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Impact on MPG by carrying more stuff.


Conrad

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I know it's a good idea to travel with empty water tank - in our motor home a full tank would add 100kg - but is there any accurate formula to calculate the impact of extra weight? Our Chasson 646 didn't have a spare wheel, so I've invested against the bother of a puncture when away from our home area by getting one and jack. Weighs I guess 30-40kg.
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I doubt it would be possible to calculate the difference as consumption is dependent on so many factors, but 100kg is less than 3% of your all-up weight, so I can't imagine it would make any discernible difference. Provided you have the payload, I can't see any reason not to travel with a full tank. I know some say it can affect the handling, but I've never noticed any difference in handling whatever the level in the tank.
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Towing a trailer increases fuel consumption markedly, or at least it did for me when I towed a large box van trailer with a heavy motorcycle in it. About 30-50% increase.

 

Speed is the other big factor; I took a new car to Italy many years ago and cruising in the German autobahns at 90mph increased the fuel consumption by a staggering 38% compared to cruising at 70mph. A cruising speed of 56mph is said to be the point at which fuel consumption starts to rise steeply once you go faster and since MHs are not very aerodynamic I would expect that even 70mph would mean much higher fuel consumption.

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My M/home has been loaded to the gunnels and till recently I towed a trailer to Spain and back. Solo but still loaded to the max permissable weight I averaged 28mpg and criused at approx. 65/70 where allowed. With the trailer( 1300kg max) still managed approx 25/26 mpg at permitted speeds. It also depends on conditions and terrain ie strong winds or as in Normandy long steep hills. This shift to smaller higher bhp engines is OK but I miss the grunt of the 3ltr, it was smooth gutsy and economical probably more so than the new breed of engines hence they are still available on the top end models ie Morello, as a a choice.Being an ex tugger bigger engines were always the choice a few years ago, as they were more stable and economical under load.
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OK, many thanks for the replies, aandy for putting extras as a % of total mass, and StuartO and spirou about speed, acceleration and aerodynamics. I nearly returned to van at first trip as it only gave 22mpg, but then realised we had a strong headwind, usually now get 27. Still can't get my head around having paid £51,000 for the dream and have to use bottles for drinking water as you can't trust the tank water to be safe. Important thing is we've sorted how to accomodate the dogs and grandchildren at night!

 

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Conrad, I have been searching for the formula off and on for many years. The only sensible information I’ve found was based on research in the US using a Ford Fusion (Mondeo) - think that weighed the equivalent of 1700kgs.

 

This research identified that tyres and power train had a bigger impact than weight. However, in the final analysis, the figures for City driving showed a 3.5% reduction in mpg for every 10% reduction in mass; Highway (which I consider to be be cross country driving) 3%; Aggressive driving 4.5%. This supports what Spirou is saying.

 

When we had a Swift motorhome and regular visits to the dealer as well as occasional holidays, empty the 2.5td gave 29mpg, fully loaded (an extra 600kg with 4 bikes, water, 3 passengers and a dog)) 27mpg.

 

My son has a 3500kg Iveco Daily van which is geared for high speed cruising. On a motorway run up to Glasgow, fully loaded mass and cruising at 70mph as much as possible he gets up to 38mpg. Unloaded on the return journey, it’s no different. On local deliveries, half loaded, he gets 25mpg rising to 27mpg unloaded. Figures based on the van’s trip computer and rounded off. His drive train is designed for high speed, long motorway runs.

 

If I go down to the A5 via Whitchurch in my motorhome loaded to 3400kg, I sometimes find the fuel consumption drops to 25mpg because heavy traffic increases stop/start on roundabouts, and 28mpg with light traffic. Going to Stoke via Nantwich (the same distance more or less), I record 28 - 32 mpg because it is easier to drive smoothly.

 

You can travel with as much water as your tank holds providing you do not exceed your plated weights (3500kg? axles?). Manufacturers are given the option of quoting the MIRO with no water in the tank, 20 litres, or full. If they choose 20l, they have to have a device which can limit the tank to 20l - my tank has a plug that can be set at empty, 20l or full. For the last six years plus, I usually carry 30l.

 

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If we run our van all but empty it does 28 mpg. If we run it loaded for two weeks away and with full water and fuel tanks it does 28 mpg. In both cases, if I add my car trailer carrying my vintage racing car it still does 28 mpg. I've regularly checked it over 10 years and 35000 miles. It's always done 28 mpg no matter what.

 

FD

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Conrad - 2019-08-11 10:14 AM

 

. . . Still can't get my head around having paid £51,000 for the dream and have to use bottles for drinking water as you can't trust the tank water to be safe.

Off topic I know, but why use bottled water? We’ve always drunk our tank water. If you think about it, the tap water at home has most probably taken weeks to reach the tap as it’s travelled through underground pipes, and we drink directly from that. And as long as the van’s tank is filled from a clean and preferably chlorinated drinking water source there’s no reason to expected it to somehow become contaminated being held in a sealed tank. On top of that the water is regularly refreshed as most of us end up refilling every three to four days. The key thing, imo, is to empty the tank down after a holiday and allow it to dry out. I leave the tank on its vent setting to assist this process. It’s inevitably bone dry within a few days.

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BruceM - 2019-08-11 2:42 PM

 

 

. . . We’ve always drunk our tank water. If you think about it, the tap water at home has most probably taken weeks to reach the tap as it’s travelled through underground pipes, and we drink directly from that. .

 

if you think about it a little more deeply, or understand the requirements of a potable water distribution system, the water from the tap has not been exposed to the atmoshere since treated but contained in a pipe where the modest amount of chlorination can't leach away.

Being held in a conventional vented tank that could often be anything but totally filled, then things are very different to water from a drinking supply tap, the reason in your home the kitchen cold water must come directly from the water main, not via a header etc.

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As others have said, motorhomes are far from aerodynamic and drag has a very significant impact on consumption. Remember that drag increases by the square of any speed increase, so even a modest difference in speed can make a fair bit of difference.
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One factor of carrying liquid in a tank without baffles is that it can affect you cornering ability dramatically as it sloshes from side to side.

The water tank also tends to be above the axle height so more weight up there can make you roll more on cornrs.

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The issue with water sloshing in the tank sounds technically correct but I have never noticed any adverse handling effects of travelling with part filled tanks. The tanks in our van are long and thin running the length of the centre of the van so side to side sloshing would be minimal and the effect would be more back to front or front to back. Not worth worrying about.

The thing that affects mpg most is speed together with how harshly you accelerate and brake. I get 30mpg easily on a long run at a steady 60-65 mph but this can drop to 25-26 mpg if I travel at say 80 mph. The overall average to date is around 28mpg and it never varies much tank to tank unless I have been travelling at higher speeds for hundreds of miles. Again I don’t spend any time worrying about a few mpg either way, I just enjoy the van. The difference between 30mpg and 25mpg over 5,000 miles is £200 and the actual difference for me over a year is going to work out way less than that so a small cost to worry about compared to all the other costs involved in running a motor home.

The best way to reduce fuel consumption significantly is to slow down and anticipate the need for acceleration and braking and limit both as much as possible.

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Billggski - 2019-08-11 4:08 PM

 

One factor of carrying liquid in a tank without baffles is that it can affect you cornering ability dramatically as it sloshes from side to side.

The water tank also tends to be above the axle height so more weight up there can make you roll more on cornrs.

 

 

I have never noticed any such effect on either of the vans I've owned, despite the fact that I always travel with the tank somewhere between full and half full. Any such effect would probably be at it's greatest when the tank is around half full which, for a 3500kg van with a 100 litre tank would mean a mass of water weighing less than 1.5% of the overall weight moving a couple of feet. I can't imagine that would have any effect on cornering.

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Ocsid - 2019-08-11 2:58 PM

 

if you think about it a little more deeply, or understand the requirements of a potable water distribution system, the water from the tap has not been exposed to the atmoshere since treated but contained in a pipe where the modest amount of chlorination can't leach away.

Being held in a conventional vented tank that could often be anything but totally filled, then things are very different to water from a drinking supply tap, the reason in your home the kitchen cold water must come directly from the water main, not via a header etc.

 

I understand your rational and it's a good point. However Defra’s advice is that header tanks are ok to drink from as long as they’re sealed against vermin, screened from light, have screened ventilation/overflow to stop insects getting in, have sufficient flow and the water is kept as cool as is practicable. This applies to residential and public buildings. Naturally, new builds will usually be mains connected for all taps.

 

Of course when I was a kid in the days when hot water in houses was fed from a header tank verses the mains, we were always told not to drink from the hot tap. I understand that was because the header tanks in the lofts were invariably open topped and could sometimes be housing the odd dead mouse and furthermore the water was fed to an immersion heater where it was warmed up to provide an excellent breeding ground for adverse bacteria.

 

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Thanks again everyone for you varied responses. For those who didn’t get through the 60 page study report Derek Uzzell quoted, gist of it is that extra 10% weight only makes couple of mpg difference, main factor as others say is speed and therefore wind resistance.

Also we’re reconsidering using bottles for drinking water, bit of a faff filling them up at site tap anyway, will at least only use it for cold drinks, especially as we can chill it nicely in fridge, tank water now if boiled for tea/coffee.

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I think that the suggestion to run with near empty water tanks is just a brainwashing exercise by the motorhome constructors to disguise the fact that their products have inadequate payloads to do otherwise. I would far rather run with a 100kg of low down water to offset the leverage of heavy roofheight beds or lockers.
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Motorhome aerodynamics are a bigger factor than weight regarding fuel consumption

A few years ago, a trip to Southampton with strong head wind = 23mpg.

The return journey, reverse route & with a very similar load, but strong tailwind = 28mpg

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