recommended 5 speed gearing

You guys really make it sound like I need to max out to 3.54 or look for deeper on my wagon.The op has 343/3spd/3.15s on 25" tires and wants to eviscerate the auto for a lightweight broad range manual. I'd say if he had 3.31s like I do in my Javelin, he'd be in that perfect sweet spot. I'll be going through my 304 later so I won't be able to chime in about power but it's a bit spooky. I've seen some gearing variations but they all seem more to do with engine torque curve and vehicle weight than anything else.

390/4spd/3.73s = Put a muzzle on that DOG!
304/3spd/3.31s = Lots of fun
258/3spd/2.72s = zips in short bursts but struggles to maintain on slight inclines
242/4spd/3.54s = Not too bad

With a mild MPFI 285/5spd/2.72s on stock 27.7" tires I have no idea what to expect for 4th and 5th gear. I assume 4th will hold for slight inclines unlike the warm 258 before it but up steep hills I'm going back to 3rd. 5th might hold on slight 1-4% grades on the freeway but they're rare and I live in a part of WA that is really flat. If I could do a 3.31 swap I would. I'm stuck between a 3.08/3.54 choice because I don't want to give up my low pinion Dana 30. The high pinion model doesn't offer any new range of gears though.

You're fortunate to have that 3.31 ratio, it's so nice.These cars were optioned real nice. Right now I'm trying to address some of the really dumb cruising behaviors I had where there was a slow loss in speed up slight inclines at 70MPH even when punching the throttle all the way to the floor. My stock 2.72 ratio never did well with anything more than low strain and would viciously lunch parts of the 3spd automatic in protest.

Despite running +27 cubes with 2x torque and 1.5x hp up front, I'm worried about the same thing happening with my manual swap. One thing I keep dancing around but never figured out how to ask, how do I calculate the target cruise RPM for my engine HP/TQ+transmission+diff ratios? I know there has to be an exact science to this but what is it?

That's a four cylinder though Buzz, it SHOULD run at higher rpm than a mid size V-8, though nowadays a 343 would be considered large. Anyway, 2000-2500 rpm is a good cruise speed for a V-8 or even in-line six. With most cams (including stock and good street performance) you want to maintain at least 2000 rpm cruising. Newer engines with electronic controls and good torque cams, and carefully geared trannys and axles, will cruise as low as 1500-1600 rpm and still be efficient, but that's with a lot of engineering. Keeping 2000-2500 rpm at low/high cruise speed is a good rule of thumb for the older V-8s and bigger long stroke I-6s (232 and larger).

Buzzman72 wrote: . So 2000 rpm is about 52 mph. And 3000 rpm is right at 78 mph. And the car cruises well between 2000 and 3000 rpm, as if that's the "sweet spot" in the torque range.

farna wrote: That's a four cylinder though Buzz, it SHOULD run at higher rpm than a mid size V-8, though nowadays a 343 would be considered large. Anyway, 2000-2500 rpm is a good cruise speed for a V-8 or even in-line six. With most cams (including stock and good street performance) you want to maintain at least 2000 rpm cruising. Newer engines with electronic controls and good torque cams, and carefully geared trannys and axles, will cruise as low as 1500-1600 rpm and still be efficient, but that's with a lot of engineering. Keeping 2000-2500 rpm at low/high cruise speed is a good rule of thumb for the older V-8s and bigger long stroke I-6s (232 and larger).