3.73 to 2.73?

[7T2]

I have been wanting a better highway gear in my truck for a while, but wasn't willing to pay the price. I currently have 3.73 gears (400SB and TH400), and I have come across a dirt cheap 12 bolt rear with 2.73 gears that should just about bolt in. Too much of a difference? All I do is drive to shows and cruise, don't need any kind of performance whatsoever. Am I going to run into any problems making this big of a change? Anybody else made this big of a numerical change?

[MrDude_1]

Other then slower acceleration and having to re-adjust the speedo... Shouldn't be a problem.. 400 SBC makes plenty of torque.
Only issue may be if you were towing something heavy, but you probably dont do that with a show/cruise truck.

[72freak]

It will e a great highwau gear but towing a heavy load is out.

[7T2]

I've been doing a little reading on here in the past hour. 2.73 may be a little low (numerically). I guess I need to take my torque converter into consideration and see where my RPMs will be at highway speed? This is for my 57, and almost 100% of my driving is at highway speeds getting to an event, then maybe a little cruising around the fairgrounds (usually at idle) depending on where I'm at. Don't really plan on doing much towing or racing.

[MrDude_1]

I dont know if you consider it an option, but putting in a modern 4 or 6 speed overdrive transmission with a lockup converter would probably work best for you.

[Clyde65]

I would say get it if it is cheap, then swap the gears to a 3.08/1 or 3.42/1 ratio.

3.73/1 with a 4speed auto or 5speed manual would be ideal.

[benoit454]

2.73's suck don't care if the motor had good torque or not they suck it's like driving a POS instead of a real truck.

Had the same motor backed by 4.10's and now it's in a truck with 2.73's gas mileage is horrible, mostly around town driving, no load, nothing in the bed, 255 60 tires. th350.

No balls at all, barely breaks a tire loose and no passing power nothing. It's like we stuck a worn out 4 cylinder in it.

The same motor would smoke 31's with the 4.10's. Tons of torque and take off power and would pull anything.

My vote 3.42's MAYBE 3.08 that's it.

[7T2]

Originally Posted by Clyde65

I would say get it if it is cheap, then swap the gears to a 3.08/1 or 3.42/1 ratio.

Good idea. If I have 3.73s now in a 10 bolt, is that as low as I can go in that housing, or will 3.42s fit? Does going from 3.73 to 3.42 make much of a real world difference?

I think 3.08 is probably where I would be happiest, but this whole thing is $50 drum to drum. I guess I can try it out and see what happens.

[7T2]

Originally Posted by benoit454

Had the same motor backed by 4.10's and now it's in a truck with 2.73's gas mileage is horrible, mostly around town driving, no load, nothing in the bed, 255 60 tires. th350.

No balls at all, barely breaks a tire loose and no passing power nothing. It's like we stuck a worn out 4 cylinder in it.

I don't doubt this, but I will be doing very little driving around town. Mainly highway speeds, so I was wondering how it would work out.

[Clyde65]

If you only want to drive it and no "showing off" or stuff like that, 3.08 is probably gonna help you more.

I have 308's in mine and although there is a little bogging in the 40-550MPH range, I rarely stay there, I run around 70+ and I notice very little if any bog. and my mileage is up over 20MPG with the fuel injected 4.8/4l60e.

[clinebarger]

Originally Posted by 7T2

Good idea. If I have 3.73s now in a 10 bolt, is that as low as I can go in that housing, or will 3.42s fit?

8.5" 10-bolt Carrier split: 2.56 and numerically lower, 2.73 and numerically higher, So yeah you can.

You will need to recalibrate the governor in the transmission when making drastic changes in gear ratios. http://www.summitracing.com/parts/BMM-20248/

[Clyde65]

Sup Clienbarger?

[Winola73]

If you're engine was built to work with 3.73's you won't be happy with 2.73's and neither will your engine. That said, I have 2.73's behind a oil burning 305 TBI and 700R4 in an '88 RCLB 2WD. It's no race truck and I wouldn't try to pull any heavy loads but for the light hauling I do with 95% highway driving I love 'em, especially on the interstate. It's in getting a trans overhaul now and I'm switching to a 1200 stall converter in preparation for the 355 grunt motor I'm building to compliment the 2.73 gears. My cruise RPM at 55 in OD and the converter locked is about 1350 with 235/75-15's. MPG's with the tired 305 averages 18.3 in slightly hilly terrain. I'm building the 355 to maximize TQ from idle to a redline of 4,000. I'm sure the 305 won't be happy with a 1200 stall but it won't be in there much longer.

[benoit454]

Ours is stock TC and it still sucks with them gears. IMO why do it why have something built and not stock with crappy gears? It's just a real waste because it just loads the engine and lugs and won't do much of anything.

If I had OD no way would I do gears lower the 3.42's

1200 aint much higher then stock anyways, some are 800-1000 or so.

I mean maybe if you build a high torque low end motor, MAYBE, and something that has like a 4-5k redline and that's it. That might work I guess. I just don't see me spending time or money on that I'd just swap gears. Around here we got alot of hills and around town stop and go.

[7T2]

I don't have overdrive. How do you tell what the stall speed is on a torque converter? I've never messed with them much, and the one on there came with the truck.

[benoit454]

I was talking about the post above me had OD.

usually you know the stal if you buy it, and if not then you run the numbers on it.

[7T2]

When I cruise 65 mph my tach is saying the RPMs are between 4000-4500, which seems high. Is there any way to verify the accuracy of a tach, short of hooking up another tach next to it?

[Clyde65]

Holy crap Batman! 4500?!
There is your mileage going out the rearend dude, it shoul be around 1700-2000, I would say a 4 speed ( or more) tranny would be a real good decision for you. FYI, the TH400 isnt known for efficiency either, very stout tranny but does rob a little power and a lot of MPG's.

[7T2]

Originally Posted by Clyde65

Holy crap Batman! 4500?!

Yeah. I'm not 100% sure that's accurate, but the tach install seemed pretty straightforward (tach wire from distributor, and yes, the tach is set to 8cyl) and the tach seems to follow what you hear the engine doing in a linear fashion. I suppose it could be reading erroneously throughout it's range of motion for some reason. I haven't been super impressed with the set of Autoloc gauges I have in it.

Then again, maybe that's normal for a TH400 and 3.73 gears? I don't know. Might be in my best interest to just get another cheap tach and compare them against each other.

The RPM's at highway speed are definitely high, but I'm not sure they're 4500 high. I'm getting around 10mpg overall, so I've been looking at all options for fuel economy. Just don't want to spend a ton of money (i.e. different trans with overdrive, replacing gears, different motor, etc.)

[Clyde65]

4500 RPM would be absolutely screaming at highway speeds. if you put a 700r4 in it, the 3.73's could stay, would be better with 3.42's but still ok. 700's need to be adjusted right so it would be best for a shop to put it in. probably around $2K for that new and built, with an appropriate stall too.

[benoit454]

that sure is alot of rpms. No need to use brakes to slow down lol.

My old monte carlo with 3.42's and a th350 with 215/65's on it ran 2500-3k max at 65 plus. usually closer to 2k for 65.

[7T2]

Originally Posted by Clyde65

4500 RPM would be absolutely screaming at highway speeds. if you put a 700r4 in it, the 3.73's could stay, would be better with 3.42's but still ok. 700's need to be adjusted right so it would be best for a shop to put it in. probably around $2K for that new and built, with an appropriate stall too.

Plus a different driveshaft, or adjusting the length on the current one...

I guess the point I'm trying to make is everything in it now works, so I don't want to get into thousands of dollars in mods to save gas. I would never see a return on my investment.

This rear end is $50. If it was a 3.08 rear it would be exactly what I am looking for. I don't trust myself to actually swap out gears, and taking that to someone I am still looking at probably $500 for the gears and someone to do the swap.

An overdrive trans would be a nice way to go, but I don't know of anything that would bolt right into place without modifying the driveshaft or buying a new one, so even with a good deal I'm still looking at $500+. I think I read somewhere that a 200R4 bolts right in place of a TH400, but I'm not sure what all is involved with that.

Based on my skills, a cheap rear end with the gears I want in it seems like the way to go, but this one may be a little extreme.

*edit* Looks like the 200r4 bolts in place of a TH350, and the 700r4 is a slightly different length that the TH400.

[Clyde65]

7t2,

most of the time, cheap isnt the "best" way to go, I am not telling you what to do, I have just been there too many times. It is better to sink a bit more money in the repair than to attempt a cheap fix. Dont think I am busting your YKW's, Im just speaking from too many learning experiences on my part.

If the truck isnt fun to drive, you wont. it will sit and rot.
As Clienbarger here on the board which would be best for your application, I think the trans is a much better choice. I have seen them around DFW for 8-900, plus install, so it isnt cheap but if you get 2x the mileage and 10x more enjoyment out of the truck, it would be worth it.

[7T2]

It's not really that I'm cheap, it's more about what my return on investment would be. As I said, everything works now, I just average 10 mpg or so. I would say if there is something I can do for less than $500 and a little elbow grease, and it's a job I'm confident handling myself, it would be worthwhile. Anything more than that and it kind of becomes an "if it ain't broke, don't fix it" type of thing.

For example: Off the top of my head the events I plan to attend this year that are some distance away will total 1000 miles of driving. At $4 a gallon and 10 mpg I'm looking at $400 in gas expense. Let's say by some miracle I get that number up to 20 mpg, I'm looking at $200 in gas, a savings of $200. If each year is similar, it will take me 10 years to recover a $2000 investment in a different transmission, that I would ONLY be replacing to get better gas mileage. Even then it's unlikely I'll hit that 20mpg mark.

Now, let's say this $50 rear end works out, and bumps mileage up to 15 mpg. Then I'm looking at $266 in gas, a savings of $144. In just this year it will already start saving me money. IF it works...

[Bad70sbchevy]

I got 3.07s in mine (I got a TH400) and I'm running 2700 rpm at 65 with 275/60R15 rear tires and a 2800 stall in mine for the same reason. Great highway gear, decent acceleration, picked up 3mpg over 4.11, but the 4.11s get way more smiles per gallon.