DIFF. better 3,07 or 2,88 for 2+2?

[Series1 Stu]

The performance figures quoted in the Jaguar manuals include allowance for tyre radius increase due to centrifugal force.

Regards

[gtjoey]

Max it out , You have exactly the set up I suggested
288 with 205/70/15
How do you like it in todays world .
Thanks in advance.
Gtjoey

[max-it-out]

GTJ - I like it - it suits my driving style , and much better than the old 3.54 that was in there before . Just got back from the Gransden air show , c. 150 mile round trip and got 28 .5 mpg - what`s not to like ? I`m still using the old Strombergs though , so acceleration is a bit subdued .

[max-it-out]

The performance figures quoted in the Jaguar manuals include allowance for tyre radius increase due to centrifugal force.

Regards

Good point - I forgot about that . The 70 mph figures might be a fairer comparison ...

3.07 @ 70 = 2780
2.88 @ 70 = 2700

Interestingly , IIRC from reading a road test , the XJ-S ( 205 /70 /15 ) was quoted as 2850 @ 70 mph ( 3.07 diff ) , so it looks like the tyres fitted on the early Es were larger .Subsequent tyre changes might explain why the performance of later Es didn`t match the originals.

[Heuer]

tyre sizes:
650 H 15 Dunlop R5: Overall diameter inflated = 696mm / 27.4"
640 H 15 Dunlop RS5: Overall diameter inflated = 683 mm / 26.88"
185 VR 15 Pirelli Cinturato CA67: Overall diameter inflated = 676 mm / 26.61"
185 x 15 Dunlop SP Sport Aquajet: Overall diameter inflated = 674 mm / 26.53"
185 R 15 Michelin XVS: Overall diameter inflated = 675 mm / 26.5"
205/70 VR 15 Michelin XWX:= Overall diameter inflated = 669 mm / 26.33"
205/70 WR 15 Pirelli Cinturato P5: Overall diameter inflated = 675 mm / 26.57"

This calculator will allow you to work out the rpm/road speed based on tyre choice, diff ratio and gearing: https://wahiduddin.net/calc/calc_speed_rpm.htm

e.g. Michelin XVS, 2.88 and 4th gear:



Michelin XVS, 3.07 in 4th gear:

[gtjoey]

So in simple terms if the original poster is still reading this
Max likes the 288 with 205/70-15 and at 70 mph
Your around 2750 rpm
Saving the earth at 28 mpg and a comfortable ride
Thank you for the information
Gtjoey1314
Wonderful pictures

[Series1 Stu]

The variations in rolling radius is pretty much negligible and can be largely accounted for in tread depth wear. You'll get virtually that much change in road speed as your tyres wear. Also in manufacturing tolerances.

Certainly less significant than perhaps people think.

Regards

[ggg1102]

Thank you all for the advice and above all for the experiences-impressions with the differentials mounted on your cars.
I understood the strengths and weaknesses of two ratios and you also gave me technical and historical notions (test on the first E-types).
I will also talk to the mechanic and I will then reveal the choice between a more comfortable ride at high speeds or greater sportiness and better hill start!
:)

[Rocknjag]

Ciao Giovanni
As others have said, it is what you want and the type of driving you will be doing
I had the 3.54 ratio in my series 2 etype. I did not like the short legs it had.
I changed it to a 3.07.
I find the car to be much better in all conditions. I like the acceleration of 1 st gear being a taller gear than the 3.54. (1st gear with the 3.54 was so short and useless in my opinion) I enjoy winding it out. I also feel the car reacts better while downshifting and upshifting through all the gears,
I am very satisfied with its performance on the highway too.
I am happy with this choice and have no desire to spend the money for a 5 speed anymore.
It feels just right. Just like what I recall a 1960’s sports car should feel like.

Saluti
Rocco

[MarekH]

You've all only looked at half of the picture and forgotten that not all 1st gears are equal. If you have a KPN or KFN overdrive gearbox fitted, then you can have the low rpm at high speed and get off of the line aswell.

The other factor to consider is camshaft choice:- the more "high rpm" the camshaft is, the less suited it is to the low diff ratio. A detuned camshaft which operates best at lower/lowest revs (at the expense of high top end power) would do move for the driving and overtaking ability than being able to brag about how much power you have, because only the car designed for the racetrack benefits from the "fast road cam" whereas most overtaking and acceleration on a public road will start from relatively low rpm.

kind regards
Marek

[Mich7920]

Hi,
If you manage the metric system :

https://www.flat4ever.com/forums/rapport-de-boite/

Mich

[malcolm]

Michelin XVS, 3.07 in 4th gear:

David, this gives c. 25mph per 1000 revs, about what my handbook says. What always makes me think is that therefore, to achieve 150mph, the car had to be doing 6000 revs, the very end of the red band in top gear on my series 2. Is that feasible? Obviously not on my car, but even original factory prepped cars?

[abowie]

What always makes me think is that therefore, to achieve 150mph, the car had to be doing 6000 revs, the very end of the red band in top gear on my series 2. Is that feasible?

Correct. And not really.

9600HP and the other press car probably had "special" engines from the Jaguar racing department to achieve these figures, if in fact they did.

[Heuer]

9600 HP had several engines fitted under Jaguar ownership and the one installed for the top speed run was probably specially prepared by the Competition Department (the front overriders were also removed to reduce drag). Much of this is documented in Porter's Definitive E-Type book. With RS5 tyres and the engine redlined at 6,000rpm 150mph was entirely feasible:



Bear in mind the D-Type had a top speed of 172.8 mph with a 3.8 engine and a 2.93 diff. My guess is that combination made its way into 9600 HP for testing as the redline of 5,500rpm would equate to 150.1 mph. The 2.93 diff was offered as an option to customers as part C16619 until late 1962 and customers could ask for their car to be breathed on by the Competition Department so Jaguar were not publishing misleading information.




So for the purists the ideal compromise between the 3.07 and 2.88 ratios would be 2.93 as it would be Factory correct. It would also put an end to which is best - the wisdom of Solomon!

Edit:
Just found the Dunlop rolling radius for RS5 tyres at 150mph which is 14.1". With that in mind:



So 150mph doable with RS5 tyres and a 3.07 diff without going into the red. On the other hand all the high speed tests were done using R5 racing tyres at 40psi which have a rolling radius of 13.95" at 150mph so 5,600rpm would be needed.

[Philk]

Amazing detail above from David (thank you).

Re the original post, at the end of the day, it is worth remembering that Jaguar changed the differential ratio a number of times to accommodate different market's speed limits. The last owner of my (USA) 3.8, changed the differential ratio from 3.54 to 3.07. Personally, I'm happy with the 3.07 for UK Motorway cruising (it also provides quite reasonable economy). With the Moss gearbox, my only criticism is that it makes 2nd gear too tall to be able to do near stationary starts, which means I will always need to do the shift down into non-synchro 1st.

Possibly, a 3.31 would have been the sweet spot for flexible town driving plus reasonable mid - high speed cruising. All a matter of personal taste though.

The Service Bulletins backing the Rear Axle Ratio changes are H.5 (Oct '62), H.11 (Sep '63) and H.15 (Apr '65)

[malcolm]

Thanks David, great info

[paydase]

Hoping not to be too repetitive, it all depends of your style of driving and of roads that you use.

When I bought my 3.8 FHC (USA origin), it was fitted with a LSD 3.31 diff that had replaced an original 3.54.
Reading all the stuff here, I thought that I would try finding a 3.07.
But my mechanic advised me first to drive the car as it was and decide later on.

And I realise that actually I only drive on samall roads (and town when taking the car and coming back).
In such conditions 3.31 is very adequate and revs in 4th are perfectly fine when at speed.
Also, my use of the car is rather conservative, not attempting to reach (or even approach) 150 mph on an Autobahn (if it ever could!), only enjoying my classic car in the country for what it is.

As was also said, the gearbox is a Moss with the quite short non-synchro first gear.
Ideal for climbing on the access ramp to my garage but not so friendly otherwise.
And also 2nd gear is indeed sufficiently short in town to rarely have to use 1st gear.

So decided to keep the 3.31.
If that can help the OP to decide for himself...

[christopher storey]

The editor of Motor ( or was it Autocar ) Charles ( can't remember his surname ? Bulmer ) gave a good account of the maximum tests, and 1. they used Dunlop R4 racing tyres and 2. more to the point he said the rpm were well beyond 6,000 rpm at maximum speed !

[Heuer]

I don't think Jaguar would have used Dunlop R4 tyres for those sort of speeds as the tyres were outdated. The stock RS5 tyres were only rated up to 130mph hence they used R5's for the top speed runs.

"Tyre design in the 1930's was barely keeping up with the demands of improved vehicle developments. At this stage Dunlop began developing specialised tyres for competition, firstly with the 5 Stud pattern in 1934, and then in 1946 the R1. By today's standards these tyres were very crude, being made of cotton fabric and genuine tree rubber; they weighed about 38 lbs. They were very popular however, remaining competitive until the R3 was introduced in 1955. The R4 followed soon after in 1956 as a wet weather version.

The next breakthrough came in 1958 when Dunlop introduced the use of nylon fabric with the R5, reducing weights by 12 lbs. per tyre. It was in this pattern that Dunlop introduced the first synthetic tread compound, now known as 'green spot' by its colour coded identification. The R6 was introduced in 1962, incorporating improved nylon casings and reduced aspect ratio. Major developments followed soon after, including the use of synthetic rubber for tread compounds and lower aspect profiles. The R6 pattern was also called the CR48 and was followed by the R7 (CR65) in 1965."

The editor of Motor ( or was it Autocar ) Charles ( can't remember his surname ? Bulmer ) gave a good account of the maximum tests, and 1. they used Dunlop R4 racing tyres and 2. more to the point he said the rpm were well beyond 6,000 rpm at maximum speed !

John Bolster of Autosport? Maurice Smith of Autocar? Bill Boddy of Motor Sport? The latter reported cruising along the M1 in 9600 HP at 6,100 rpm which equated to 155 mph.

[gtjoey]

We have to REALLY remember
Back in 1961 there were no such things as polygraph tests!
If we had one I think the entire story would be different
150 MPH
245 HP
I just have a suspicion that the Almighty Pound might have played a roll in these test results