I am currently topped out at about 70kmh in my 1964 f350 with 4 speed transmission. I'd like to gain about 30kmh without sacrificing to much in torque. I use the truck to haul hay and wood but very rarely need 1st gear. It currently has 4.88 in it. What would be my best replacement thinking about switching to 4.10 or 3.73.

 

The best plan will consider what engine you have and what final ratio you have.
What engine do you have? Consider how much torque you have to pull through higher gears. (I moved up a tire size or two on my Tacoma for more clearance, but I miss how it jumped off the line, and find myself dropping into 4th for hwy passing etc much more - fortunately it is my son's daily driver now)
What tire diameter do you have? Or, do you know what RPM you're running at 70kmh?
There are many rpm calculators out there. Here's one: https://spicerparts.com/calculators/...rpm-calculator

I have a T-18, and like you, rarely use 1st. 34" tires that put me at 62 mph (100 kph) at roughly 3000 rpm Generally I'm hwy cruising between 3000 and 3400 rpm
I have a 460 motor w/ mild cam and it moves off the line in 2nd gear just fine, but less engine would be telling. I wouldn't really want to always start in first, since it is not synchronized, and it's a big step between 1st and 2nd. And I do like the granny every now and again, from pulling stumps to cruising slow in a parking lot w/o working the clutch a lot.
If you've got the inclination, I think many folks here would suggest adding an overdrive - but from what I've read, that's its own can of worms.

 

You need to provide more information on the truck and usage. Without any additional information I would think 3.73's would be a mistake that you regret and even 4.11's may make for an underpowered truck unless lightly loaded and/or level terrain.

 

I have a 292, and 235-80-17 for tires.

 

By specs, your tires are 31.8" in diameter. I'll assume they actually measure 31" on your truck at your inflation. I am assuming your 4th gear is a 1:1 ratio.
The math puts you somewhere around 2400 rpm at 70 kph. 2400 RPM is not that high for an old truck geared like this.
The math says you'll get to 95kph at 3200 rpm. Maybe it gets noisy and rattles - but you can work on that too. Like I said, I'm running over 3000 rpm on the hwy, and I shift around 4000 rpm - no problem in that if the engine is in OK shape

If you do not have a tachometer on the truck, I suggest you get one - maybe a real cheap one just for temporary purposes strapped to the steering column. You might find that running your truck up to 3000+ on the hwy gets you close to your goals.
Anyway, give it some thought and some testing. Put 4.88 and 4.10 numbers in the calculator and compare to your actual driving experience. Your 292 may not be able to span the torque/speed/rpm combination you're thinking you want.

 

IMO, 4.10 would be the highest ratio for a F350 with 292. Would require and ring and pinion gear set and a carrier, if these pieces are still available for a '64 Spicer 70. Should be done by a reputable rear end shop, (if they still exist anymore) The Spicer 70 with 6 lugs was available for the first time with a 4.10 ratio in 1966 and they were rare.

 

I am doing a complete rear swap on my 64 F350. I called Randy's Gears and spoke to one of the techs on what was available for the 1st gen Dana 70. Not much they told me, 2second gen is getting limited, third gen was still in good supply. I started to look around to see what would work. I found that a 1995 F350 cab and chasse 10.25" dual rear is close to the same width. I have to move the axle mounts and move them inboard about a 1-1/2 to accept the springs. Also the new pads need to be 2" wide for the width of the springs. I made my pads from 2" square tubing. You don't want the dually pickup rear. Its way to wide. The best part is The one I scored has bigger brakes and is a 4.10 ratio. I like the idea of 8 lug wheels so I got 6 wheels when I bought the rear axle. The fronts are being changes to 8 lugs via the axle adapters on the market.

 

That's what I was afraid of. Problem is, it's almost 2023, not the late 60's or early 70's when you could easily purchase the bolt on parts you need. Back then there was also a Watson overdrive auxilary box that you could purchase that went in between drive shafts and not to mention the Clark 264V0 5 speed overdrive ( .8 final ratio) bolt in that was available from the Ford parts dept. Today most will go to the pre '73 F 350 8 lug Spicer 70 ( 4.10) complete rear axle assembly in a wrecking yard and then deal with the 6 lug front or try to fabricate an adapter to connect a NV 4500 5 speed overdrive, not sure if it exists yet, but maybe someday soon.

 

Or you could go a completly different route and install a TKX 5-speed overdrive. I love mine. I have a complete install write up that you can search. It wasn't that hard.

 

When I swapped to 3.54 on my '66 250 I purchased the newest upgrade parts avail and also upgraded the axles to 4340 units. Cost a few bucks but done. Very little stuff for the early spline count stuff.
I bought my stuff from Randy's R&P also. Now Randy's Worldwide

 

For what its worth, in my 65 F100 240 I6, I have a T18, 3.25 gears, and (oem'ish) 28in tires. I find in my daily 11mi suburban commute, this setup performs well.
Yes, I won't win any races but I can easily keep up with rush hour traffic.
It originally had 3sp column shift and 3.70's. I felt this combo was way under geared. 70mph was at 3000rpm. Running down the freeway, the engine always sounded like it was about to fly apart. The engine is of questionable health, but it runs ok.
Now it runs 2700 rpm at 70. It's only 300 rpm, but it makes a big difference.
I do live in Central Florida and there are NO hills to speak of.