2.0 HDi, 04 reg) but found that fuel consumption wasn't the main objective to find our cruising speed. It's the NOISE! Gosh is that Van loud! 60mph is quite ok but anything above that is just too much for a long journey IMHO
 
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