At moderate highway speeds of say 60-70 mph my 1963 3.8 Mark 2 delivers no better than 10 mpg. The car performs very nicely and doesn’t smell of unburned fuel. I would appreciate knowing the mileage other owners get.
Best regards,
garth
I Greetings All,
Garth, there’s a lot missing here.
Manual or automatic?
Is there an overdrive?
What rear end ratio?
Big difference between 60 and 70.
Too many things here missing.
My opinion is that even with a DG250 10 seems low, but there’s a lot of factors that could be dragging it down.
Hi Garth, whilst I have not checked our 63 Mk2 3.4 auto yet. I know that our 72 XJ6 auto manages 21 imperial mpg at motorway 60-65 mph. The Mk2 definitely is a little better than the XJ6 as the DG250 is direct in third gear - like a manual. Paul
I get 15 miles per UK gallon UK gallon or 17.5 US gallons on average
Dave
Manual 3.4 with overdrive…
Dave
Greetings All,
The mileage you are getting seems low for a car with Manual and OD.
What is your rear diff ratio?
Did your car come originally with the manual?
What are you ticking over for RPM at 60MPH in overdrive?
With my 1965 3.8S with MOD, 9:1 compression, I have gotten as much as 19.5 MPG (US gallons) on a highway trip with speeds at about 60 MPH.
'Twas a long time ago but my 3.8S DG on a night run managed IIRC about 25 imp MPG
10 MPG almost indicates a leak before the fuel is getting to the engine?
I am getting around 100 smiles to the gallon , that’s UK gallons , as for mpg , who knows 
This seem about right to me! Paul
On my 1966 MK2 with a 3.73 and OD I been been getting better mileage as time progresses beyond the rebuild. I have a heavy lead foot and 12 is the normal around town for romping the engine and waving to the ladies! I recently got 15.7 on a mountain excursion. I truly think on a highway trip I would get 17 MPG US. I will be going on a 1000 mile anniversary trip through Yellowstone in September and I will post my figures to the forum at that time. What could possibly go wrong???
I am in need of a differential rebuild and am leaning towards a 3.07 as I do not need this much power off of the line.
Gerard
Loveland, Colorado
Thanks to all who responded to my enquiry. I don’t know what my rear-end ratio is and don’t know how to determine what it is . The first and second gears seem quite low so low in fact that I often shift from first to third and then directly to fourth and overdrive. There are some tags on the differential cover. The only one I can make out reads BPL. There are two other metal tags I will need to remove in order to determine what info is on them.
The car original gear box has been changed to a syncromesh. and has an overdrive. The car has always had a manual transmission. The tires are 195 x 15 rather than 185 x 15 so the speedometer is off a bit.
Presently I am preparing the car for the Going to the Sun Rally in Montana so it is at the moment on my lift about to have a new high pressure power steering hose installed. When it is back on the road I will check on the suggestions that have been made and get back to all of you.
If you are going to change the diff ratio I would advise against the 3.07 as when you take into account the O/D the effective ratio becomes something closer to 2.88 great for cruising but my 3.8 ‘S’ struggles a bit off the line on a hill start.
I blew my diff after replacing the input seal and not realising that it had a crush tube, I just cranked the torque up, ah the delusions of youth 
this was a good many years ago now. If you can get the auto 3.54 diff with LSD (not the drug) I think you would find this a better compromise. Just my 2c
Well that seems to answer your question.
Was the Synchromesh tranny ratios the same? What model did the synchro unit come from?
What gearbox number?
You might actually benefit from a different rear end ratio.
I would agree that the 3.54 is a good compromise. This ratio is what was fitted to non overdrive cars.
Thanks for all your interest and helpful suggestions. I am the third owner of our very beautiful saloon so their is a lot I don’t know about. The car runs beautifully with good performance so I need to determine what the transmission and rear end combination is and then make some choices about what needs to be done.
I just about have finished prepping the Mark 2 for our trip to Montana where we will join about thirty or forty other classic and sports cars and their owners for the Going to the Sun Rally. This will be our sixth time for the rally. Since we will trailer our Jag behind our Expedition I’ve had to check over the tires, brakes, and electrical components. The Expedition is ready to go thanks to the Ford Store in Morgan Hills, California. It’s about 1,100 miles to Bozeman, Montana from our home in Saratoga, California, the rally is about 1,200 miles and then back home so that’s a lot of driving. Anyway, great cars, great people from all over the west, fantastic food and scenery.
Robin, how about letting us know about your trip to the Yellowstone.
Best regards to all,
Garth
Is this another Robin on the site that’s going to Yellowstone? I’m in New Zealand and not likely to go to Yellowstone in the near future. Wouldn’t mind but can’t be bothered with the extra security to get into the States now 
Hi Garth, I’m resurrecting this post with a Mk2 data point hot off the press - today. We have a 34 Auto - the DG250 that locks up “direct” in top gear. Drove 156 miles (confirmed by GPS) on 29.3 litres of premium fuel. In and out of the same garage and pump this morning and this evening. Steady driving between 60 and 70 mph and a little bit of town work. That’s 24 imperial mpg. Sits about right with the 21 imperial mpg I get out of the good lady wife’s '72 XJ6. FWIW, Paul
I have a 1967 Mark 2 with all matching numbers from the factory engine and four speed all synchromesh with overdrive transmission. On a 260 mile round trip on which I filled the car at the same pump at the same filling station starting & returning., I managed 21.5 MPG US. Most of the trip was on a wide open freeway at speeds between 60 & 70 MPH in overdrive. I’m not displeased with the result considering the car is 50 years old, with 40000 mile on the engine which I had rebuilt back in 1989.
On our 1963 3.8 MKII (3.31 with OD) we regularly got 20mpg at highway cruising speeds. This was in the days before ethanol was in the gasoline.