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Rear end ratios/speedometers

Rear End Ratios/Speedometers

Hello all ! The rear axle of my MK 7M needed to be rebuilt.I also wanted to
choose a different ratio as the current one was a bit  high for long
motorway trips. To choose the correct one , I built an Excel Spread-Sheet (
1 page , 100 KB ) which after entering the time ( seconds ) measured over a
few miles or Km at a constant RPM, and the axle ratio used calculates : -
the tyre circumference; - for each gear, the speed ( in MPH or KMH ) at
different RPM.... for different axle ratios I thus decided to fit a 3.77:1
( instead of 4,55:1) , which works fine. This also allows to calibrate the
speedometer or to measure the impact on speed or RPM of fitting tyres of
different sizes or profiles. Anyone interested, please let me know, I will
send a copy of the Spread-Sheet. - Michel Gosset

To all, Michel Gosset's spreadsheet is now available on the web site. It is
a zipped file, about 24 KB. Check it out of the "Library"
( or download directly
( - Dick White

Question: would my 54' DHC have taller gears than a roadster? were there
more than one gear ratio available to the 120's? I missed the last
conversation sorry. How do I determine what we have? Ratio wise? are there
numbers stamped on the housing or something? - E.W. Blake, 54' DHC 120

Dear Edgar, re gear ratios for XK120s, I think that if you wanted different
ratios, you changed the rear axle ratio, not the gearbox, but there was an
option (I would think pretty rare) later for close ratio gears.  Here is
the info, but I do not know how you identify the box from the outside or by
number (unless there was a CR after the chassis number, which I do not
think I have ever seen for a 120): Close ratio gears: SB 95B - Apr 53:
Close ratio gears:  close ratio gearboxes were now announced to meet the
needs of those using 120s in competition; they were available only for JH
and JL gearboxes.  What was supplied were alternative Constant Mesh Gears,
which resulted in closer intermediate gear ratios, sub-assembly constant
mesh pinion C 6658 for UKL 9/17/6 and constant mesh wheel C 5826 for UKL
7/19/-.  The ratios were then compared as follows:
        Standard:       Close ratio
1.      3.375           2.98
2.      1.982           1.74
3.      1.367           1.21
4.      Direct          Direct
Rear axle ratios were variable, but usual were 3.64 and 3.54  I think (3.77
was initially said to be "standard" on OTS). You can jack up a rear wheel
and start turning and counting to see what you have.  No difference DHC vs
OTS generally. I do not know how to identify the ratios from the external
casting, but someone on this list will I hope tell us.  The following may
be of some interest, and I think there was a while back a thread on rear
spring packing pieces: SB 90 - Mar 51 Rear Axles (ENV):  axle casing - Note
that the number was stamped on the top side of the axle nose piece. JHS.1 -
JHS.1602:  casing part no. A.254/1 used for axles C 3452/3821/4460/4462
(various ratios): these had a packing piece C 3829 between the rear spring
and the spring seat on the axle casing. JHS.1603 - on:  casing part no.
A.259/1 used for axles C 3452/1, 3821/1, 4460/1, 4461/1, 4462/1.  Casing
has a modified spring seat with which the packing piece is not used.
JHS.1-JHS.1839:  longer rear axle shafts (butted together) used (part no.
A.259/11). Thereafter:  shorter shafts A.272/11, with spacer block. Due to
the different lengths, the shafts are not interchangeable. - Regards, John

The 3.77 ratio Salisbury axle (standard) is the one I have in my car. BTW -
if you are going to jack up a rear wheel and rotate and count to determine
your ratio, remember the factor of two created by having only one rear
wheel turning. (divide wheel rotations by 2 when comparing to the drive
shaft rotations) The other Salisbury ratio noted in the manual was 3.31. If
anyone knows where one of these is available for either sale or trade, I
would like to change mine to 3.31. We do a lot of driving - some on
interstates - and I really don't need the torque that's now available in
4th. I'd much rather reduce my rpm by 14%. I have heard that the gears cna
be swapped between the two Salisburys so I would be interested in buying or
trading just the gears also. - Bruce Cunningham, '53 XK120 OTS

John, I could be wrong but I seem to remember a ratio, in the form of a
fraction, stamped on the axle casing. 13/49 seems to ring a bell. I could
be wrong. When I get a chance I'll stick a '120 over the pit and take a
look. - Roger Learmonth

Another point re rear axle ratios:  if your car is original, you can tell
from the speedo number what ratio was fitted. - Regards, John Elmgreen

The rear axle ratio is indeed stamped on the case, actually it is the
number of teeth in the crown and the pinion. Look for a number like 49x13
(for the 3.77 ratio) or 43x13 (for the 3.31 ratio) on a Salisbury axle.
There were other ratios offered, but I don't have the tooth counts at hand.
For the ENV axle the numbers will be 51x14 (for the 3.64) or 59x18 (for the
3.27). The gears can be swapped, but only with another of the same TYPE
axle, i.e.
Salisbury 2HA, Salisbury 4HA, or ENV. - Rob Reilly

Hello Edgar, To determine which rear axle ratio you have, you can: - if you
are lucky find a small plate on the axle showing the respective numbers of
teeth on pinion and crown (50/11) -lift the rear axle and count the number
of revolutions of the propeller shaft for a revolution of the wheel.This is
a bit dirty and not very precise -also take off the axle cover and count
the teeth . Very dirty ! - run your car in top gear ( not overdrive) for a
Km at a steady RPM (say 3000 RPM) and measure the time ( say 32.7 sec).
Your speed is : 3600 / 32.7 = 110.09 Km/h ( If measured on a mile, divide
time by 1.609) ... then measure the circumference of your rear tyres ( say
231 cm ) Your axle ratio is : ( 3000 RPM X 231 cm  ) / ( 110.09 Kmh /
0.0006) =  3.77. Regards - Michel Gosset, 150 DHC, MK7 M

regarding counting revolutions - the precision depends on the number of
revolutions you choose to count i.e. - the more revolutions you count, the
better the precision -  provided, of course, that you don't lose count or
miscount in the process
 - Bruce Cunningham, '53 XK120 OTS

John and all, A couple of months ago, I posted the following question
regarding XK-140 speedo serial  numbers. Since I didn't get a response,
maybe it's time to revisit the question: Old email - forwarded:
> My O/D XK-140 has speedometer part # X51691/35 with the number "1200"
>centered immediately below. Since my speedo shows evidence of rework by a
>PO, I can't verify that it is correct for the 4.09 O/D axle. The parts
>manual identifies only speedometer part numbers for the 3.54 and 3.31
>axles. Can anyone verify the correct part number for the 4.09 axle? -
>Sincerely, Mike Carpenter

A useful reference booklet is Jaguar Spares Division publication number
J.27, titled "Data for Ordering Speedometers". I have 1st Revision (March
1960) and 2nd Revision (January 1963) both of which list all "Z" and "SN"
prefix Speedos, but not "X" prefix unfortunately.  Does anyone have a first
Edition (April 1959) ?; what speedos does it list. There are 30 different
"Z" prefix, and 20 different "SN" prefix speedos listed for various XK150
permutations in 2nd Revision book. - Roger Payne

Mike, the X51691/35 speedo is listed in the 120 parts catalogues as being
for the 3.54 Salisbury, but there is nothing on a speedo for the 4.09 axle.
The 4.09 axle IS listed in the axle section. May one suppose that the usual
kind of people who bought 4.09 axles were short track racers who didn't
bother about the speedo, so the factory didn't offer them? Incidentally the
speedo is only dependent on the rear axle ratio, regardless of whether you
have the overdrive gearbox or not. So you're wondering if the PO modified
the speedo, and how can you check the calibration (without sophisticated
equipment)? It's a math problem. Measure the tire outside circumference and
convert inches to miles. Divide by 4.09 to get how many miles per one
driveshaft revolution. Run it with the gearbox in top gear but the
overdrive switched off. Compare tachometer readings (multiplied by 60 and
the number of miles per revolution) to speedometer readings. - Rob Reilly

Rob, Thanks for the information. My XK-140 parts manual indicates that the
4.09 was standard with the "C" Series -140, however, the manual does not
list a speedo S/N for the 4.09. I have not checked my actual ratio. Guess
that's next. - Mike Carpenter

4.09 rear axles were standard I think for 140 and 150 overdrive cars. Re
the calibration problems, check out the good old XK handbooks for details
of speeds at RPM, if I am not mistaken, taking account of the changes in
rolling circumference of the tyres at different speeds. - Regards, John

The procedure described has possible error in measuring the circumference
of the wheel as it is difficult to determine the effective radius of a
wheel that is being deformed at the point of contact with the road. The
most accurate measurement of circumference will be obtained by marking the
point of contact with the road then rolling the car a few revolutions and
measuring the linear distance on the pavement and dividing by the number of
revolutions. Of course the ultimate test of the speedometer is timing
travel at a constant speedo reading over an accurately measured distance
and calculating the actual speed. The mile markers on most interstates are
pretty accurate but "measured mile" signs are probably better if you can
still find them.  If you are using tyres that are not pretty close to the
original equipment, you could be introducing more error than you might
think. - Bruce Cunningham

to Mike Carpenter, Jaguar Spare Parts book lists following Speedometers for
XK140 with 4.09:1 rear axle ratio: In miles per hour: SMITH X.51691/35
(early cars) Jaguar part # C.5389, 51-118-503-16 (later cars) Jaguar part #
C.5389; In kilometers/ hr: SMITH X.51691/36 (early cars) Jaguar part #
C.5390; 51-118-503-18 (later cars) Jaguar part # C.5390. - hope this helps,
Roger Payne

Well, I misinterpreted something I read in a magazine, so let me correct
myself before you guys keelhaul me. Referring to the factory parts
catalogue J.18 for Mark VII, XK140 and 2.4 Litre with overdrive: The
X.51691/35 speedo is correct for XK140 with overdrive when combined with
the right angle speedo cable drive and the 4.09 axle. Apparently the right
angle drive has its own reduction which cancels out the 4.09/3.54
difference?? The 4.09 is the only axle listed in this book for 140, and the
/35 speedo for miles and the /36 speedo for kilos are the only speedos
listed for 140. This book is for overdrive only. Mark VII and 2.4 do not
use the right angle drive and their axles are 4.55, so they have different
speedos. - ROb Reilly

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