I find it curious that the compound first gear/SLR is applied somewhat universally. Is it not reasonable to assume that vehicle weight and torque characteristics of the engine will have an impact on the "desired" compound gear/SLR? For instance, a 390 in a lighter first generation Cougar/Mustang will benefit far more from a 9.73 compound gear than a fully dressed Galaxie--correct?
I am thinking a bit more optimistically now, though, particularly upon reading Heo's comment. The compound gear/SLR info is helpful, but nothing replaces real world experiences from those who have a similar set up in their Galaxie. I suppose if I want to be safe, I should probably dig out my cam card and try to figure out torque/RPM levels, as it seems reasonable that it will have an impact on off-line performance with respect to the compound gearing, weight, etc.
Am I wrong here? Or am I simply over-thinking the issue?
You are assuming we use math without experience, however with your screen name I would expect nothing less
In general, the heavier car with the flatter torque curve will like the wider ratios and deeper SLR.
Sounds sort of snarky, but the truth is, the 2.32 gear worked well in NOTHING on the street. So although you are correct that application matters, other than a peaky small block in a very light car, the close ratio that forces the tall 1st gear is rarely needed and forces some odd gear choices with any big block.
The same was true in the Rock Crusher Muncies in Chebbies. Maybe in radical small block vehicles where the power was in a very narrow range, but that was it and it generally takes a 4.30 gear or deeper to make those run, ironically you end up in the same SLR area when you gear for an engine like that. (4.30 * 2.32 = 9.97). What happens is, you either gear for the top end or you gear for 1st, they just don't match well for a daily driver (and I would add rarely for all but the wildest drag cars, but doesn't really apply here)
It also can bite you if you go too far. I have a buddy who decided to go with a Super T10 in a drag car, 3.34 1st and 5.13 gears, for the life of me I cannot get him to re-gear the rear or even better, the tranny. His 17:1 1st gear is killing him in the 60 ft, so don't think we just go for max SLR, it is generally between 10:1 and 12:1 for a street car and will go higher for certain combinations of drag car as stated above. In his case, there is no doubt that if I could get him to a more standard 1st and second gear ratio that he would run quicker (or a taller rear but, in his case the wide ratio is SOOOO wide that it's hurting a pretty radical 440 inch Rat)
So I do not run a Galaxie, but let me tell you some specifics about a relatively heavy Mustang.
1 - My Mustang, fully dressed with A/C, carbed 433 cid at the time, 2.32 RUG-AE2 and a 3.70 gear. Street driven in Vegas, fun enough to drive, but 1st and 2nd sort of soft not impressive, on highway, very buzzy, long ride from Vegas to Bishop CA required silence because we couldn't hear each other talk or the radio (SLR = 8.58, final 3.70, avg mpg in the 14s)
2 - Same Mustang, same engine and tranny setup, 3.00 gear for a run from Vegas to LA for Knott's show. Great-ish on highway, real bad in Huntington Beach stop and go. Drivable, but very blah. (SLR = 6.96, final 3.70, avg mileage in the 14s)
3 - Same Mustang, 489 FE, carbed, probably 150 more HP built to match gearing combo, same fully dressed, 3.70 gear, TKO-600 with 2.87 1st and .64. MUCH better traffic manners, in town mpg went up a little, highway went WAY up, but was driving too fast with tall rear gear ratio. (SLR = 10.61, final = 2.36, mpg 14s)
4 - Same Mustang, EFI, even more peak power but milder, added 4.11s, in town....even happier....granted 5th gear is an equalizer on the highway to offset the higher SLR, but no downside in 1 through 4, the car is much happier it stop and go traffic and accelerates like a monster when you drop the hammer. 1st gear is by no means too deep and is very nice due to torque multiplication, (SLR = 11.79, final = 2.63, mpg high 14s)
In all cases above, 275/60-15 rear tires, so pretty tall compared to what most Gals will run.
The end result is, within reason, more SLR is better until 1st gets too deep or a more peaky torque curve cannot recover from too wide of a ratio. I would focus on no less than around 10:1 SLR and an acceptable cruise RPM in high gear. Sometimes that forces you to a 5 speed, like it did me.
I will say though that I truly agree that tire size and torque curve do matter, however, in regards to tire size, you'll only go up, and odds are the Gal doesn't have room for a tire that will cause the estimate to change much.
In my 4200 lb 445 truck, the tire size turns a mathematical 11.78 SLR into a equivalent 9.58 when compared to the Mustang size tires. Right now I am torn between changing my gears to 4.11 or adding a 5 speed and gears. The reason being is a truck 4 speed is TOO wide of a wide ratio because it's really doing it in only 3 gears. Otherwise just a gear change would be fine.
As far as torque curve, the more radical the motor, it is true the more likely you'll need a closer ratio. However, I'll argue that the wide ratio is "narrow enough" for any FE I have ever seen on the street even ridiculously radical ones, so unless someone builds a significantly destroked TP with a caveman radical cam, I just don't see a benefit of a close ratio tranny and the resulting tall 1st gear
