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Desert Orchid

Desert Orchid

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Aug 2, 2005
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My fuel consumption has deteriorated significantly in the last few months. It seems to have the garage flummuxed. I can't help thinking it's something dead simple.

I think the road wheels simply aren't running as freely as they used to/should. In an effort to save as much as possible in running costs, bearing in mind I do 33,000 miles per year and pay for it all myself, I 'coast' whenever I can. There's a stretch of motorway which is fairly steep downhill for about a mile. I usually hit the top of the hill at 70mph and slip the car into neutral. By the time I get to the bottom, I'm usually doing about 73mph. Lately, I've been losing speed on this same stretch, hence my theory.

The garage propped up the car and spun the wheels and concluded they weren't binding. They say it might be the gearbox but I don't see how the gearbox would affect the freewheeling of the car. I don't want to put the car in and give them the opportunity to spend unnecessary time and expense on the gearbox, which I'll ultimately have to pay for.

My question to the panel is, is it possible the gearbox ("it might just be a wee bit tight", said the garage) would cause the car to lose momentum when freewheeling?

At the moment, I've worked out that I'm buying two gallons more per week than usual, which will fairly add up over a year.
 
Not the tyres is it ? Have you changed brands and are they the right pressure ?

Inflating tyres to their proper pressure can improve mileage by about 3.3 percent, whereas leaving them under-inflated can lower mileage by 0.4 percent for every one PSI drop in pressure of all four tyres.


That may not sound like much, but it means that the average person who drives 12,000 miles yearly on under-inflated tyres uses about 144 extra gallons of fuel doing so.
 
The tyres usually need changing once a year. I usually check the pressure after they've been changed and then again fairly regularly although not every time I fill up (every fourth night). I'd be inclined to rule out tyre pressure but I didn't realise the impact could be so significant.

Also, in this instance, we're talking about over 10% deterioration in consumption. My on-board computer tells me how many miles the tankful will get me. It's usually 800m+ but lately it's showing under 700, averaging 680-690. (I usually fill up at the same garage after doing the same 85m journey.)
 
Get your local council weights and measures to check out the pumps ??? Its not beyond some people to make a few bucks by short delivery? :o

Or try another garage being that all other possibilities have been tried ...and no if free wheeling there can be no drag from the gearbox as at that moment in time when the clutch is depressed the gearbox stands alone........... ;)

So if its not pads binding and its not deflated tyres and you have regular service ie. bearings are greased, there's nothing else that mechanically that can go wrong ...have you tried changing air filters????????? one bunged up can increase consumption
 
I can't imagine a garage running the risks involved in having inaccurately calibrated pumps.

Would there be a difference between freewheeling in neutral and freewheeling with the clutch depressed?

The car is serviced every 12 weeks or so. I'd be angry if the air filter wasn't changed then.

I really think there's direct link between the consumption and the car losing momentum on a hill where it used to gain speed. Aren't all modern bearings sealed units? I thought it might just be a question of greasing the bearings but I'm sure I read somewhere you can't do that with sealed units.

Thanks for all the suggestions so far. Keep em coming...
 
If the gearbox wasn't fully disengaging properly as the garage suggest then it would surely be turning the engine over, this would show on your rev counter.

A boot full of junk adding extra weight can have a significant impact on fuel consumption but is unlikely to to be cause of your car not picking up speed downhill.

As Merlin suggests tyres would be the first thing that I checked. Not only the pressures but have you change to a different profile last time they were changed?
 
I'll ask the garage about that. I give the guy some room for his own judgment when it comes to the tyres. He knows if he can get me a Pirelli for only £5 more than a budget tyre I'll pay for the Pirelli and he always automatically employs his 'cheapest in town' pledge. If both tyres need changing (usually the case) he gives me a further discount.
 
Uhhhh.... I thought that if you had a prang if in motion in neutral, you were guilty of careless driving? Aren't you supposed to be engaged in a gear when moving?

You're very rigorous about getting your car serviced! Mine's lucky if I remember a midyear one, let alone four per annum. That could save you a bit if you cut out a service.

Also - have you put on a lot of weight? :lol:
 
I'm aware of the legalities of the situation, auntie K, but it's a fairly safe strretch and I'm always extra wary when I do slip it into neutral for that reason.
 
Ah, so it is illegal, then? I thought it was, but it's a longgg time since I took a test, although I wouldn't have thought such a basic rule would've changed.
 
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