Speed and revs, 60mph = 4100 revs!

[max-it-out]

My car also has the very high 3.54 diff so....

Not sure if this is correct ( maths was never my strong point )...The xjs I used to have had a 3.07 diff. and 70 mph was 2850 rpm in top ( 1-1 ).Using these figures , I have hit upon what I believe to be the ideal solution to my problem -

A 5 speed box with 5 th giving 0.75 would allow 70 mph cruising at 2400-2500 rpm and 5000 rpm would equate to 142 mph.This should be perfect because 5000 is really the max I would want to rev a 4.2 and also 140-145 is about the max a car like mine with a standard engine could reach.

All I need to do now is find a suitable box with that ratio ( and stump up the £ )

[mgcjag]

Hi Mark...just search 5 speed options you will fing lots of info...Steve viewtopic.php?f=9&t=7226&hilit=5+speed+option

[PeterCrespin]

Did David ever sort his rev counter out? It looked 99% obvious to me that with gps-confirmed speed and ridiculous reported revs he simply had an iffy tach. As it happens, the 3.54 ratio was only the middle of five D-type options. But then why have a race car if you can’t use max revs every lap at the fastest parts of the circuit?

[david.coan]

Thanks for the prompt, as i started this thread i should have posted my conclusion.
I had a 3.07 crown and pinion fitted to replace my 3.54 diff this summer, and i ended up with the IRS having new shocks with height adjustment, all new rubbers, uprated brake calipers etc etc on the principle of ‘while its out and you’r in there u might as well’. The end result was fab, i did not appreciate just how much the irs with all new bits would transform the feel of the car too. The tacho was also sent away to Speedycables to convert it to work with the electronic ignition, as the rvi tacho paired with a 123 dizzy ignition was causing my rev reading error, and for a very reasonable price of £105 and 2 weeks later it came back not only expertly calibrated but also looking good as new. The speedo i have checked against my gps speed readings and its a very good match.
The tacho and speedo readings across a large speed range now very closely match the table in the handbook for a 3.07 diff and it just feels much better for higher speed cruising than it did with the 3.54, it almost feels like i have an extra gear. Add in the refurb on the IRS and the transformation was significant.

[John ball]

I had already worked out the gearing on my XK120, by using mathematics the long way. But the website posted by Heuer at post 8 is very good. I have a tremec T5. Thus 4th gear being 1/1 with a 3.54 and Blockley tyres of about 702mm diameter calculated out to be 23.2 mph per 1,000 rpm, which is exactly what the car reads and 5th gear with a 0.80 ratio gives 29 mph per 1,000 rpm. The original poster now with a 3.07 diff should show about 26 mph per 1,000 rpm with his four speed box. This is a good compromise gearing, as mine is quite low geared in 4th and really quite high in 5th. 5th really is an overdrive that can only be used on motorways.
Some people had 0.63 with their T5’s which in my opinion is way too high geared.

[christopher storey]

John : if you think your XK is undergeared in 4th you should try my Mark IX which is 19.4 mph/1000 !

In the late 50s/early 60s my Father used to cruise his at 90 mph = approx 4650 rpm, admittedly usually only for the length of the M1 which in those days was about 67 miles . When after 100,000 miles of this treatment over about 3 years it ran a big end, it was sent back to Brown's Lane, and was returned a few days later with a note that new shells had been fitted and that the crankshaft would have passed inspection for installation in a new car!

[John ball]

Talking about running big ends etc. When I had the car down at CKL, they recommended using Valvoline VR1 oil as they said after using that oil on a tear down, it was always very clean inside. Now I always use it. It is more expensive but does have a high level of Zinc which is important. I did a survey on shear quality of all the usual oils and it is the best.

[Kingkitesurf]

The tacho was also sent away to Speedycables to convert it to work with the electronic ignition, as the rvi tacho paired with a 123 dizzy ignition was causing my rev reading error, and for a very reasonable price of £105 and 2 weeks later it came back not only expertly calibrated but also looking good as new. The speedo i have checked against my gps speed readings and its a very good match.
The tacho and speedo readings across a large speed range now very

I asked speedycables as well as I have the exact issue with the tacho and the 123ignition. They came up with 250 GBP ex vat and a 6 to 8 weeks fixing time. Is that a Brexit revenge? I am in the Netherlands

[mgcjag]

Check out https://www.spiyda.com/smiths-rvi-rvc-c ... board.html they have various conversion options

[david.coan]

I would also go back to Speedycables and query the work and price they have quoted for, why would it be 2-3 times what they charged me just a few months ago ? Hope it goes well

[politeperson]

I once fitted the guts of a mid 70s XJ6 rev counter into the case of an S1 +clock rev counter using the S1 Smiths dial and the XJ6 pointer. I cound not use a rev generator because I was using a big valve head which lacked the rev generator bolt holes. The signal came from the coil and the whole lot cost £40 in ebay bits. I will post a picture of the face if I get a moment.

On the subject of cruising revs, I have a 911 which does just over 90mph at 2,000 rpm. That is around 45mph per 1,000 rpm. It really motors.

Then again I love the S2 with the 2.88 diff with 4 speed Jag box in the silver car. That didn't feel flustered at 120 mph on the Autobahn and was still accelerating reasonably at 137 mph on the Nurburgring. I cant remember the exact figures, I will have to remind myself.