minor tid bit…but while on this topic…yes, there is the compression of the weighted tire, decreasing effective diameter, and there is the increase of diameter at speed due to centrifugal forces, and tire pressure affects both, as does the little tread wear from new. But these things do matter…in modern All wheel and four wheel drive cars it is recommended to change to new tires on all 4…as differences will create excessive wear in the limited slip drive parts that will “think” one or more tires is always slipping.
Nick
Jaguar give two figures for revs at a specific speed in the Service Manual. One is the theoretical figure and the other is the figure allowing for tyre growth. The tyres are quoted as Dunlop Road Speed 16 x 6.0 inflated to 35 lbs. sq. in.
For instance:
ENV 3.64 (standard ratio)
70mph in 4th = 3164 rpm/3105 rpm
ENV 3.27
70mph in 4th = 2842 rpm/2789 rpm
Salisbury 3.77
70mph in 4th = 3276 rpm/3217 rpm
Salisbury 3.31
70mph in 4th = 2877 rpm/2822 rpm
The speeds/revs are given for all gears and speeds from 10mph tp 140mph in 10mph increments. The 3.27 at 120mph is revving at 4872/4578rpm. The 3.64 is 5424/5096rpm. The 3.77 is 5616/5279rpm.
yes Chris good info on the factory specs at the time. I have no knowledge of how the old rubber bias ply 600-16 might expand at speed compared to todays tires…interesting tho that it was that many rpm. The 3.54 data was inserted into the later “Handbook”…at 35psi, quite a high tire pressure for those tires.
the 3.54 at 120mph 5268 calculated and 4951 actual at speed with the factory fitted tires.
The 3.54 data has an error: note that at 60mph = 2364 calculated ERROR? (should be 2634?) ,use 439 rpm per 10 mph. (and 2602 actual )
I prefer to look at it as how many miles per hour, per 1,000 rpm.
Nick
Wouldn’t help him. Won’t fit a Dana 44. Chrysler had their own rear axles. No Dana 44 in any Dodge/Plymouth/Chrysler passenger car.
at some time, I’d like to change my ratio…but I am not sure what my differential is…seems to be a complete replacement and not a Leys original to Jaguar…hmmnn…can anyone ID this…I have tried before…here it is again: my 53xk120…with US Pat numbers…WHAT IS THIS DIFF trying again to ID this mystery differential…what can anyone tell me about this differential:. On the left lower flange facing rear is cast raised US PAT 2018188, and above that on the left upper flange US PAT 1968618 (or similar…hard toread), and on the right lower flange facing rear, is a very small circled A, under it small “44”. And above that on the upper flange is cast 18418. it is a 10 bolt rear cover with the rear cover extended part for internal gear clearance about 3 in wide, the iron fill plug faces rearward on the right side of the bolt on cover.,iron drain plug on bottom, it has a very clear raised “69” on a smooth flat on the top of left side (visible only from above), On the underside of nose is stamped B53 135. and found on top right rearface flange this no…18418 cast.No other marks or cast / foundry marks,No Leys mark, No “C part#, and it may or may not be on its original axle tubes , no marks at all on either axle tube. It is an “open”…jacked up one wheel turns…other does not. I know it is a 3.54…verified by wheel/drive revs. It is likely from 1960s…maybe 70s…has to be prior to 1983. There are no axle tube numbers. Interested in manufacturer, when, what it may have been on, any detail
Nick
Diff ratios is always a personal choice, but I’m with Tony here - I like to keep the RPM/road speed relationship somewhere in the original ballpark as this is how the car was designed to operate. Given that with the number of cameras and monitoring devices now in the UK, not to mention the overcrowded roads, it doesn’t make much sense to concern yourself too much with anything over 70-75mph in my opinion.
I went through all this when restoring my Cobra, and ended up going with the original 3.54 ratio with the four-speed close ratio T10 (direct top). There is no point in having a CR gearbox with a long diff ratio as far as I can see, and it is much kinder to the engine to keep its normal rev range to somewhere up around 2500 - 3500rpm. I think a lot of people in the UK have now got so used to the ubiquitous diesels that they keep the revs low. The main consideration has to be to ask how the car will be driven, so all sorts of relevant factors should be considered - gear and final drive ratios, rolling diameter, characteristics of engine output (cam profile, CR etc.). The only real exception to this is my Cooper S, which does have a pretty long final drive ratio for a Mini (3.31:1) - but it only has 10" wheels, so has an excuse.
Dana/Spicer model 44. How it got there? 
Bare center section castings could have been shipped in bulk to the UK, then machined and built there. There’s virtually no difference between contemporaneous Salisbury 4HA and D/S 44 differentials.
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The issue is here how do you drive. Our cars were designed for a context, that did not include spending most of their time at constant steep in three-lane highways, which is what I I have in The Netherlands. I would see it differently in the Alps, or the Pyrenees, but as you say, you need to see how you drive.
thanks Mike…I have tried to find the US pat # on it, on some other identifiable diff…as to what auto it may have come from…if it did…I do not see other modifications to fit it on the axle tubes, brakes etc. I have not heard of another XK120 with similar…all so far have the Leys marks. Maybe my local jeep crawler guys will know…main thing is to find out if I can rebuild it and with what parts that can be still found. It does have that little A 44…and this on the lower nose B53 435
Here is the stamped serial number on a 4HA Salisbury from FHC 681114. The car was built in April '53 so I am inclined to think the C53 means March of '53, 1 month lead time.
Broken crown bolts, awaiting rebuild.
Would a date of Feb '53 make sense for yours?
Here is the rear axle in my 2017 Jeep Wrangler.
Cover is the correct shape for a Dana 44.
my car is S673776, factory dispatch March 19, 1953…so the diff date code makes sense.and that makes it interesting: if this was factory fittes with the odd US Pat #s and no Leys mark diff. It does appear to be original to the car, looking at all the fittings etc…and it would be a big coincidence to have a later replacement have the B53 date code that makes perfect sense…hmmmn. That is why I have been wondering if any other March 1953 car may have a diff unit similar to mine with the US Pat #s and A 44, with the possibility that for a while the factory used a different lot than the Leys?? Looking at yours Rob…and mine…in the larger view…almost same…possible spotting very minor differences if from different manufacturing foundry…as the US Pat #s are cast in. ??
Nick
Some more information (and possibly a final decision on final ratio) → SimplyPerformance (no affiliation, bla bla) stock a 2.88 from Jensen (not a Jaguar part) that fits directly in the 4HA. Jaguar options below 3.07 would have the wrong pinion offset and need machining to the input bore.
I go that way, will order and do the mod after summer.
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I have a 2.88 locker 4HA in my XK120 with a 4 sp. E type synchro box and a 4.2.
It goes very well. I must post a video sometime.
The casing was MK2 Jaguar with the brackets removed and 5x5 PCD steek wheel hubs made to fit.
It is not an easy process.
Width is the issue, it has a few inches more that an XK axle. However it looks a bit meaner with the tyres closer to the wheel arches.
I have Michelin Pilot X tyres which are very tall.
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Mine has what looks like a LEYS mark.
Some other mark too blurred to make out.
That’s an indication that the sand in the foundry mold was a little too dry that day, it crumbles around raised letters.
Other foundry smudges.
Nick, when you open yours up you may see other marks like these.
Following up on the topic…
I have booked today a new diff and crown and pinion. Unfortunately it seems that i can only install a 2.88 with LSD… not that 8t bothers me much, as I prefer this to the 5 speed… it will be a Quaife. All-in All-out ca. 2.5keuro.
Just like mine then.
I promise to make a film of it driving today, the sun is shining and I should finish work early.
Then you can see how it goes and how the revs match the speed.
It is a 2.88 with LSD and E type 4 speed synchro with 16 inch wheels and very tall Michelin Pilote X tyres.
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Look totally Fab James.
Thanks Friends.
I will never be a professional Youtuber, I admit.
No matter what I do, my Gopro sound quality sucks, even with an external mic.
Click click click.
However this is me going up the road earlier today as promised.
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I love the little detour through your front drive… 