Originally Posted by jocko
Since you say you are putting a TH400 in this package - does that mean you already have the trans? And the 3.08s?
If yes, then disregard this post.
If not, then read on.
While a TH400 and 3.08 is a great setup, it lives in the world of tradeoffs compared to more modern, and readily available, parts. In other words, TH400 and 3.08 is PRIMARILY a cruising setup. TH400 1st gear multiplication with a 3.08 net you decent highway mileage at the expense of bottom end acceleration. Whereas a 4.10 rear end will snap your neck with a stout motor but you'd be stopping for gas much more often and be taching out at high rpm at highway cruising speeds. i.e. you have to make a choice between the two when using a non-O/D trans.
The answer to your dilemma is a slightly deeper gear set and an O/D trans. While most folks tout the benefits of highway mileage for an O/D trans - the less-often-discussed benefit of an O/D trans is the presence of a deeper 1st gear IN ADDITION to the highway mileage improvements. Take a look at the changes in highway cruising rpm TH400 vs 2004R below - but also note the huge difference in 1st gear torque multiplication.
The only reason I recommend a matched set of O/D trans with rear end gearing is that, while it can be done of course, an O/D trans with a leftover rear gear that was originally designed for acceleration (i.e. a 4.10) or cruising (i.e. a 3.08) will cause problems at the extremes when an only an O/D trans is added to the mix. In other words, if you have a 4.10 and add an O/D, now you'll have neck-snapping acceleration, but it will want to shift halfway through the intersection you're crossing. On the other hand, if you add an O/D trans to a 3.08, you might be lugging the motor (i.e. too low rpm) at the lower highway speeds (50-60 mph).
That is why the factory combos make sense - the milder, "in the middle" rear gears take advantage of BOTH ends of the range in a vehicle.
This is why the 700R4 (or 2004R) + 3.73 gears are SO popular. Best drivability out there. If you lean a little more toward highway cruising than chirping, then a 3.42 might be more appropriate. With your motor, burnouts won't be a problem either way.
Tire size is only a small factor unless you extremely change from stock. Bigger tires equate to reducing the numerical gear ratio of the rear end. So, if the car was produced with a 4.10 gear and 29" tire - and you swapped on 31" tires, you would be effectively be changing you rear gearing to maybe a 4.08, or 4.02, or whatever... Bottom line, it becomes more of a "highway gear" with taller tires. The opposite is true of a smaller diameter tire. Do nothing but switch to a smaller diameter tire and you effectively increase the rpm required to drive the same speed on the highway that you were at before (and keep in mind, you could only really confirm this with a gps - your speedometer will be incorrect!)
Anyway - sorry to waste space if you already have the TH400 and 3.08, but felt it worth discussing since you were kind of asking about the how the whole shebang works together.
All the stuff below is based on a 29" dia tire.
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