3.08 to 3.73

[mk3barti]

My 76 has an open 3.08 12 bolt that I'm contemplating changing to 3.73 posi. 350/350 in the truck with fairly aggressive cam, intake, carb, headers, etc. Truck is mostly around town use with a bit of freeway. Looking for anyone with experience doing something like this as I know it will rev like heck on the highway, but should be more suited for the motor. 275 60 15 tires as well. Thanks

[MikeB]

Yeah, the RPM with 3.73 gears and 28-inch tires would be approx 2750 @ 60 mph and 3200 @ 70 mph (plus any T/C slip). That would be a bit much for me on the highway for anything more than a few miles. Compared to your 3.08, you'd be waiting for the trans to shift into a higher gear!

Unless you plan on going to an OD trans sometime in the future, I think 3.42 would be a better choice. Having personally gone from 3.42 to 3.08 in a 55 Chevy sedan, I can tell you there is quite a noticeable difference in highway revs and acceleration.

I'm thinking about changing my 82 C10's current 2.73 ratio (8.5 inch 10-bolt) to either 3.23 or 3.42 . And I'm afraid 3.42 might be a bit too much on the highway, even for my 29-inch tires. But, hey, 3.42 would work well if I ever go to a 700R4.

[RodsGreen10]

My 76 came with the exact same rear end. I put a 3.73 in mine with a True Trac limited slip. And I love it. I have a 385 stroker in mine and it is north of 585hp. And it is perfect for me I am always in the power band with my cam. Burn outs from hell. I like to let her eat. Anything I have or have had I always put a 3.73 in it. I don't daily drive my truck either.

[Alex V.]

With the modifications you've made to the engine, do you know where the torque and horsepower peaks are at now? Also, have you put a higher stall torque converter in? The more mods you've made the more you need to tailor the ratio to the rest of the drivetrain. Off the top of my head, if you want good highway performance I'd want the RPM @ 65 MPH to be right on or just below the torque peak as long as that's below about 3,200 RPM - unless you have a very high stall TC. 3.73's are going to have you wound pretty tight with 28" tires, but if you want it to be snappy around town and can live with it winding out around 65 MPH and being comfortable at 55 on the freeway then it may be your ticket. For a stock drivetrain I'd say 3.42's is a much better bet, but the existing mods and what areas of performance you put a priority on may dictate differently.

[mk3barti]

Cam is the edlebrock 7102 which from what I've read does good mid to high range. 3000 bm hole shot convertor. Performer intake with 1407 carb. Mild port work on heads and long rune headers. 2.5 exhaust. 3.73 is probably a better fit for this set up from the sounds of it

[slotard]

I have 350/350, 4.10, 30.5" tires which works out to very similar effective gearing (9% shorter gearing, 9% taller tires). It's pretty miserable on the freeway IMO, but it's just a basic work truck engine (minor headwork, Comp Cams 252H, Edelbrock Performer intake, non-horrible manifolds, dual 2.25 exhaust) that doesn't like to spin as fast as yours (cam is rated for 800-4800 vs yours 1500-6500). Even with the higher powerband, I'm not sure how well you'll like it without overdrive for sustained higher freeway speeds, especially with the looser converter. Around town you'll love it, though.

[mk3barti]

Thanks. Looser converter meaning slip? I assume that this is happening now because my engine revs higher than my rear end and tire combo should by calculation. I do not fully understand the slip thing with a higher stall converter.

[wilkin250r]

I imagine you know how a standard clutch works, correct? Your torque converter basically uses fluid pressure like clutch pressure. The faster your torque converter spins (higher engine RPMS), the more fluid pressure it creates.

Just like closing your clutch pressure plate, it will start to "grab" and begin transmitting power from the engine to the transmission. Same thing with a torque converter, the faster you spin it, the more "grab" you get.

However, it never really "grabs" 100% (unless you have a lock-up converter). There is always some amount of slip, and that amount of slip is also related to the "stall speed" of the converter. A torque converter with a stall speed of 3000 rpm will obviously have more "slip" than a converter with a lower stall speed of 1500 rpm.

If you want, I can easily explain the "stall speed" of a torque converter.

[DANTIP]

Your 3.08 uses a 3-series carrier and you can get 3 or 4-series 3.73 gears. If you want to be stop-light quick, a 3.73 gear will be better. If you like highway driving stay with the 3.08.

[cadillac_al]

With my 3.08's and 30" tires 70 MPH is pushing it pretty good on the interstate. People are still blowing by me. I can imagine with 3.73's you would have to get used to cruising 55mph.

[mk3barti]

Thanks for the great information guys, that is helpful. Right now I run 2700 ish rpm at 60 mph with 3.08's and th350 which would translate to slip based on what I am reading here.

[slotard]

If we assume slip % stays the same going from 3.08s to 3.73s (which isn't quite accurate, with the new gears you're doing less torque at higher rpm so I'd expect less slip, but not horribly far off), your new rpm at 60 will be (3.73/3.08) x 2700 = 3270rpm.

With 1:1, 3.08s, 28s, no slip, 60mph would be around 2200rpm. 2700/2200 = 1.23, so you're getting around 23% slip. That's quite a bit, but with a 3000rpm converter I guess that's to be expected.

If you want better performance at low speed AND better highway mileage, you'll need a transmission with more spread (meaning gearing in top gear divided by gearing in bottom gear). TH350 has a spread of 2.52. TH400 has a spread of 2.48, not much difference. 200-4R has a spread of 4.06. 700R4 and 4L60E has a spread of 4.40. 4L80E has a spread of 3.31. And then there are the 6 speeds, and now 8s, but those are significantly more complex.

The downside of a wider spread (from a behind the wheel perspective, neglecting strength) is that, given an equal number of gears, RPM will drop more on a shift. A looser torque converter will help with that, but if it doesn't have lockup it'll increase cruising RPM and hurt mileage. With more gears, you can get a wider overall spread with the same gear spacing, but that adds cost/weight/complexity.

[mk3barti]

Again thank you, that is great information.

[MikeB]

Originally Posted by mk3barti

Thanks for the great information guys, that is helpful. Right now I run 2700 ish rpm at 60 mph with 3.08's and th350 which would translate to slip based on what I am reading here.

Yeah, a lot of that is T/C slip. At 60 mph with a stock T/C, you'd see around 2200 coasting, maybe 2400 under load. I hope you are using a big auxiliary trans fluid cooler!

Stock to 2000 stall would be a better fit for a 3.08 axle and non-lock up trans.

3000 may be a bit much even for 3.73 gears, but I have no experience with that combo.

[Firebirdjones]

3.73's work fine for me in 3 different vehicles, my 79 1 ton 454 included. I've criss crossed the country on several occasions with that gear, no overdrive, and usually a 28" tire. It'll cruise at 65 mph ~2800ish rpm without complaint. Doesn't bother me a bit. My 1 ton truck knocks down 14 mpg highway like that. I've done pretty long 4,000 mile round trips this way.

I have 2 cars here with 3.08 and 3.07 gears, 28" tires. It's easy to cruise 75 mph in those at about 2800-ish rpm. They work okay but not my first choice for any performance work.

Guess I'm not that queezy about gears on the highway, used to daily drive 4.56's with a Muncie. Drive that around daily for 5 years and then get in a car with 3.73's and you feel like you just jumped in grandmas car

[slotard]

14mpg with 4.54, 3.73s, 28s, no overdrive? I certainly wouldn't have expected that kinda mileage with that setup. I wish I got that with 350/350/4.10/30.5", which is about 1% different effective gearing (bigger tires just about cancel out the shorter gears).

[Firebirdjones]

On that particular combo I run the original 9.5x16.5's which are right around 30" tall, but yes it knocks down 14 mpg if I keep the cruise speed around 65 mph. 3.73's with a 400 turbo. I've worked on the combo though to provide good drivability and optimizing everything. Dialing in the carb with a wideband, tweaked the distributor curve along with adjustable vacuum setup, smallish headers and 3" exhaust, and a lot of other little things that add up. It looks like a stocker but a lot of tweaking.
It works well for what it is considering the truck weighs 5300 lbs and is shaped like a brick. My wife daily drives it back and forth to work around town and knocks down just over 12 mpg on a weekly basis in stop and go traffic.

[barry1982]

Just my opinion, but sounds like the 2700 RPM is already fairly high. Lower gears would almost make the truck un drive-able on today's highways. I made that mistake a few years ago. Put a 4:11 posi in my 72. Lots of fun around town, but 3650 to hold 60 MPH on the highway. Gas consumption was ridiculous.... Would be okay for a "weekend" fun truck, but at the time it was my only transportation.

[SkinnyG]

I went from 3.07:1 gears and open to 3.73:1 and Detroit TrueTrack.

Possibly slightly better fuel economy in doing so, but you need to understand:

350/TH350, 231°@0.050" cam, 108LSA. 10.9:1 compression, 2500 stall converter. These gears put the cam where it's actually ~working~ efficiently.

The old 3.07 gears were a dog with this cam. The tack ~now~ reads over 3000 on my daily commute. It's fine.

Because the truck is so much better with the 3.73's, I will ~not~ be rebuilding the 4L60 I have on a shelf - no point giving me an overdrive and dropping the cam out of its power range.

[mk3barti]

I think I'm going to give 3.73 a go. Truck is a daily or semi daily in the summer which is mostly city driving. I think getting the cam in the proper power range will outweigh some revs on the freeway, like what SkinnyG is saying.

[Firebirdjones]

Sounds like some of the rpms reported are really way off compared to stuff I have here.

I have a chevelle with 4.10's, a 28" tire, and I run a 10" 3600 stall converter in it. At 2800 I'm going exactly 55 mph. I can push 3,000 and cruise at 60 mph. I'm no where near 3650 rpms at 60 with that combo. Been running 4.10's in that car since 1985.

[Firebirdjones]

Originally Posted by mk3barti

I think I'm going to give 3.73 a go. Truck is a daily or semi daily in the summer which is mostly city driving. I think getting the cam in the proper power range will outweigh some revs on the freeway, like what SkinnyG is saying.

Probably a good choice considering it's a heavy truck. If you want any performance out of a heavy vehicle, gear multiplication is a good thing. I find 3.73's are a nice all around gear that does everything pretty well, dragstrip use isn't bad (I like more gear for that), highway is fine for me, around town is about perfect.

Another consideration is pump gas. If you are building an engine on the edge with pump gas, rear gear and weight of the vehicle play a factor in how tolerable it will be. If you lug the engine with a tall gear and work it harder around town you might find the engine a bit more sensitive to timing, especially on hot days. I've experimented with this as well. However lots of variables with this scenario.

If you find yourself driving the highway more and not liking the 3.73's as much, then an overdrive or gear vendor would be another nice addition later down the road.

[mk3barti]

Good to know, thanks. I had a Charger when I was much younger with a 440 / 727 and 3.91 and it would howl pretty good on the highway. I never thought to add a gear vendors if I don't like it.

[MikeB]

Originally Posted by Firebirdjones

Sounds like some of the rpms reported are really way off compared to stuff I have here.

I have a chevelle with 4.10's, a 28" tire, and I run a 10" 3600 stall converter in it. At 2800 I'm going exactly 55 mph. I can push 3,000 and cruise at 60 mph. I'm no where near 3650 rpms at 60 with that combo. Been running 4.10's in that car since 1985.

Yeah, those numbers are pretty close to what the calculators show for your combo. I'm surprised you don't see more RPM due to slip of the 3600 T/C. ??? I would have guessed another 300 RPM at least. I have found that cruising along on a level highway, my stock T/C slips 100 RPM or so. More under load; none when coasting.

[tucsonjwt]

The Gear Vendors website says that a gv unit will only improve fuel economy on a non-lockup torque converter vehicle. Modern lockup transmissions will not experience better fuel economy, but will have additional gears for improved towing, etc. My 83 C20 454 granny 4 spd with 4.10 rear axle has a gear vendor and it went from 9/11 mpg to 11/13 mpg after I had the gear vendor hooked up, which is the 20% fuel economy improvement that GV advertises. I would not get a GV just for fuel economy because they are very expensive to install (like $3-4K even for a rebuilt unit.)