Mark 2 - 2.4 Liter Rear Axle

My 1966 Mark 2 - 2.4 runs over 3000 RPM at 55 MPH. I have the 4.27 : 1 Ratio. Is there a reasonably balanced solution to this, that will not have a deleterious impact on the other gears? I do not want to change transmissions.
Thanks for any help you can provide
Gene Kahn

take a look in the jaguar service manual and check the figures you quote. You should also verify your tachometer is reading correctly as they frequently don’t.
If you are still not happy your choices are limited (this assumes you don’t have a slipping clutch!)
alternative Jaguar transmission which includes an overdrive. Alternative aftermarket 5 speed transmission, alternative ratio differential. From memory the base model 2.4 non overdrive was the cheapest model at the time. the O/D option was offered for a reason!

Thanks Phil,
Yes, I checked that the ratio is 4.27: 1. All Mark 2 rear axle ratios are shown below in this excerpt from the 2.4 Operating, Maintenance and Service manual.

Rear Axle
Type: Hypoid. semi-floating.
Cars fitted with a synchromesh gearbox only 4.27: I
Cars fitted with an overdrive 4.55 : 1
Cars fitted with automatic transmission . .4.27 : 1

The clutch is new < 200 miles, adjusted properly and works perfectly.
The tach & speedo were just rebuilt by Nisongers and calibrated.

Additionally,
The manual states that at 60 MPH shows 4:27 Ratio and 3296 RPM.( Confirmed with GPS speed unit & trailing modern car)
In short, the motor is screaming at 60 MPH.
OK, you mention an alternative ratio differential. Given that I don’t want to change the original transmission , Given the above information, can you recommend a different differential ratio that will provide a reasonably balanced solution to this, and will not have a deleterious impact on the other gear ranges
Thanks again,
Gene

We have all become too used to modern high gear ratios. Your 2.4 , being by a wide margin the shortest stroked Jaguar XK engine , will come to no harm at all with its 4.27 ratio, and if you consider that even a 90 mph cruise would be less than 5000 rpm - and how infrequently you are likely to crusie at that high a speed, then changing the ratio has little point. If you go for a higher ratio e.g. 3.77 or 3.54 you will get a quieter cruise, but at the price of a very marked lack of acceleration in the lower gears . The real answer is an overdrive box which will give you about 21mph/1000 rpm in overdrive

I should add that in the 1950s in my father’s Mark VIII, and later IX, both of which gave 19.4 mph/1000 rpm, we commonly cruised at 100 mph = 5180 rpm

Thanks Christopher, Thank you for your practical advice. I think that’s what Phil is saying as well.
I think I’ll forget about the change as the car is so original
Gene

That’s the right decision!

Which transmission, manual or auto?

I’d be more likely to change out the diff before installing an OD.

I own a RHD 2.4 MOD that I imported from the U.K. In '82.

They went the other way on my car I’m ticking over way under 2000 RPM at 60 MPH.

Your car have Solexes?

LHD 2.4 4 speed
Yes on Solex

Am I reading this right?

There were only 2 ratios offered on the MKII?

That applies to both US An UK models?

This is all that appears in operating manual on the topic
Thanks
Gene
Hypoid. semi-floating.
Cars fitted with a synchromesh gearbox only 4.27: I
Cars fitted with an overdrive 4.55 : 1
Cars fitted with automatic transmission . .4.27 : 1

Part of the equation is tire circumference.
The 4.27 ratio was used in the Mark V and Mark VII which have 6.70x16 inch tires, more circumference than your 6.40x15 inch tires, hence a lower engine rpm for any given road speed.

I think the point here is that they are is using the smallest engine, I know that my '66 S Type came with the 3.54 diff when it was an Auto.
I changed the trans to a 4 sp M/OD and then blew the diff gears and the only set I could source at the time was from an E type @ 3.07 Car would cruise at 1800rpm @ 60Mph

I have never understood why Jaguar partially defeated the purpose of an overdrive by changing the final drive ratio to a lower ratio when o/d was fitted. My stock MOD 3.8S would do fine with a 3.54 ratio rather than the 3.77 which it has. Were they after improved acceleration, less stress on the clutch from a standing start?