1000 = 1K or special K :)

It can easily chug along at 20mph with the 3.9 rear in 4th as I noticed today
in cool melbourne weather but to be comfortable 25mph up all in 4th.
The pulsar electronic dizzie with the higher comp would help a tad.

What's a K body? Rover K? Chrysler K? Or do you mean a B-body (Datsun B10/B110/B210, etc)?

just over 6000rpm in 4th, an extra 200cc transforms a K.

i didnt know stock a12 was capable of 100mph

Im surprised myself to say its torquey but it is compared to other A12s I have driven in K bodies.
Also I avoid Melbourne city but traffic is unavoidable sometimes in the burbs.
It can pull in 4th from 25mph to 100mph comfortably so it not even close to ideal with a 3.9.

since when is an a12 torquey? i'd think you'd be stressing an a12 more trying to push it around city driving with such a tall ratio. worth a try i guess

This is the dilemma and the reason everyone with an A series wanting/needing an overdrive box.
No one wants cruising over 3500rpm especially so with an a15, to think 3700 rpm drives me crazy
with an a12 Id rather cruise at 2500rpm where I know this stock and torquey a12 loves to be.

I guess it all depends on where ones priorities lie.

For me, ... well I couldn't really give a rats rectal orifice about dazzling acceleration, & I do not much like seeing the tach pegged on 3,500 rpm [or more] at a paltry 62mph [100 kph]
It makes too much noise & wears the engine too quickly.
Bugger that for a joke.

For my street hack I would prefer to use the A15 [which I have] plus the tallest diff & tallest tyres that are practical.
The torque of the A15 with GX induction & a mild cam will still provide far more than adequate performance in urbane areas & the tall gearing will keep me percolating along a highway speeds while my car behaves like the clever Datsun engineers intended as it sip sip sips the fuel.
It's a good feeling when you pull up to the pump after a 350 mile [530k] drive with a 7.7 imp gallon [35L] tank & fill 'er up for change out of $40. [at todays prices.]

I would split my mileage up as less than 10% urbane driving & over 90% Highway cruise.
It's horses for courses.

Lags the issue with yours losing poke could have been the camshaft?.
The a12 I have is slightly higher comp and churns well at all speeds
and overtakes with ease 100kmh and over.

It actually runs out of first too quickly so I know with a 3.7 it should
be a little better. A 3.54 will be the beez neez and its no obsession
just prefer using the torque rather than hp around town not to mention
hating first gear and almost able to take off in 2nd comfortably.

I remember the a15 in my coupe with 79mm bores been a real stump puller
with mild cam but useless without a 5 speed & 1st might as well not exist.
The stock camshaft made it even worse with all torque.

Using the calc with 165/70/13 and 1:1 final at 100kmh the 3.9 is at 3700rpm
no wonder I wasnt happy with the noise!!! With 3.7 this drops to 3500rpm so
will need an overdrive if using an A12

a10 piston velocity at 7000rpm redline is 826.000 m/min 3500rpm is 413m/min
a12 '' '' '' '' 980.000 m/min '' '' 490m/min
a14 '' '' '' '' 1078.000 m/min '' '' 539m/min
a15 '' '' '' '' 1148.000 m/min '' '' 574m/min

So with 3.7 rear at 1:1 and A10 piston travels
77 metres less than A12,
126 metres less than a14
161 metres less than an A15
to cruise at 100km/h

All changes with a rare overdrive 5 speed.

10000rpm if 5th, i thought victas went out to 15000rpm f1 style? ;)