3.54 or 2.88 Diff Ratio with 16 inch wheels. Your opinions please

I can go down either route as I have a 4HA Salisbury axle on my 120 roadster.

The car will be used for long journeys but some acceleration is essential. I want to go to Germany, France and Italy from England.

I am not interested in hill-climbs or circuit racing in this old car.(not this year anyway).

I like 80/90 mph +.

I have a 3.07 E type and also 2.88 E type on 15 inch wheels and 4 speed synchro gearboxes.

I much prefer the 2.88, as it just gobbles up the road on long journeys. However the wheels are considerably smaller, so the gearing must be different.

On the 120 I have a fresh 4.2 engine running 170 psi on each cylinder until the 3.4 is ready. I also have the 4 sp. synchro E type gearbox. I have disc brakes. The rear calipers are 1 5/8 pistons, the front XK150 calipers with a small servo.

So what I am saying is that it should go and stop well.

Is 2.88 over doing it? Will 3.54 be more “in the zone”?

I have no experience on 16 inch wheels and tall tires, so I very much welcome comments from those that have done it.

By the way, I am using 16 inch Dunlop steels from a MK9.

If I get it wrong, I could swap the diff out later, as I have both diffs in the shed already rebuilt, ready to fit, so not big problem.


Go 2 88…That live axle is inches from your behind.
I have 307 and love it but Im waiting on a 5 speed.
The 288 and a 4 speed would be ideal.

I think the 4.2 just wastes rpm with a 3.54. It’s good in town. 4th gear all the time in the E Type.
It’s also incredibly snappy in top gear but who needs that. If you already have a 4.2 in a light car with a 2.88 and like it you will definitely want the same in the xk. Even more so if you like long distances. The two bad things I heard about is that the engine could be louder at some rpm and of course hill starts with larger wheels. If both are ok in the E why not in the XK. I can’t see how you would be lugging the engine and if changing it with the new engine is no big issue for you even better.

Unless of course one of the many RPM/speed calculators you can find on the internet give you uncomfortable results with the larger tyre circumference.

3.07 might be a good choice with 16" wheels and plenty of torque. You’d have improved acceleration compared to the 2.88, but still quite relaxed cruising ability.

Hi James…how about a 4 syncro with o/d on a 3.07…with the 16in wheels…even the 3.54…both popular choices with C type replicas…Steve

I think some may have been thinking 15 inch xke ?? with 16 inch, and tires that are in the range of 27.5 or more diameter…(most 600-16 are)…I much prefer something taller than the 3.54…I liked 3.27…but I am thinking to change mine to 3.08, 3.09 or in that range. There are rpm to diff / tire size calculators on the web…fun to play with…you can find say what diff, with what tire diam to have 2000 rpm at 60 mph …I think 2.88 is ok for 15 inch xke…a bit tall for the 16 inch. Since it is a Dana…there are many ratios available…I’d want to be in that 3.08 to 3.21 range. Comfortable cruise is what I like…so as said…whatever diff gets you to around 2,000 rpm at 60 or so…Nick

It is a bit of a conundrum. In theory I am leaning towards the 2.88. I am always open to experienced advice though.

I did the Spicer calculator for 27.5 inch tall tyres (16 inch 185s), in top gear (4th)-

The results for a 2.88 are
1,000 rpm 28mph
2,000 rpm 57 mph
3,000 rpm 85.6 mph
4,000 rpm 113 mph
Pretty useful I would say on a 1,200kg car.

The results for 3.54 are
1,000 rpm 23 mph
2,000 rpm 46 mph
3,000 rpm 69 mph
4,000 rpm 92 mph
Not bad either really.

I would have thought a 4.2 engine would be quite happy pushing the car along at 85 mph at 3,000 rpm. It has load of torque with the 2.88. Anyone disagree?

As a point of interest, our 2019 Mercedes shows 1,500 rpm at 80 mph in 9th gear. Crazy gearing, but works very well. This is the most relaxed cruised I have ever seen. The motor must pump out loads of torque at the engine speed.

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I would say it depends on the state of tune of the engine (where in the rev range the torque lies), where you drive, how you drive… If you are in the US and most of your driving will be on dual carriageways, maybe the 2.88, but acceleration will certainly be sluggish and the gears may not be well placed. Here in the UK, with country roads and many small towns, you would rarely be using top gear in urban areas so would be driving a lot on the indirect ratios.
My Mustang has a 3.50:1 rear axle ratio, with 235/60 15 tyre size, but this is with a 0.73 5th gear T5. That gives a long cruise similar to James’ Merc, but around town I tend to keep it in 3rd. I don’t know about the US, but we are getting an increase in 20mph limits here in the UK. I don’t know how you would drive through a 20 limit comfortably with 2.88 gearing.
My 140 (3.4 SE) has a 3.54 diff ratio with overdrive. These aren’t diesels - I like to keep the revs up and get the ignition timing all-in.

Yeah but again the 5th gear drops you 650 to 800 rpm…Its wonderful.
This is the problem.
We are used to 6 speed, 7 speed , 8 speed and 10 speed.
The overall perfect ratio for modern cars the last 10 years has been 6th and 2,000 rom at 70
Now its on average 1,750 at 80 ish.
3,000 rpm at 80 to4,000 depending on the ratio is ANNOYING.

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In the automatic, 1700 kg 4.2 XJ I‘m often waiting for another gear when it’s in top.
The power is definitely there and it’s turning ~3000 at 60 mph. I want a far higher ratio.
3.54 must be too high if you do long distances and there’s always 2nd and 3rd for 20 mph zones.

My W203 Mercedes turns 2100 at 60 (not unlike the numbers for the 2.88), finally runs out of breath at ~5100, that’s just under 250 kph and the engine is definitely weaker than the XK. It’s a 4 cylinder. Just fine. I don’t see how the 2.88 could be too high unless you have really steep hills.

it is not black-3.54 or white 2.88…there are diff ratios in the 2.92, 3.07, 3.09 range. Play with those with your tire diam. I am one to like 2,000 rpm at 60-65 in 4th. Nick

But he has black and white in the shed, both rebuilt (the envy) and as it is I think he should try the 2.88. Guess he is the first one to try that ratio on an XK with a 4.2.

With the ENV diff you have basically only two ratios to choose from (unless you go the other way to better acceleration and higher revs when cruising). They are 3.64 (standard) and 3.27. There are a lot more choices with the Salisbury: 2.45, 2.88, 3.07, 3.31, 3.54, etc). Not all of these are so easy to find, however.
My early 120 OTS now has the 3.27 ENV, and cruises at about 3000 at 70mph. With the 3.64 I had to use about 3500 to cruise at 70mph. The engine is a bog-standard 3.4, except for an XK150 B-Type head, 8:1, and 16" wheels with Michelin X tyres. It is quite happy tootling around town at 30mph or even 20mph in 3rd, even with the 3.27.
I would imagine the bigger, torquier 4.2 would happily pull a 3.07, or even a 2.88. I believe a lot of E-Types used the 3.07, though they were running on 15" wheels, not 16".

My 2.88 E-type goes like not snot on 15 inch wheels and a tuned fuel injected engine. The 15 inch Michelin XWXs (205s) sit about an inch lower than a 185 80 16.

I think a 2.88 should be fine in an XK120. The 4.2 engine I have in mind will put out well over 210 bhp and plenty of low end torque, so I think it should go pretty well.

I am leaning towards the 2.88 at the moment.

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Worst case will be a swap when the 3.4 goes back in or if you don’t like it. I think you will like it. The only problem that could arise is hill starts but as you said the XK is a light car. And exhaust drone at certain speeds but…

The stance on your car looks delish!

Thanks! Took a bit or torsion bar fiddling!

I made great progress yesterday on the axle front.

Having discovered that XK140/150 axles that resemble boat anchors are £1,500+, I decided to purchase an MK 2 Jaguar 4HA axle which was £70. This will be used to house one one of my diffs.

It measures 4ft exactly in width.I hope it should clear the bodywork with MK9 hubs and steel wheels.

Half an hour later and the MK2 brackets were off.

I positioned the car 6 ft in the air, then put the weight of the rear on the springs that resting on 5 ft high stands, so the springs were now a bit squashed flatter.

I put the clean axle casing in position, lifting it on top of a couple of 3 inch Morose spring perces that cost £30 a pair. These are formed from 3mm thick steel and they dropped onto the spring mounting points perfectly.

I was then able to rotate the axle housing to my best guess for the pinion angle. This was achieved using a pitchfork off the floor of the work shop.

I was aiming a couple of degrees below the center line of the drive shaft, as the axle twists up a bit under load.

I achieved this buy placing a 25mm long tube between the pinion flane seal hole and the top side on the gearbox cross member. The had welded angle iron at 90 degrees so I was able to judge the relationship between the tube and the direction the axle was pointing.

With the diff centralised and in the correct rotation, a Tacked the perches in place, then welded then solidly on the bench.

Next I have to tackle the anti tramp bars and what appears to be an enormous ladder bar which forms a cage around the pumPkin.