T5 transmission upgrade

I am planning to do a T5 transmission swap on my 58 XK150dhc, I love the car but the gearbox is not real friendly to city driving. My car has an original overdrive and I plan to keep all of the parts. My rear end ratio is 4.09 ratio. The 5th gear options on the T5 are .63 or .73, any suggestions? The best source seems to be Driven Man for 'a complete kit. I’ve been told an experienced mechanic can do it in 30 hours so I’m figuring 60 should be reasonable for a rist timer.
Any comments would be appreciated.

I have a '68 Mustang with a T5 conversion which I fitted myself. I initially chose the 0.63 5th gear ratio, with a 3.50 final drive (the car has wide 15" large profile rubber on the back, which can’t be that far off the XK’s). I found this gave me a very long top ratio, with a big gap between 4th and 5th, as you would expect. The 400bhp stroked 393 could pull it with ease, but it wasn’t great for touring as I kept wanting to drop it into 5th which would drop cruising revs to 1500rpm, way too low for a petrol engine. 2-3 years later I swapped the 5th ratio to 0.78 and the FD ratio to 3.25. This evened out the ratios across the range very nicely and we’ve done 40,000 miles touring Europe in it since - with, whisper it, an average of around 22mpg. Once did 650 miles on deristricted Germany motorways at 90-95 when it returned 26…

I can send you a few pictures of my T5 install in my 150 ots, pass on your e mail,

Not sure how relevant it is to an XK but I’m almost through converting my Mk2 to a T5 and have a detailed build thread going over in the saloons forum.

You need to change the diff from 4,09 to 3,54
Otherwise you will switch between 4th and OD on twisty country roads. With the 3,54 OD becomes a motorway only gear.
0,7 to 0,8 is good. 0,63 will be too high
I have the T5 in a 3,8 E and a 140.
The car becomes a modern car whereas the Moss is very period correct for an XK, esp the 120 and 140.

Thanks, I’ve been advised don’t try to do the swap without removing the front harmonic balancer and pulling the engine , also remove the interior to get to everything. Did you go for 5th gear ratio of .63 or .73? My car is not an S


I have a T5 in my 120FHC. I have a 3.54 rear axle, which is what the later 120’s have. The early ones are lower geared with a 3.77.
In forth gear which is straight though, the gearing is about 23mph per 1,000rpm. But the 3.77 gives on 21mph. So, my advice is not to go for the 0.63 fifth gear ratio as it will be far too high geared and have too little pull. I chose 0.80, which gives plenty of pull and is a good O/D style ratio. It is about 28mph/1,000 so near 2,500 at 70mph.

I agree with John, 0.63 is too long. If you can use a shorter ratio with an axle ratio of between 3.25 and 3.54 that will give you a more even spread of ratios. You really don’t want to be slugging along at 1500-2000 rpm for long periods with an XK engine. Your top gear acceleration will be pretty sluggish, too - 3.2 rear end with the 0.63 5th blunted the performance of even my old Mustang’s stroker, and that pushes out over 400ft.lbs. of torque. Which you’ll not easily see from an XK, with the best will in the world. And, given the extent of the work I’ve just had to have done on my XK’s rear axle (and others I know have had the same), you don’t want to put too much strain on that, either.