Yes, I fitted a 3.54 to our 3.4. Makes it nicely long-legged in o/drive, like a modern, but I too suspect that 3.07 might be a bridge too far.
I found it didn’t much difference to the mileage, yes a bit better but not as good as the ratio percentage ! But far more relaxing at 75 mph cruising. You’ll find you have to drop down to 3rd if you want to pass something quickly, whereas with the 3.75 you rarely needed to change down at all, just flick out of o/drive.
On a 400 mile run I got 27 mpg Imperial at average 62 mph incl stops, just myself on board. With 2 pax + luggage then around 25. Of course running around locally is much lower, probably sub 20.
I concur that a 3.07 would be a struggle for the 3.4, I have one in my 3.8 ‘S’ and on a hill that used to struggle a bit, however with the overdrive engaged it made a car with very long legs
10mpg… That sound about right. '74 XJ6 S-2 essentially the same engine; Ok bigger displacement but not much. I’ve removed all of the smog stuff; replaced the PCV with an oil catch can and re routed the crank case into the intake. My point is the engines are pretty much the same and I get 10mpg if I’m lucky around town it’s between 8 or 9. Particuraly painful when I am forced to run the high octane stuff.
My 3.4L Mark 2 runs a 3.54 rear axle with an overdrive. This equates to 28mph per 1000 RPM in O/D. 2800RPM at 80 MPH.
My last round trip run from Boston MA to Stowe VT for the British Invasion was 432 miles. On the delightfully uncrowded interstates of rural NH and VT I ran at 75 - 85 mph for most of the way.
My mileage was 14.8 MPG with which I am pretty satisfied.
Thank you for your kind note. The Mark 2’s are a very nice old car and I
always wanted one from when they first came out. It took a while but now I
am retired and own one I can enjoy the car very much.
The MPG does drop considerably from 70 MPH to 85 MPH. I have a 1967 Mark 2 3.4 with factory fitted all synchro gearbox. On a recent club event, 220 miles round trip on uncrowded interstate, at speeds between 65 & 70 MPH in overdrive, the car achieved 21.2 PMG. At 85 MPH, I’m certain that number would have plummeted . Despite their distinctive shape, Mark 2’s do not have a great aerodynamic drag coefficient. but they certainly make up for that in other ways… I’ve enjoyed mine for over 28 years now, and like BESM, I too, enjoy the car very much.
If a US spec 1974 XJ6 probably an automatic gearbox and these Borg Warner Model 65s were not particularly efficient. An older design, only three speeds and no torque converter lock up. And there is a bit of a weight penalty between a Mark II and an XJ6. But still, your 10 MPH (US gallons?) seems low unless you are in very heavy traffic all the time.
No traffic. The car is a pup, the carbs have been recently rebuilt. So that’s a plus; and has made a huge difference in performance and drivability. I don’t get stranded anymore. But the car is a pig when it comes to fuel. I run a high octane 92 and unless there’s a leak someplace that’s about what I average. even on the highway. I drove to Seattle and back 120 miles round trip, and put 10 gallons in the tank when I got back. Ok, 12mpg. I have no idea how fast I was going cause the speedo doesn’t work either. Fee miles!!! right?
Using objects on the highway (well not on the highway but next to it) of known distance, and some math. the engine is turning about 2500 rpm at 60 miles an hour.
I’m in the process of getting a 3.4’S’ Auto back on the road after the timing slipped and bent all inlet valves, so I will hopefully be able to respond with some relevant MPG figures.
The head is in being hot tanked as I type and they are going to advise if I need to replace the guides, I’m guessing yes as there is quite a bit of movement when the valves are in the guides
I have a rebuild kit for the carbs and am still playing with the idea of having the dizzy rebuilt to spec and installing a Pertronix pickup.$$$$
Might also look at adjusting the cams slightly, just have to look up which way to go, advance/retard for better pick up in the lower rev range.
Any suggestions?
I installed a Pertronix in distributor in my Series II. it wasn’t that expensive. I got it off Amazon, and it was cheaper than paying somebody to rebuild my points distributor. I am very pleased with the results. I did notice the cap and rotor doesn’t last as long as the points cap and rotor. Not really sure why the cap and rotor are what came with the Pertronics; I was surprised. Net result money well spent. Took one gremlin out of the leave me on the side of the road equation.
I had a Pertronix electronic ignition installed on my '67 Mark 2 fifteen years ago. Engine starts up immediately, idle is smooth, acceleration great, and, as mentioned earlier, highway cruising in overdrive yields as much as 21.2 MPG.
