Are speed ramps killing my fuel economy? - MichaelR
Hi there.

I'm a very infrequent poster but daily reader to the forum, which is why you've probably not seem me about much :)

I drive a Citroen Xantia 1.9TD. Recently, I'd been becoming worried that there might be a fault with my car becuase I was getting low fuel economy - usually about 32mpg.

Each weekday, I'll drive a mile down the single track lane linking my estate with a junction onto the A38 - an NSL dual carriageway. I'll then drive along this dual carraigeway for 5 miles, before pulling off and driving for another mile through very light (I rarely have to stop bar a single set of lights) urban traffic. I repeat this in the evening, and will often pop down the mile long lane in the evening to the shop etc, and back up. I don't drive around with my foot firmly planted on the accelerator. Now, that sounds like pretty untaxing usage - I'm never usually stuck crawling through rush hour traffic. I was very concerend about my low economy.

Last month, I travelled up to Southampton from Plymouth, which is a mixture of dual carriageway, and NSL A road's, with some steep hills. I wasn't hanging about. I also drove up to Didcot via the M3, out for a jolly in the New Forest, crawled around in traffic in Southampton, a city which simply has far to many traffic lights, and went for a trip to Fareham using standard urban A roads. I then returned to Plymouth. All on the same tank of fuel, which I calculated to be about 49mpg. Significantly improved over my usual usage.

Now, the single track lane I mentioned, which I'll negotiate anything up to 4 times each day, has 7 speed ramps on it. Not mild ones, or cushions, but full width ones, which are quite harsh - a family members Golf TDi with Sport Suspension grounds on them, for example. I usually have to slow to about 10mph to take them. It occours to me that the reason for my shocking fuel economy when at home could well be the slowing down and subsequent speeding up from using these speed ramps. Am I on the right lines here? Could it make such a difference?

Also, what effect are these ramps having on my car? I'm driving over in excess of 14 of these stupid things every single day :(

Are speed ramps killing my fuel economy? - Armitage Shanks{P}
I may be on the wrong lines here but some of your short drive(s) must be done from a cold start? This will give poor fuel consumption. The two journeys you are describing are, broadly, the difference between urban and extra-urban driving. I can\'t find any figures for your car but looking at Citroens, a 2.0l HDI does 40 in town and 67 on the open road! I think the difference is just that between your two journeys. Slowing down for traffic lights etc is just the same effect as slowing down for stupid road humps. (See an earlier thread about Construction Regulations for humps)
Are speed ramps killing my fuel economy? - Andrew-T
If you have eliminated all other causes for increased consumption, it must be down to the cautious progress over the speed humps. That said, I haven't found that diesels are anything like as affected by short-journey driving as petrol engines. SWMBO's 205 Dturbo spent a lot of time on shopping runs but still managed over 50mpg; that only improved to about 57 on long trips. The driver of a LWB diesel Mercedes van used for his local joinery business claimed 50mpg the other day.
Are speed ramps killing my fuel economy? - Ben79
Firstly, with so many speed humps, I bet you are glad you drive a magic carpet.

Secondly, I presume the car has been serviced correctly recently, the fuel and air filters aren't dirty.

Thirdly, speed humps do increase your consumption, as you are braking and accelerating in 2nd gear, without the humps you would cruise past in 4th or 5th.

Fourthly, how many miles has your Xantia got? Do you use big brand diesel like HJ recommends? Have you tried injector cleaner?

We should be campaining for speed humps to be removed, they increase local pollution, break your cars suspension and create congestion.

Town planners should build housing away from main roads and do everything in their power to keep the traffic flowing, use of roundabouts, bridges, intelligent traffic lights and more roadworks at night away from residential areas.

Are speed ramps killing my fuel economy? - MichaelR
The car has 151,000 miles on the clock and I have recently changed the air filter etc. As far as diesel is concerned, I always use Sainsburys diesel usually as the filling station is right at the end of the road. I've not tried any injector cleaner, and yes, the first trip down each day is always from cold.

Thanks for the help so far :)
Are speed ramps killing my fuel economy? - SteveH42
Short journeys will always use more fuel than longer ones - I average about 35 on the way to work, but can get in excess of 50 on a run, more if I take it easy. The speed ramps won't help though - it is acceleration that guzzles fuel, so having to slow down and then pull away from each hump will use a lot more fuel than if you could just cruise along at a fixed speed.

BTW, not the Michael Rodgers of uk.r fame by any chance?
Are speed ramps killing my fuel economy? - Marcos{P}
On the short run to work my M.B. E320 CDI will do about 20mpg but on a longer run it will virtually double.
This is my first diesel and the cold start fuel consumption is awfull but once warmed up improves dramatically. After speaking to several people this seems to be a common trait with diesel engines.
Are speed ramps killing my fuel economy? - kal
Is your merc an auto or manual?
Are speed ramps killing my fuel economy? - MichaelR
The very same Michael, Steve :)

Thanks for your advice people :)
Are speed ramps killing my fuel economy? - Marcos{P}
My Merc is an auto so I suppose that would make a difference as well.
Are speed ramps killing my fuel economy? - Armitage Shanks{P}
I recall that, because most Merc autoboxes change up and down at optimum rpm and speed combinations, some of them are more economical than manual boxes. That said, there are so few manual Mercs around it would be hard to establish the facts. I can't find any figures in the charts relating to comparisons between manual and auto boxes,only diesel and petrol engines of the same capacity where, obviously, diesel does better.

Value my car