Stock `66 contemplating 3:73, running Deestone 28", opinions wanted

[Spring Grover]

Morning. Bone stock `66, running 7.00 x 15 Deestone. Think they were listed as 29" when I bought, they stand 28" with a tape measure. I love the look of these tires on this truck. With that being said, on road only, country back road driving. Like the title states, contemplating installing 3:73 gears, she needs a clutch anyway, good time to do it. No tach but she's screaming over 45.
Like to hear the yays and nays.
What say you?
tia
Mark

 

[SteveH]

If you insist on 28" tires (pretty close to stock) then 3.73s would give you some RPM relief on the highway. If you putter around back roads, I wouldn't bother.

 

[SteveH]

Adding a note - while the engine may sound like it's screaming, and FJ40 like yours at 60 mph on 28" tires is turning ~3000 rpm.

On my '78, I installed 33x9.5 Kendas, which means at 3000 RPM, at an indicated 60 mph, I'm actually going ~69 mph. This is called a 'rubber overdrive' and one way to fix LC gearing without opening up diffs. I understand your desire to keep those cool, small tires, however.

You might put a tach on your truck (temporarily) and see what RPMs you are getting at different speeds. 3000 RPM is considered by many to be the safe, practical max RPM for driving an F/2F engine that is in good condition.

 

[pb4ugo]

You can find rpm calculators on the internet. Note your current rpms at certain speeds. Then run the numbers for 3.70:1 gearing on the calculator and use your actual measured tire sizes. Changing a clutch and changing gears in 2 diffs are 2 completely different tasks. The front diff is the more labor intensive swap. You might consider servicing the knuckles and repacking the wheel bearings while you're there. The rear is the easy part. 66 40's diff might be different than later model diffs. You might need to get new fine spline pinion yokes if you're buying ring and pinions. I think new pinions are going to be fine spline.

 

[1911]

I'm running 33" tires on (stock for my year) 3.70 diffs, though I also installed an H41 transmission with a 5.3:1 1st gear, paired with an Orion transfer case. I love this combination; I'm only pulling about 2,500 rpm to do 65 mph, and still have low gears for the trail.

 

[brian]

I'm running 33" tires on (stock for my year) 3.70 diffs, though I also installed an H41 transmission with a 5.3:1 1st gear, paired with an Orion transfer case. I love this combination; I'm only pulling about 2,500 rpm to do 65 mph, and still have low gears for the trail.

I'm in the 33s+370s camp as well....even with an h42, it's still a great combination.

 

[65swb45]

It’s a 10% diff change. Borrow a set of tires from a friend who has tires 10% bigger and see how you like it.

I’ve had a few clients over the years who have ‘loaned’ tires in my driveway for each other to try.

 

[Spring Grover]

It’s a 10% diff change. Borrow a set of tires from a friend who has tires 10% bigger and see how you like it.

I’ve had a few clients over the years who have ‘loaned’ tires in my driveway for each other to try

-That's not much, for the effort and expense anyway. I feel you just put the project in perspective Mark. I have a set of 31" I could throw on for testing purposes, but I like this tire, easy to get in the truck, and looks nice.
-To the others that chimed in, thank you, good points all. I think some calculations are in order, just to see what rpm I'm running, at what speed, and just keep it in the limits. Seems to be the prudent thing here.

 

[65swb45]

-That's not much, for the effort and expense anyway. I feel you just put the project in perspective Mark. I have a set of 31" I could throw on for testing purposes, but I like this tire, easy to get in the truck, and looks nice.
-To the others that chimed in, thank you, good points all. I think some calculations are in order, just to see what rpm I'm running, at what speed, and just keep it in the limits. Seems to be the prudent thing here.

That’s what the like button is for.

 

[Grayscale]

I wouldn't ever measure a tire's height to figure out rpm, the circumference translates to actual work. The pressure of the tire and weight of the car will change things. To convert 7.00 15 to something like 29x8 gives you a reference point that you can verify with a a cloth tape measure, but you seem to have some numbers from the manufacturer. Tire Size Calculator - https://tiresize.com/calculator/

You'll get your rpms back down with a fresh clutch.

My opinion: I'm a fan of leaving it as it is. Any 40 is worth using to it's greatest potential, in a properly maintained, road-worthy state. There is plenty to keep up without introducing conversions on an rare-ish antique. If you must keep up with traffic, then the list of mods doesn't end with gears and / or tires... For example, acoustic insulation, lift kit, disc brakes to keep up with the bigger treads and higher speeds might also be in order. Maybe there is low-hanging fruit to harvest, but, eventually you'll have a special account just for web-ordered ladders, as the classic-40-aspect of the vehicle slowly diminishes into a pile of leftover parts. For me, time spent with the classic is more valuable than time updating the classic.

 

[70fj40]

I believe I would spend my money on a tcase rebuild, new clutch, knuckle rebuild and re greasing the axles before I did any re-gearing. Those items will likely give you a noticeable difference driving it. Take it from someone who didn’t do those things first, I wish I had because they made a big difference in my 1970.

 

[Spring Grover]

My opinion: I'm a fan of leaving it as it is. Any 40 is worth using to it's greatest potential, in a properly maintained, road-worthy state. There is plenty to keep up without introducing conversions on an rare-ish antique. If you must keep up with traffic, then the list of mods doesn't end with gears and / or tires... For example, acoustic insulation, lift kit, disc brakes to keep up with the bigger treads and higher speeds might also be in order. Maybe there is low-hanging fruit to harvest, but, eventually you'll have a special account just for web-ordered ladders, as the classic-40-aspect of the vehicle slowly diminishes into a pile of leftover parts. For me, time spent with the classic is more valuable than time updating the classic.

I feel the same way with this vehicle. That has to be the best definition of a 40 purist owner I ever read Grayscale. ASIDE from the drum brakes, which for me had to go, I love the original 3 on the tree drive. You're absolutely right. One thing leads to the next, and the next is boxes of original parts, and I already have boxes from another `66 build I did.
Onto the next project. Thanks for your input gentlemen.

 

[ace10]

That old girl is beautiful as she is.

My '67 FST is all original, too. And just gets driven occasionally on back country roads.
I went up to 29"ers, and didn't really notice any meaningful relief from the screaming engine.
I've accepted the limitations, including the drum brakes, and drive accordingly.

 

[Roastchestnuts]

I have a 64 with a 2f and 3speed on the column. it has kenda kevlar 33x9.50 and stock 4:11 gears and some dobinsons leafs I think the 1.75 inch springs (I wanted the least amount of lift). what I am getting at is with 33s I still am very sketched out driving faster than 55. Not because RPMS but because going faster seems like a death wish. These are tractors that "can" go faster but that does not mean you need to keep up with 70mph traffic. I actually eventually want to put 16 inch rims and 750 16 size tires. I think they would be about perfect for my use. here is mine

Side note: I love everything about your fj40! love the tires, old school caps, wheels, and the cacti!

 

[cruisermatt]

you should get a used 3.70 3rd member and put it in the rear and take it for drive just too see how how it is. You’ll either like it and want to change the front too or not notice a difference and take it out and probably be able to sell it to.

Also, a lot of the noise on these early trucks is often old rear end bearings or ring/pinion’s especially if they’ve sat for a long time and stuff gets rusty

My other favorite “ hidden mod” is the later flywheel and diaphragm clutch setup.

 

[Roastchestnuts]

you should get a used 3.70 3rd member and put it in the rear and take it for drive just too see how how it is. You’ll either like it and want to change the front too or not notice a difference and take it out and probably be able to sell it to.

Also, a lot of the noise on these early trucks is often old rear end bearings or ring/pinion’s especially if they’ve sat for a long time and stuff gets rusty

My other favorite “ hidden mod” is the later flywheel and diaphragm clutch setup.

I have that flywheel set up from you