'70's XJ12 Real Gas Mileage

Our 4.2 gets up to 22mpg (imperial) freeway/highway running. It has a 3.07 diff.

That’s pretty good. XJ12 had 3.31 diff. 4.2 man o\d S1 I owned regularly returned 24mpg open road, ditto 3.8 and 4.2 E Types. Best E type mileage was 29 mpg on inaugural open road trip when I was being particularly careful, but still cruising at legal limit. 200 mile one way journey, I thought the fuel guage was faulty as it was still showing half full. Quite chuffed at that.

The diff fully explains the difference at the time, Andreas - the slower the engine turns for a given speed, the less petrol it uses just to run itself. At that speed the engine develops enough rev and power - the secret of modern car economy is umpteen gears to ensure just enough rpms to propel the car at the desired speed…

Certainly, 20 years ago, the car was much younger, wear takes its toll, and adjustments may drift off, injectors may not be perfect - and whatever. Tyres play their role, inflation and quality - as does driving style. More than marginal changes in fuel consumption warrants some attention…

xj6 85 Sov Europe (UK/NZ)

Imperial vs USA gallons is an important distinction in these conversations :slight_smile:



I do seem to recall a road test in which they remarked that the V12 was less affected by nursing the throttle, so speed had relatively little effect on mileage ( within reason,obviously) so was as (un)economical at 50mph as 70mph or 80mph.

Funny you mentioned that, I always thought that the V12 operates in a Lt/h mode rather than Lt/Km…!
An other observation is that it will sometimes have worse mileage at 40Km/h than at 120Km/h…
The most economical speed is at 80 to 100Km/h though.

My 1973 DDS with carbs and without aircon has averaged 16.2 MPG (imperial), which is 13.5MPG (US) and that is mostly cruising in the 50 to 75MPH range.
Since the advent of E10 petrol here in UK it has dropped to 15.7MPG and with supermarket petrol her at £1.65/ltr or £7.50 per gallon it’s getting really painful to fill up !
I believe those blessed with aircon lost at least another 2.5MPG due to the rather inefficient system back in the day!

Best regards,
Mike (1973 DDS S1)

One thing I’ve experienced all my oldie Jags (XJS, Series III sedans, XJR/6) is that 70 mph seems to be the magic point where fuel economy drops off significantly. Perhaps that’s where aerodynamics start to become a factor.


Yes, double the speed and quadruple the aerodynamic drag, as I suspect many of us know.

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At a steady and loooong period of 65 mph, Tweety got an honest 25 mpg: raise that to 75 and boom!

19 mpg.

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Yes, but the engines have their sweet spots… at a constant 40 kph the engine will be far less efficient than at 120. And somewhere above that you start requiring serious power and even faster and most newer engines (old ones too?) will eventually go into full load enrichment…

Not proposing that we drive at 40, just noting the physics. I would still be in second gear in my cars at 40.

I’ve always felt the bigger XK engines worked best approaching peak torque, around 2,500 to 3,000rpm. Depending on gearing, that could be somewhere between 50 and approaching 80mph. I also think my XJ performed better at those speeds when equipped with the 4.2 rather than V12. In fact an auto UK spec pre-HE XJS has much the same performance as an SWB S1 XJ12, but the faster manual version has markedly poorer top gear performance when compared to a manual 4.2 S1 XJ6, until the 70-90mph increment, and then only by 0.4 sec. Even the 80-100mph increment is separated by 1 second. BTW these XJS figures are from the famous test in which the car was started on the starter motor in top gear and accelerated on to 140mph as a timed run.
IMO, the man o\d 4.2 was a remarkably rapid car in real world driving,probably the best Jag I’ve ever owned.

Yes, gearing versus engine torque curve and road speed is something to consider. The V12 isn’t suited to a 3 speed automatic. Not, at least, if utilizing the available power is a consideration.

And the 2.88 diff used in many V12 cars really blunts acceleration.