www.jec.org.uk

Hi, my car is a '61 3.8 with DG auto and LSD.

To me, the car sounds quite 'busy' and noisy (mechanically) at about 70 MPH - which I think is about 3000 revs (the speedo needle tends to flick around a bit at that speed, so hard to be sure) - I generaly cruise around 2800 to 3000 revs which I think is about 60-65mph? Does that sound right?

I think the issue here is probably low overall gearing ,and the fact that the car is 51 years old and is not as refined as a modern car (which my ears are more used to) - From what I have read, gearing wise, the auto falls somewhere between a MOD car and a (rare) manual with no overdrive - so probably quite low geared. It is a real pity it isnt higher geared in top, because there is plenty of torque to allow for that...

I have new door seals all round but no under bonnet sound insulation.

(I may check the speedo using my Satnav - see how accurate it is)

Any views/ comments/ thoughts appreciated.

Cheers
Buzz

Hi Buzz

The diferential in your car should be a 3.54:1 ratio, the revs for 70 MPH sound about right at 2.8 to 3k allowing for error. If anyone has any books and confirm then please post.

regards

My Jaguar Manual shows 70 mph at 3221 revs on a 3.54 diff (3.4/3. and
70 mph at 3430 revs on a 3.77 diff (3.4/3., with ov-drive 2670 revs
70 mph at 3886 revs on a 4.27 diff (2.4 ltr)

My car was originally a 4 speed without over-drive and has the 3.54 diff, but with an over-drive box it now does 70 at 2500 revs.

the Haynes manual says auto box top is 1:1 so above revs will tell you which diff you have in or the little plate on one of the diff bolts (number of teeth on pinion and crown wheel).

Best of luck

Brian

Hi, I have done calculations from the workshop manual and completed a word table as my speedo is not very accurate at all speeds so that i can use the rev counter to check.

MPH 1st RPM 2nd RPM 3rd RPM 4th RPM O/Drive RPM
10 1509 980 658 496 386

20 3018 1960 1316 992 772

30 4527 2939 1975 1489 1159

40 3913 2630 1983 1543

50 4873 3275 2469 1921

60 5833 3920 2956 2300

70 4550 3430 2670

80 5180 3906 3038

90 5805 4377 3405

100 4837 3763

110 5285 4113

120 5730 4458

My car is a 3.8 manual with overdrive, everything else standard.

This may help a little.
Regards,
Paul

Sorry, table looked Ok when posted but then when looked at the actual response it has shifted all columns. Basically use the last figures at each speed to read under the overdrive column and then work across from that, so that for say: 120 mph fourth gear is 5730 and o/d is 4458 etc.
Regards,
Paul

Guys, thanks for your help - appreciated.

I realise that my satnav has a speed read-out - these are meant to be highly accurate.

My plan is to take the car onto a nice quiet dual carriageway and log the RPM at various road speeds.

Cheers

Buzz