[Saloon-lovers] 420 G / MK10 rear diff ratio question

Saloons
1

I’m about to look at a 420G automatic and was wondering what the
rear diff. ratio is. Almost everything from Jag with an IRS at that
time had 3.54 ratios behind an auto. (except the XKE), so with the
14’’ wheels and modern tires, the engine rpm’s now might be a little
high at highway speed. The common fix on XKE’s is to put in a 3.31
diff. With 14’’ wheels on the G, the outer tire radius is even
smaller, so i’m hoping this big sedan came through with a 3.31 or
even a 3.07. I’m assuming this car is a North America car and not
some import where the ratios could be anything.

I have the feeling i’ll be looking for a 3.31 or 3.07 diff in a
month or two.

Thanks for any help.
Phil.–
PhilW
Virginville, Pa., United States
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2

In reply to a message from PhilW sent Sun 17 May 2009:

Hello Phil,

Yes, 420G auto, 3.54 ratio, 64.8 mph @ 3000 rpm.

Paul–
The original message included these comments:

I’m about to look at a 420G automatic and was wondering what the
rear diff. ratio is. Almost everything from Jag with an IRS at that
time had 3.54 ratios behind an auto. (except the XKE), so with the
14’’ wheels and modern tires, the engine rpm’s now might be a little
high at highway speed. The common fix on XKE’s is to put in a 3.31
diff. With 14’’ wheels on the G, the outer tire radius is even

–
PS
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3

In reply to a message from PS sent Sun 17 May 2009:

Oh, crap. The modern 14’’ tires are smaller than originals, so my
RPM’s will be even higher than that. Maybe i’ll skip right over the
3.31 and go to a 3.07.

Thanks,
Phil.–
PhilW
Virginville, Pa., United States
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4

Phil:

I put the 15" XJ6 Series II wheels with Michelin tires on my 420G and not only do they look terrific, but the car never rode better. I don’t know if this helps with the rpm situation but it may be a good alternative to redoing the diff.

Rick Cusack
'67 420G

17 May 2009:>

Oh, crap. The modern 14’’ tires are smaller than originals, so my
RPM’s will be even higher than that. Maybe i’ll skip right over the
3.31 and go to a 3.07.

Thanks,
Phil.

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5

In reply to a message from Richard Cusack sent Sun 17 May 2009:

Rick,
I have been wanting to get a set of newer cast wheels for my 68
420G, to ‘‘save’’ my originals (and a very cheap way to get a whole
new set of tires and wheels as Jag owners buy the ‘‘bling’’ 22’’
wheels).

Could you post a picture of your G with the XJ6 wheels? What year
were they from?

By the way, what is the bolt pattern for the 67 420G, and what
later model cast wheels will fit?
Thanks.–
The original message included these comments:

Phil:

–
baloo – 67 420G w/ 502c.i. 68 420G RHD orig & rest’d
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6

In reply to a message from baloo sent Sun 17 May 2009:

Past postings by other indicate that the diferrential gears are
Dana 44 by Ford and readily available in a wide variety of sizes.
P.–
Peter J. Smith, 1966 3.8S MOD
Carson City Nevada, United States
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