I will be swapping the 4+3 in my 85 to a ZF this winter and will be changing the 3.07 gears out as well. While here are many "which gears are best" threads out there, I can't find one that quite answers my specific needs.
The car currently hits peak horsepower at 5,200-5,300 rpm and I shift at about the same.
Future plans are to swap in my LS, but in a couple of years so I am trying to find a rear end ratio that I can live with for now but will ultimately be correct for the LS. (I really don't want to pay for gears twice.)
The cam I'm planning on running in the LS has an advertised operating range of 2,800-6,800 rpm.
As luck would have it the speedometer gears currently in the ZF are correct for the 3.54 ratio, which is one of the ratios I am considering. I've read that the 4.10 is the gear for something that runs in the rpm range of my future engine but may be a little much for my current one. 3.73 looks better for my L98.
Does anyone have any suggestions as to which gear would be "optimal" or am I dreaming that I can get away with only changing gears once for two engines with different peak power RPMs?

 

You always have to ask yourself will you really do the swap when you think you will? Things happen, plans change. I'd gear for your L98 if its going to be a while. I'd also think about a more modern transmission if I was going to LS swap....

What are your goals for gear swap? As in, optimal for street driving? road race? drag racing? Because the last ones are more about which gear and rpm you need to be at in certain places. On the street, do you have the traction to utilize the lower gears?

Which engine is your LS, and what cam specs? THe rpm range is a baseline, but will still vary a lot, depending if you put that cam in an LS7 or a 5.3.

 

You are on the right track of looking at where your car is making its HP to determine the best gear. The ZF 6 speed is also geared really low numerically compared to other manual transmissions. The first gear in a ZF is a 2.68 which is like 2nd gear in some transmissions. My 01 Silverado has a 5 speed with a 4.0 first gear ratio. The ZF also has a very low O/D ratio at .5. Most transmissions have .68 as the O/D. So if you go with a 3.73 or even a 4.11 it won't have as high final drive RPMs as you would with a higher final O/D ratio. The .5 really reduces the final RPMs. Also the low 2.68 1st gear requires a steeper rear end gear to get it moving. My 92 6 Speed LT1 has 3.45 gears. My friend's 89 Auto Z51 has the 3.07 rear end. With the 700R auto he has a 3.04 first gear ratio in that trans. Mathematically the gearing works out pretty close between my 6 speed with a 2.68 ratio and 3.45 gears to his auto 3.04 ratio and 3.07 gears. So a lot of the gear is used up in how low the ZF is geared. My 92 6 speed with 3.45s cruises at 70 under 1700 RPM for example, and the RPMs won't change L98 vs LT1 (just the HP). I would guess 3.73s would be 70 at 1900 RPM or so. 4.11s would probably put you around 2100-2200 RPM doing 70. 4.11s in a classic would put you at 3500+ RPMs doing 65...

 

Cam specs (for the 6.0 LS)
Intake Duration at 050 inch Lift:
222
Exhaust Duration at 050 inch Lift:
234
Duration at 050 inch Lift:
222 int./234 exh.
Advertised Intake Duration:
271
Advertised Exhaust Duration:
286
Advertised Duration:
271 int./286 exh.
Intake Valve Lift with Factory Rocker Arm Ratio:
0.600 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio:
0.575 in.
Valve Lift with Factory Rocker Arm Ratio:
0.600 int./0.575 exh.
Lobe Separation (degrees):
115
The engine will be turbocharged, as well.
The intended use will be somewhat all-around. It mainly gets used on the street but is also intended to do some drag racing and (hopefully) autocross.

 

I think 4.11s would be just fine with a ZF. It won't hurt your street driving that much if you intended it for that too. 4.11s in a ZF are like 3.73s with anything else. My 01 Silverado 5 speed came with 3.73s. I dropped it down to a 3.42 due to the fact that my 1st speed is a 4.0 1st gear ratio. There was no point in having that steep of a first gear and my 5th speed o/d is only .68. It is all math to determine where you want your RPMs to be and you need to look at each gear ratio in the trans as well. There are sites with gear calculators where you put in your ratios, tire size and will tell you what RPM you'll be at what speed.

 

I always forget the OD ratio on the ZF is a .5. You would be doing about 74mph at 2000rpms with 4.11s. Now that being said, to maintain 45 you could run 5th gear at about 1700 rpms which would probably be a little low for that combo, or 2400 in 4th to also do 45 mph. With the 3.54 it would be about 2000 rpm in 4th for 45 mph.

With a turbo combo like that having a little less rear gear wouldn't hurt in my opinion. Having a 3.54 would, for me, be about perfect for what id drive and i wouldnt be shifting as much on the auto cross. Also at the drag strip... I'd expect you'd be trapping in the 130s. With 4.11s you'd probably have to grab 5th gear or be on the limiter well before the 1/4. The 3.73s would probably be ideal for the strip. You would probably kiss the limiter as you broke 1/4. Again there is no right answer. I'd look long and hard at a 3.73 ratio if it was solely drag strip and around town. The overdrive of the ZF gives you a lot of flexibility. Now on what you have now... the 3.73 is probably a lot more than you'd need.

 

That's where I'm having a difficult time making up my mind. Both engines have a pretty substantial difference in (proposed) shift RPM. I may just go with the 3.73 as i think its the best compromise between the two plus if the new engine makes similar hp and torque in the lower RPMs as this one does (just with a lot more higher in the RPMs) I think the 4.10 will make traction "difficult" in lower gears. If I already had the engine built and on the dyno it would make gear selection an easier choice.
The plan for the current engine is to keep it for two more seasons before the swap unless I hurt it first. I may do so earlier if I can manage to collect all of the required additional parts though. That LQ4 is sitting on the stand taunting me...

 

The low numeric value of first gear in the ZF makes me think the 4.11 is a better gear as well. In my 01 Suburban 8.1L with the 4L80 it has 3.73 gears. The first gear ratio in the 4L80 is 2.48. That suburban is a dog off the line. I don't know why GM kept that low of a ratio gear in so many trucks when the Allison trans has a 3.09. So to compare the ZF with 3.73s and a 2.68 is ok, but not really that much a step up over stock if you're looking for performance. I would say the 3.73 is a good ZF ratio if you want to do more daily driving and have a little benefit for occasional light track use. If you're going to mostly track it and light street use the 4.11s could be better. But again, do the math to determine what the best ratio is for you. There are standards along the lines of you don't want your RPMs spinning more than a 9:1 ratio in first etc. I don't recall what those exact standards are as they vary by type of suspension set up, car, weight, towing vs. racing, etc. You can/should research those as well to get your set up optimal for what you want.

 

Here's a handy gear calculator that you can play with: https://www.andysautosport.com/learn...ors/#GearRatio

For best results, clear the static Tire Size fields and Tire Diameter (in inches) field and instead fill the dynamic Revolutions per Mile field appropriately for the intended tire size (typically about 810 revs/mile for a 275/40R17 or 315/35R17 tire). Rev/Mile specs for most tires can be found within the tire specifications on sites such as TireRack.

This one is also useful, especially for direct comparisons, but you have to input all the details yourself: http://www.gearcal.com/

For reference, I have a 4+3 with 3.73 gear and shift around 6500 rpm, but I run a 769 rev/mile tire. If I were using a standard 810 rev/mile tire, I'd likely prefer a bit less gear for my combination. Coincidentally, my tire w-3.73 behaves exactly the same as a standard tire w-3.54 within the precision of the calculators.

Here's an old log showing pretty good correlation with the calculators: https://datazap.me/u/tequilaboy/dyno...48-444-414-435

Note: The vehicle speed was calibrated for the tire size and gear ratio with a Dakota Digital SGI-5, so it may be a bit laggy. The tach signal is also pretty noisy as well. Might even be a little clutch slip, but overall, pretty close to what was predicted.

3.54 should work well for both of your future configurations as will the 3.73 with a slight bias towards the high rpm case.

Interesting to note is that there is only a 300 rpm (maximum) difference between the two ratios for a given speed and gear up to about 6400/6700 rpm. 3.73 gives you the option to run a taller tire if desired. 4.10 is just plain silly without a 28" tire. Higher numerical gears are also harder on all of the downstream components, spider gears, side gears, stub axles, half-shafts, u-joints, spindles, and even the tires.