Gear/Tire Size/Motor/Tranny Combo

[MtBraun]

I often see parts of the above addressed, but I'm trying to determine a rear end/tire size combo that will fit the engine/tranny combo we plan for the 84 swb.

Engine will be a BB for sure, likely a stroked 427T mated to a TH400. Tire size will be either 38 or 40. I'm going back and forth on 3.73 or 4.10 gears. With the kind of torque we expect with this build it may not matter relative to drivability. There's about a 200 RPM difference between the two gear ratios at 60MPH.

I found a 14 bolt FF with 3.73 gears I can snatch for $100 which is the cheapest I've found. This would work well with current set up since we already have 3.73 gears in the 84 and would only need to get 8 lug hubs for the front.

I'm looking for what people have actually installed and used. What worked best and what would you do differently if anything? If you can't tell by the combo this is NOT a daily driver. Probably end up being a pavement queen with the occasional big tire smoke show.


Thanks to @vkh for his calculator spreadsheet. That is very helpful.

 

[4WDKC]

I often see parts of the above addressed, but I'm trying to determine a rear end/tire size combo that will fit the engine/tranny combo we plan for the 84 swb.

Engine will be a BB for sure, likely a stroked 427T mated to a TH400. Tire size will be either 38 or 40. I'm going back and forth on 3.73 or 4.10 gears. With the kind of torque we expect with this build it may not matter relative to drivability. There's about a 200 RPM difference between the two gear ratios at 60MPH.

I found a 14 bolt FF with 3.73 gears I can snatch for $100 which is the cheapest I've found. This would work well with current set up since we already have 3.73 gears in the 84 and would only need to get 8 lug hubs for the front.

I'm looking for what people have actually installed and used. What worked best and what would you do differently if anything? If you can't tell by the combo this is NOT a daily driver. Probably end up being a pavement queen with the occasional big tire smoke show.


Thanks to @vkh for his calculator spreadsheet. That is very helpful.

I know this chart isnt real world experience but it helps. my trucj had 342s and 33s when I bought it, even with the 3+ first gear of the 700r4 I couldnt brake torque it to spin the tires. Maybe you can use the similarity of the 342s and 33s to 373s and 38s

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[Frankenchevy]

I know this chart isnt real world experience but it helps. my trucj had 342s and 33s when I bought it, even with the 3+ first gear of the 700r4 I couldnt brake torque it to spin the tires. Maybe you can use the similarity of the 342s and 33s to 373s and 38s

You must be registered for see images

are those numbers rear drive shaft rotations per minute at 60mph?

 

[75gmck25]

About 2000 rpm is a good highway cruising RPM, but for most modern trucks that is in 4th gear and .7 overdrive at 60 mph. In high gear at 60 they are more like 2850 RPM, which is what you will have with your TH400.

I think you need more than 3.73 gears if you want to run 38's or 40's. Most folks would be deciding between 4.10 and 4.56 gears with tires that large. However, you will also have a big block helping keep up that low end torque, so maybe it will work with 3.73 gears.

Bruce

 

[MtBraun]

I know this chart isnt real world experience but it helps. my trucj had 342s and 33s when I bought it, even with the 3+ first gear of the 700r4 I couldnt brake torque it to spin the tires. Maybe you can use the similarity of the 342s and 33s to 373s and 38s

You must be registered for see images

What are the colored bands supposed to represent?

 

[MtBraun]

About 2000 rpm is a good highway cruising RPM, but for most modern trucks that is in 4th gear and .7 overdrive at 60 mph. In high gear at 60 they are more like 2850 RPM, which is what you will have with your TH400.

I think you need more than 3.73 gears if you want to run 38's or 40's. Most folks would be deciding between 4.10 and 4.56 gears with tires that large. However, you will also have a big block helping keep up that low end torque, so maybe it will work with 3.73 gears.

Bruce

That's precisely what I'm wrangling with in this equation. I expect 500+ lb./ft of torque so I think a guy can give up some gear.

 

[MtBraun]

About 2000 rpm is a good highway cruising RPM, but for most modern trucks that is in 4th gear and .7 overdrive at 60 mph. In high gear at 60 they are more like 2850 RPM, which is what you will have with your TH400.


Bruce

Help me understand this part. I thought third gear on a Th400 was 1:1? In the calculator that results in 2037 RPM with 3.73 gears and 40" tires at 65MPH.

 

[4WDKC]

What are the colored bands supposed to represent?

http://southwestpowersports.com/GEARCHANGE.HTML

 

[4WDKC]

Help me understand this part. I thought third gear on a Th400 was 1:1? In the calculator that results in 2037 RPM with 3.73 gears and 40" tires at 65MPH.

it is d=1:1

 

[bucket]

With gears that tall and tires that big, there won't be any smoke show.

I run 4.56's with smaller 36" tires and it's about right. You should be looking at 4.88's or 5.13's, especially if it's a truck built just for show and/or fun.

 

[Honky Kong jr]

it is d=1:1

Hey where’s the Vette and your truck or are you into kittens now?

 

[shiftpro]

That's precisely what I'm wrangling with in this equation. I expect 500+ lb./ft of torque so I think a guy can give up some gear.

It's a mistake to think torque will make up for gears. Even if this is true, it puts extreme forces on everything between the crankshaft and rear wheels.
Gearing down not only puts your usable rpm in the correct range, it also takes stress off of.. like I said, everything behind the crankshaft.

 

[MtBraun]

It's a mistake to think torque will make up for gears. Even if this is true, it puts extreme forces on everything between the crankshaft and rear wheels.
Gearing down not only puts your usable rpm in the correct range, it also takes stress off of.. like I said, everything behind the crankshaft.

I see your point. But why not just build up everything behind the crankshaft? From what I can tell the 14 bolt won't be a weak link, so what would be? The driveshaft and TC?

When I look at the gearing/rpm tables, it looks like a 4.10 gear puts my cruise speed at about 2200 rpms, which should be right in the sweet spot of that engine combo. I understand its more complex than cruise speed, but I'm trying to balance power utilization with drivability.

 

[QBuff02]

My '82 is a 1 ton with 454/th400/4.10 gears combo and with 33's my highway cruise rpm at 55ish is 2,600-2,700rpm. I think I'd be at an absolute minimum of 4.10 for those tires.. 4.56 or 4.88 would be a better choice. A lot of guys around here that go 38" tires or north, generally run somewhere between 4.88-5.13's..

 

[4WDKC]

Hey where’s the Vette and your truck or are you into kittens now?

they are there, you dont see them?