i have a '64 chevy van (3500 lbs and aerodynamics of a large brick). 200 4r trans, 4.11 12 bolt posi and a 292 with clifford cam 206 @50 and .513 lift. 26" tall tires. i am very pleased with the performance around town, pretty quick for what it is but freeway leaves me feeling as if it should shift one more time. roughly 2000 @ 50, 2500 @ 65 and 3000 @ 80. before we get into the whole mileage thing, i really dont care about that nearly as much as giving up anything off the line. first gear gives me a final ratio of over 11:1. it had a slight improvement over the stock 3.36 gears but cant help but think it is too low, like granny gear? am i making a lot of noise without taking advantage of the torque? rear has to come apart so now is the time to swap gears but on the fence about keeping 4.11's or installing 3.73's. dropping a couple hundred rpm's and better mileage would be a bonus but not my main concern.
4:11's seem high to me for the torque monster 292, but I'm an idiot.
I think you'll like the 3.73:1 gear. Still plenty quick around town and just tall enough on the highway.
My 270 likes 3:55s and that is what I'll use behind the 292 I'm planning for my '68 flatbed. It will have a 700R4 so the OD will take care of highway speed. The 270 has a T5 and will easily run 70-75 all day.
when i had the 3.36 rear, 55-65 would constantly shift from 3rd to 4th and back, always thought 3.55's would drop it just enough. i have upgraded intake, carbs and headers since and have lumps to install along with decking the head for another 1/2 point in compression. that in itself may/may not get me over the hump. the 5% reduction from going to 3.55's would! i have several concerns though. first, i plan on using a taller tire (eventually) and that will add back +/-2%, i have the 200 4r, not the 700r4 so first gear not as low, i did have a 700r4 long ago and do remember losing a little off the line when switching to the 200 4r, and last and least of concern, if i ever decide i want a bigger cam, i might need the little extra from 3.73's. i have given a lot of thought to 3.55's but feel i am safer with 3.73's. running the interstate, i am usually between 2400 and 2800 rpm's. screaming by todays standards but normal for those not having overdrive. i can live with the 4.11's but would like a little lower highway rpm's. i am not racing at the track, just want quick acceleration. at some point, gearing can be too low and actually hurt, not help. are 4.11's at that point?
I think 3:73s would be a good choice for you especial with the larger tires you plan on. I ran 4:11s for a while. The cam in my 270 loved 3,000 rpm. It still does but that's 90 mph with the 3:55s and OD. That would eventually be a problem of some sort.
Here is a gearing calculator that is stored and downloadable on my website. It uses Excel to run it.
https://www.12bolt.com/gearing-calculator.htmlI would aim for 25-2600 rpm at higher rpm cruise. So at 75-80.
The other question, How well can you stop at that speed?
3.73's put me right there. 2460 @ 75, 2624 @ 80. very first upgrade was converting to discs up front, converting to dual master cylinder and replacing everything front to back.
I fully agree on stopping but my pickup has stock brakes. I try to leave distance and in traffic I slow down. It is a fact that the car in front of me can stop quicker than I can. My '53 pickup stops far better than my '70 Blazer which has the same size shoes and the exact same drums. When it gets back on the road it will have a power booster which will help some.
other than adding a third bottle of friction modifier, i havent done anything yet. i will pull it apart once it is parked for the winter. meanwhile, been watching the tach on my '03 silverado and rpm's are at/just under 10% lower at comparable speeds. other than mileage, is there any disadvantage to running 2500-3000 rpm's for several hours? had a c40 many years ago that turned 5500 rpm's at 50 mph. did that twice a day for an hour and a few 3 hour trips while i owned it. i have more money in my current engine than i paid for that entire truck so it doesnt ever see 5500. is 3000 rpm for an hour straight even a concern or is that where everyone ran before we had overdrive trannys?
I have 128 possible gear ratios without changing the ring gear and I will still run about a 3.73 on the street.
When my pickup still had the 4:11 rear gears and the 270 GMC I ran 3, 000 rpm all day on trips. The cam I have loves 3.000 and even now with 3:54s and OD it will creep up there. 3,000 rpm now is 90 mph so I try not to go there. That is about twice what the truck was designed for.
changing the gears is secondary, i have to tear it apart and gears are cheap enough so now is the time (over the winter). i love the 4.11's everywhere except the interstate so still on the fence but from the day i put it in, i felt it was too low. i would probably live with it if i didnt need to look at the posi/clutches. i have two issues. started making a lot of clunking at low speed turns and left side doesnt always engage. end play on that axle is 0.125 so i am assuming the clutches are gone. the second might be totally unrelated but had slight torque steer from the time i put this rear in. about the same time it started making noise, the torque steer noticably increased. at highway speeds, the slightest increase in throttle pulls to the left, let off and to the right. it is somewhat random but more likely after driving a while. it rarely does it when everything is cold and doesnt always do it after driving a longer distance, also goes away and wont reappear rest of the trip. doesnt seem to be effected by speed nor rpm (if constant), just the slightest increase in rpm. at slower speeds, where i can actually here it, i can hear it clunk and pull left. the times the rear makes noise, the pulling starts. days with no rear end noise usually doesnt pull left. cant help but think there related issues.
I've encountered this numerous times in my general repair career. Most of the time, the clunk and the pull can be relieved by changing the rear gear lube and adding GM "anti snap" additive!!
If you're not dead set on swapping gears, or even if you are, try this first!!
thanks, your the first that also encountered the pull. in fact, several so called experts swore it was an unrelated issue. i did not change the fluid, i simply syphoned enough out to fit the posi fluid. adding one helped slightly so i added a second and that made a noticable difference. when i put the rear in a couple years ago, i used whatever gear oil i happened to have and a bottle of gm addative that sat on my shelves 20+ years. what should i use? how much addative and does the stuff with addative already added have enough? not dead set on changing gears, was more of 'while its apart' thing. 373's probably a better match but love the quick acceleration with 411's. the real issue was the pulling. i spent endless hours trying to find the cause. just recently started clunking and fairly certain the two are related. at the very least, i need to pull the cover for a visual inspection and 'if' i dissasemble farther, i will likely swap the gears.
The additive is now called GM PosiTraction Rear End Additive and is available in four ounce bottles at you local GM dealer's parts counter or from Amazon.
Drain the old axle lube, inspect the gearset, and refill the housing using the additive and topping it off with new lube.
Then take the vehicle out to a large parking lot and drive it in tight circles, a few times to the left and then a few times to the right, Keep the steering at full lock and go just fast enough that you're almost spinning the tires. A half a dozen loops in each direction and then repeat.
This also works for the Jeep QuadraTrack transfer cases!! You Jeep guys take note!!
When you're done, drive the vehicle around for a while!! This lets the lube cool a little and washes away the deposits and stuff that build up in the posi assemblies!!