Doctor Diesel

Musing on cruising

I found the article in the August 2012 edition regarding ‘The Extra Mile’ and the consequences of the proposed 80mph speed limit most interesting. It got me thinking about what is the most efficient speed at which to drive.

My Dad was convinced the most fuel efficient speed to drive at was 56mph. However, when my Dad learned to drive, cars generally only had four gears, had mostly small petrol engines, were shaped like bricks, and you had to operate the windscreen wipers manually!

Dad and I used to discuss this particular subject at quite some length, but he was unfortunately a confirmed petrolhead, and I never managed to convert him before he passed away last year.

Today’s cars have between six and eight gears, with efficient diesel engines designed for hard work, and mostly have a good co-efficient of drag factor.

With modern cars having high motorway gearing and much reduced co-efficients of drag, the most economical speed to drive might well be higher than 56mph. What do you think?

Matthew Birkinshaw.

Yours are very interesting thoughts on the optimum cruising speed, Matthew. I think it is very much a case that the best cruising speed for fuel economy depends on the car, the engine, the gearing etc.

If you think about it, aerodynamics have a significant effect, as they use up most of the energy in the fuel at higher speeds. So, in the extreme case, the optimum cruising speed for a Hummer, or a petrol V8 Range Rover, is quite possibly as low as 40mph.

At the other extreme, a high-geared slippery little number like an Audi TT 2.0 TDI quattro could well be 60mph or even more. Also, the more flexible the engine and the better the low down torque, the more it is possible to hold a high top gear, which of course ought to favour diesel over petrol.

My car, with a six-speed gearbox, is an interesting case. I don’t reckon that in rolling country there’s actually any significant advantage in using sixth gear. On dead flat roads, maybe yes, but when frequent uphill inclines appear, it’s just not really worth the bother of constantly changing up and down between fifth and sixth. I just stick it in fifth and leave it there, holding typical engine speeds of 1,800 to 2,500rpm.

I have tried to measure the difference on dead flat roads at various speeds between sixth and fifth, but you just can’t get a long enough unobstructed run to draw any firm conclusions – although sixth is obviously best on motorways and dual carriageways, at 60mph and over.

I don’t think many people are aware that sixth gears can be too high for cross-country motoring, and I have a pal who tows a sizeable caravan with a Citroën C-Crosser considerable distances around France.

I have been trying to persuade him to try slowing down a touch and to use fifth instead of sixth when towing, as he doesn’t seem to appreciate that the dropping back to say 55mph from 60mph could offer him some sizeable fuel savings – and that can be significant when you’re only getting 24 to 26mpg!

At 55mph, he’s probably more in the meaty and most efficient part of the torque band than he is in sixth.

I could ramble on longer, but I must not. Thanks for your interest Matthew.



Letter one - motorway cruising - 31

Shocking revelations

I feel that my recent experience with my Mk5 Golf 4Motion could be of interest to your readers. On switching on one morning, my multi-function display advised that my off-side stop bulb had failed. I wasn’t sure whether this constituted an offence, bearing in mind that my car has a central stop light warning, but regardless I went ahead to change the bulb.

Working on the rear lamp units was certainly a lot easier than my experience with the front lamp units and the offending blackened bulb was soon revealed. There were two apparently standard tungsten bulbs plus a curious small central bulb which was obviously for the indicator. As I recall, this was rated at six watts instead of the more normal 21 watts.

It was then that I became aware that the two main bulbs were not dual (5w/21w) filaments (that one used to encounter – Doc) but were both single filament bulbs rated at 21 watts! I replaced the unit, but was left wondering how on earth the rear end stop lights could possibly function with such an odd bulb configuration.

I subsequently contacted my local VW service department and the magic word CanBus emerged! Apparently my Mk5 heralded the introduction of this new computerised wiring system which is now universal, with the possible exception of LED systems. I have yet to encounter anyone who has ever heard of CanBus.

I have long felt that Diesel Car Magazine should devote more time to technical details, i.e. tyres and transmission systems, as opposed to focusing on new car reviews, and I feel strongly that CanBus should be at the top of the list!

One other thing: you quite often dwell on the fact that diesel cars take a long time to warm up. I have never felt that my car was slow in pushing out some heat and it was only recently that I became aware that it was fitted with an auxiliary electric heater. Is this unusual? (Not according to Tom Jones! Doc.)

I was certainly never advised that this was an option, but it would rate very highly on my list of desirable optional extras. Keep up the good work.

Ron Montgomery

Very interesting subject indeed, Ron. Thanks for the feedback regarding your thoughts on the relative space given in Diesel Car to technical features and things like new car reviews. Ian Robertson will obviously be interested in your opinions on this.

We do have a wide spectrum of readers and we have to try to meet quite varying interests. CanBus is a rather complex technology and not everyone wants to know what goes on under the bonnet and beneath the carpet!

CanBus is, as you discovered, a computerised control system for car electrics that essentially uses two control wires, Can high and Can low, to communicate with all car control systems, using coded signals. It saves a huge amount of weight and cost compared with conventional wiring, where an individual set of wires was run to each unit, like an indicator unit, or an electric window.

With CanBus the ‘high’ and ‘low’ relate to the signal speeds involved, Can ‘high’ being used for vital control systems like engine control units, ABS, and safety systems, ‘low’ being used for lighting and instrumentation. But enough for just now – some of our readers are already dozing off Ron. Thanks for raising the subject though, which we’ll be revisiting one way or another in a future issue.

Regarding electric pre-heaters, these are a standard item on many Volkswagen Group and other cars. They used to be diesel powered on early cars, like diesel Tourans, but there were a few problems with the hot exhaust gases setting light to dry grass in camp grounds, and leaving mystified owners wondering why their trainers were apparently melting when they stood alongside the car!


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