Classic Jag diff ratios and impact
Discussion
I have a 1975 Jaguar XJ coupe that I am restoring and modernising as I go along. It had a 6.0 v12 from a 1990 Jaguar XJ RS in it when I bought it with the standard fit GM400 3 speed gearbox from that donor car also. The diff fitted is a 3.54 which is the original one from the XJC from the factory.
I have found a brochure for the XJRS which states that the car weighs about the same as the XJC and with the 6.0 engine and a 2.88 diff it achieved 0-60 in 6.5s and a combined MPG of 22
What impact on those kind of figures would the 3.54 diff be having? Before it went in for restoration at 70 I was achieving about 16mpg and have no idea what the 0-60 was
My understanding is that the 2.88 diff makes the cars acceleration more lazy and improves MPG by lowering RPM at motorway speeds. Is that right?
As part of the restoration I have updated the gearbox to a 700r4 which is the same gearbox as the GM400 but with the extra 4th gear with a 0.70 ratio.
Just wondering if the 3.54 makes it a faster car or whether it’s worth swapping to the 2.88? I don’t really understand diff ratios
I have found a brochure for the XJRS which states that the car weighs about the same as the XJC and with the 6.0 engine and a 2.88 diff it achieved 0-60 in 6.5s and a combined MPG of 22
What impact on those kind of figures would the 3.54 diff be having? Before it went in for restoration at 70 I was achieving about 16mpg and have no idea what the 0-60 was
My understanding is that the 2.88 diff makes the cars acceleration more lazy and improves MPG by lowering RPM at motorway speeds. Is that right?
As part of the restoration I have updated the gearbox to a 700r4 which is the same gearbox as the GM400 but with the extra 4th gear with a 0.70 ratio.
Just wondering if the 3.54 makes it a faster car or whether it’s worth swapping to the 2.88? I don’t really understand diff ratios
It depends what you really want tbh.
And how much you plan to high speed cruise vs using the acceleration.
I have a Camaro which uses the 4L60e transmission. It’s essentially a latter 700R. It has tall gearing with a 2.87:1 diff. It’ll do 160mph in 3rd. So 4th is just cruising. It pulls 40mph/1000rpm in top.
It goes a lot better than you think it would even with the tall gearing. I had an XJ-S V12 HE before this. An early HE example with a claimed 295hp. The Camaro is significantly faster. So I’d think the taller gearing would blunt a V12 Jag maybe a tad too much.
My Dad used to have a 6.0 V12 XJ40. That had a 4 speed 4L80e transmission. Which is one up and stronger than the one in the Camaro or the one you are using. I think the XJR x300 also used this box. This was very much more on par with my Camaro performance wise. I think it had the 3.55’ish gearing.
For the Camaro the factory offered a 3.23:1 sports gearing. Although most aftermarket kits are 3.73:1.
So IMO I think both gearing setups will work. But the shorter (numerically higher) diff ratio will probably make the car more fun while still offering sufficiently good high speed cruising ability.
And how much you plan to high speed cruise vs using the acceleration.
I have a Camaro which uses the 4L60e transmission. It’s essentially a latter 700R. It has tall gearing with a 2.87:1 diff. It’ll do 160mph in 3rd. So 4th is just cruising. It pulls 40mph/1000rpm in top.
It goes a lot better than you think it would even with the tall gearing. I had an XJ-S V12 HE before this. An early HE example with a claimed 295hp. The Camaro is significantly faster. So I’d think the taller gearing would blunt a V12 Jag maybe a tad too much.
My Dad used to have a 6.0 V12 XJ40. That had a 4 speed 4L80e transmission. Which is one up and stronger than the one in the Camaro or the one you are using. I think the XJR x300 also used this box. This was very much more on par with my Camaro performance wise. I think it had the 3.55’ish gearing.
For the Camaro the factory offered a 3.23:1 sports gearing. Although most aftermarket kits are 3.73:1.
So IMO I think both gearing setups will work. But the shorter (numerically higher) diff ratio will probably make the car more fun while still offering sufficiently good high speed cruising ability.
With the old 400 3 speeder hooked up to the 3.54 the car would accelerate quite well with a cruise at 80 showing around 2800rpm. I guess with the 4th gear being 0.70 that will drop to about 2100 with a corresponding increase in fuel consumption. I was just surprised at the claimed 22mpg with the 3 speeder in the XJRS brochure and the 0-60 time was quicker than I expected so was wondering what would be different with my setup, allowing for age or course
Tbh though. If you have any concern over mpg, a Jag V12 is not the car to have 
No idea what mine did. Never bother to work it out. Although if it was doing 11mpg I’d have been impressed.
I raced mine against an XJR-S at Santa Pod. Lost every time. But only by a car length.
No idea what mine did. Never bother to work it out. Although if it was doing 11mpg I’d have been impressed.
I raced mine against an XJR-S at Santa Pod. Lost every time. But only by a car length.
Edited by 300bhp/ton on Saturday 16th February 21:31
300bhp/ton said:
Tbh though. If you have any concern over mpg, a Jag V12 is not the car to have
No idea what mine did. Never bother to work it out. Although if it was doing 11mpg I’d have been impressed.
I raced mine against an XJR-S at Santa Pod. Lost every time. But only a length.
I’m not concerned in the slightest if you could see how much money I’m pouring into it. Just interested to see how they made those figures... assuming they are true of course No idea what mine did. Never bother to work it out. Although if it was doing 11mpg I’d have been impressed.
I raced mine against an XJR-S at Santa Pod. Lost every time. But only a length.
Bigger diff ratio number better acceleration, higher revs when cruising, higher fuel consumption.
Lower diff ration number, slower acceleration, lower revs when cruising, lower fuel consumption.
A gear speed calculator will help you visualise the combination of diff ratio, gearbox ratios and tyre wheel combination as the rolling circumference can make a big difference.
Lower diff ration number, slower acceleration, lower revs when cruising, lower fuel consumption.
A gear speed calculator will help you visualise the combination of diff ratio, gearbox ratios and tyre wheel combination as the rolling circumference can make a big difference.
craigjm said:
I don’t really understand diff ratios
That's worrying. It's a simple number which when multiplied by other numbers gives further numbers. We call it "maths". YMMV in other countries like the USA.A car needs to be geared to reach its theoretically achievable top speed at about peak power rpm in top gear. With the RS engine that should be about 155/160 mph at 5250 rpm which means about 30/31 mph per 1000 rpm.
Assuming the original 205/70/15 tyres and the ratios that google tells me for the R4 box (3.06, 1.63,1, 0.7) the numbers with the 3.45 diff will be.
1st - 7 mph/1000
2nd - 13
3rd - 22
4th - 31
So spot on.
Edited by Mignon on Sunday 17th February 14:21
Mignon said:
craigjm said:
Assuming the original 205/70/15 tyres and the ratios that google tells me for the R4 box (3.06, 1.63,1, 0.7) the numbers with the 3.45 diff will be.
1st - 7 mph/1000
2nd - 13
3rd - 22
4th - 31
So spot on.
How does tyre size impact? The tyres will be 225/45r18 front and 245/40r18 rear from what I can remember. How does that change the calculations above. The gear ratios you have are correct. 1st - 7 mph/1000
2nd - 13
3rd - 22
4th - 31
So spot on.
So in theory if they were both new cars weighing about the same and the RS does the 60 dash in 6.5s how much quicker would it be with the 3.54 diff, four speeder etc? I know there would be a difference in wind resistance between the two models but if the different set up was in the RS so that remains equal
The new tyre diameters are not much different from the OE ones so little change.
To answer any 0-60 questions you would need the services of a mathematical genius with a sophisticated computerised vehicle performance simulation program but what do you think the chances of ever meeting someone like that are?
To answer any 0-60 questions you would need the services of a mathematical genius with a sophisticated computerised vehicle performance simulation program but what do you think the chances of ever meeting someone like that are?
Mignon said:
The new tyre diameters are not much different from the OE ones so little change.
To answer any 0-60 questions you would need the services of a mathematical genius with a sophisticated computerised vehicle performance simulation program but what do you think the chances of ever meeting someone like that are?
Yes the rolling radius has been kept quite similar so i thought that would be the case. The new GPS speedometer in the car can measure the statistics if I want it to when the cars finished so will just wait for that and then give it a shot. Thanks for the figures above To answer any 0-60 questions you would need the services of a mathematical genius with a sophisticated computerised vehicle performance simulation program but what do you think the chances of ever meeting someone like that are?
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