454 Gear Ratio Advice Needed

[slaterson]

Hey all, recently bought a '68 SWB with a 1980's 350 and TH400, manual brakes and steering and no posi. My eventual goal is a big block 454 (or similar) with 500+ HP. mated to a Tremec T56 5 or 6 speed, possibly Holley EFI, posi rear end, power disk brakes (at least fronts) and power steering with air conditioning.

My goal for this spring is to add some nice wheels/tires and power disk brakes. Unfortunately, I have found a crap selection of 6 bolt wheels so I am planning to convert to 5 lugs. The local scrap yards have a good selection of posi rear ends but I am wondering which gear ratio would be best for my planned big block setup? Performance trumps fuel economy. Planning on 20" AR Torque Thrust wheels with 275/40 rear tires (not sure of total tire height). Also, would rear disks really be needed? .. Any other advice? ... Thanks, love the forum so far!

[big block 72]

3.73 ratio would work well with that combination and is a common factory ratio. With the t56 you should still get decent economy while still being able to roast the tires at will.

[Mike C]

Figure out what trans you are going to run, figure out what the overdrive ratio in top gear of that trans is. Figure out how tall a rear tire you want to run, then do the math to figure what rpm you want to run at cruise speed.

An early T56 with the .50 6 gear I would run 4.56. 4.10 at a minimum. A 5 speed trans I would run 3.73-4.10.

I'm swapping a 427/TH200 4R into my '69 and I think I have decided on 3.73 for optimum highway cruising rpm, but I may do 4.10 still. Decisions, decisions, decisions.

[slaterson]

Anyone else have some advice on this?

[ryans69chevy]

I have a 454 waiting to go into my 69. I'm gonna go with the tremec magnum 6 speed with a .63 final gear. I just got my rear end last fall built with posi and 3.73 ring and pinion. I talked to quite a few people and that was their recommendation on gear ratio.

Discs in the rear would be sweet. might not be totally needed, but it's only money right!? Ha! Are you looking for discs?

[jocko]

Like Mike said, choose your trans first. You may not need different gearing. Keep in mind that aftermarket o/d transmissions not only have an overdriven highway gear, they also tend to have a deeper first gear - so the coolness of a 70's 4.11 (or 4.10 in the case of our trucks) doesn't mean what it used to - the trans does the multiplication for you. Generally speaking, the more torque your motor makes, the less gear (numerically) your rear needs to get the truck moving. This is why a factory 3/4 ton had a 4.10 standard on the small block (it needed the extra torque multiplication for a load) and a 3.54 D60 on the 396/402. I'd submit that a 3.54 or 3.73 would be about right - 5 and 6 speeds and o/d transmissions don't need radically high or low numerical gear rations - again, the trans does all the work. If you were keeping a stick TH400, for example, then I'd be asking you what your ultimate goal is with the truck. If it is pulling/hauling, the a 3.54 is fine with the hp numbers you're talking about. If you want to race, then a 4.10 or 4.56 might be the ticket for off the line accel in a heavy truck with a TH400. Anyway, anything short of racing, I'd stick with a 3.54 or 3.73 (at the most) for your combo. If you choose a trans, let us know and we can run the numbers for you on cruising rpm and off the line torque multiplication for comparisons.

In Ryan's case with an o/d gear that low (numerically), I'd definitely not go below a 3.73 (to a 3.54, etc) or you will be lugging the motor (cruising rpm too low) at highway speeds. 4.10 would even be doable - but it will probably go about 10 feet from a dead stop before it's time to shift to 2nd gear, or you'll end up just starting off in 2nd. Anyway, lots of ways to skin the cat - most o/d transmissions don't have a .63 (it's nice for tip top speed if that's what you want), most have ..68-.70 or higher ratios. There is a reason that a bunch of factory late model trucks with o/d transmissions use 3.73 from the factory.

[slaterson]

Originally Posted by jocko

Like Mike said, choose your trans first. You may not need different gearing. Keep in mind that aftermarket o/d transmissions not only have an overdriven highway gear, they also tend to have a deeper first gear - so the coolness of a 70's 4.11 (or 4.10 in the case of our trucks) doesn't mean what it used to - the trans does the multiplication for you. Generally speaking, the more torque your motor makes, the less gear (numerically) your rear needs to get the truck moving. This is why a factory 3/4 ton had a 4.10 standard on the small block (it needed the extra torque multiplication for a load) and a 3.54 D60 on the 396/402. I'd submit that a 3.54 or 3.73 would be about right - 5 and 6 speeds and o/d transmissions don't need radically high or low numerical gear rations - again, the trans does all the work. If you were keeping a stick TH400, for example, then I'd be asking you what your ultimate goal is with the truck. If it is pulling/hauling, the a 3.54 is fine with the hp numbers you're talking about. If you want to race, then a 4.10 or 4.56 might be the ticket for off the line accel in a heavy truck with a TH400. Anyway, anything short of racing, I'd stick with a 3.54 or 3.73 (at the most) for your combo. If you choose a trans, let us know and we can run the numbers for you on cruising rpm and off the line torque multiplication for comparisons.

In Ryan's case with an o/d gear that low (numerically), I'd definitely not go below a 3.73 (to a 3.54, etc) or you will be lugging the motor (cruising rpm too low) at highway speeds. 4.10 would even be doable - but it will probably go about 10 feet from a dead stop before it's time to shift to 2nd gear, or you'll end up just starting off in 2nd. Anyway, lots of ways to skin the cat - most o/d transmissions don't have a .63 (it's nice for tip top speed if that's what you want), most have ..68-.70 or higher ratios. There is a reason that a bunch of factory late model trucks with o/d transmissions use 3.73 from the factory.

Thanks for that great info.. I am unsure of the trans I will eventually be using, it will most likely be the T56 5 or 6 speed. Initially, this rear end will be coupled to my current 350 with 3speed auto and it may be a year or so until I swap to a 454. I just want to pick a posi rear end with a gear ratio that will work good now so I can covert to 5 bolt and get some nice wheels...and also work well with my future big block plans. Based on the response so far it sounds like the 3.73?? (Btw, learned today that my planned tires are 29" tall)

[slaterson]

So I guess I'm leaning toward the 3.73.. I plan on sourcing from some local scrap yards so if I can't find 3.73 would the 4.10 be a good 2nd choice or should I go the other direction (3.54 or similar)? Any other input or suggestions? Btw, will not be used for hauling or pulling, strictly a sport/performance truck..

[Mike C]

3.07 and 3.73 are the most common ratios found in the 12 bolt truck. There are some 3.42 as well as some 2 series gears in the '70's sand very early '80's trucks before they went 10 bolt. 4.10 are obviously 10 percent more gear than 3.73 so 10% more rpm in whatever gear for a given speed.

But if you are looking for a "bolt in" solution, the 3.73 is it assuming you have a coil spring tuck.

[slaterson]

Found a 10 bolt 3.42 posi rear end (complete axle) on craigslist that I can pick up for cheap. It came out of a 1997 Tahoe. Would this be a good setup for a planned 500hp. 454 big block with tremec t56 and 29in tires?