3.73 to 2.73?

[Bad70sbchevy]

BTW 1700-2000rpm sounds way to low for 3.73s, at least above 3200 at 65mph depending on tire size and torque converter.

[Clyde65]

Originally Posted by 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...

I see your point and as a computer geek, I would put pen to paper and think that is a good investment as you state, however, how much enjoyment are you gonna get out of it when its getting 10 since nothing is broke, IDK about you but if I have something getting 10 and something getting 20 I will more than likely take the 20 and do my running and commute with it. however, if I can have something that gets 15 and something that gets 20 and I can keep the miles low on both, then the $2K I spent would be worth it. Ultimately you have to do what you see fit for your situation, know that either way you decide, we think it is a great idea and we will help anyway we can.

[Winola73]

Originally Posted by 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.

1200 is "lower" than stock, stall speed for my truck is 1600.

You obviously don't comprehend what I'm building and I'm not going to try to explain it. I've done a similar build before and I'm building it not having it built. It works, there's no "maybe" to it and it suits my purposes just fine.

[Longhorn Man]

a couple things stand out to me here...
your tach is wrong, WAY wrong. You are probably in the 2500 to 2800 RPM range.
NOTHING with give you a 100% MPG increase short of a late model eng/trans swap. Your ambulance is tall enough that aerodynamics is your main problem here.
I suspect, if you put a 2.73 rear in there, your MPH will be the same, possibly even worse. You'll have the RPMs too low unless you wind it up to 70 or 75, and by that time, you are trying to punch a huge hole through too much air to be able to do it with good MPG. You want 2200 to 2600 RPM with an old school engine, and with the height of your rig, I would shoot for the upper end of that.
With all this being said, if you install it yourself and need no other mods, or minimal mods, you can stab it in there and swap it back out if you feel the need. Another point, is that it is cheap enough that you may be able to get all your money back if you decide to unload it later.

[7T2]

Originally Posted by Longhorn Man

your tach is wrong, WAY wrong. You are probably in the 2500 to 2800 RPM range.

Yeah, probably. I need to get that figured out. The only reason I know it off the top of my head is because my speedo doesn't work, so I measured speed with my GPS and made a note of where the RPM's were at 65, since they seemed to be repeatable, and they were showing between 4000-4500 on the tach. Like I said, that seems high, but if I get the tach there in final gear it always works out to the same speed range.

Some other good points you made there about the wind resistance and size of the vehicle. I may just have to live with 10 MPG.

[Average Joe]

Heck for 50 bucks give it a shot! Worst case your out 50 bucks but still have a spare diff. You may want to confirm that the other rearend accepts the same size U-joint before you commit or you may be modifying your driveline.$$ Keep us posted if you decide to go for it I'd love to hear the outcome.

[MrDude_1]

Originally Posted by 7T2

Yeah, probably. I need to get that figured out. The only reason I know it off the top of my head is because my speedo doesn't work, so I measured speed with my GPS and made a note of where the RPM's were at 65, since they seemed to be repeatable, and they were showing between 4000-4500 on the tach. Like I said, that seems high, but if I get the tach there in final gear it always works out to the same speed range.

Some other good points you made there about the wind resistance and size of the vehicle. I may just have to live with 10 MPG.

if you have a GPS you know your true speed.
measure how tall your tires are.
you know your rear gear ratio and your transmission ratio...

so you can work backward to calculate your RPM.... and see how far your tach is off.

[Jay W]

Check out this calculator. It will be very helpfull showing your speed and rpm/ shift points for a wide range of trans and you can input any geer/tire hieght you want.

http://www.americantorque.com/graph-rpm-vs-speed/

I currently have a 350/TH400 and 3.08 geers in a 69 chevy. stock motor with intake and headers and at highway speeds the 3.08 is perfect. Drove 600miles got about 16-18 mpg not great but i was cruzing at about 75mph on a fairly hilly twisty road in through british columbia. For $50 I say do it and see how you like it, if it doesn't work you can sell it or swap geers. I am looking to drop the 3.08 open rear for a 3.73 posi to get better stop light to stop light fun, just need to find one here in western canada.

[crazy longhorn]

I guess I have a "freewayflya"....3.40 gear, on a 29" tire,with 6 gears of Richmond on the "row". At 65mph, there is no point to shift into OD....that pulls 1800 on the tach, but my 383 is not into the sweet spot till about 22- 2400 (about 85mph). @ 3150 on the tach in OD my old longhorn is cruisin @ 105 mph(a little too fast for a "brick in the wind',as far as mileage. My old B ville(03), that car gets better mileage running 82 mph ,than it does @72mph....the engine is in the "sweet spot" IMHO, if you get that baby into the sweet spot, tween 70-75 mph, You will see good things. A mid 3's gear will give the best of both worlds....still fun to play with, but not too "tight" on to hyway.....just depends on what you want. best of fun,crazyL

[LMN8UR]

I've got the 2.73 rear gears, the overdrive tranny, and a torque monster 383. With my 255/70/15's 70 mph is 1400 and change. I haven't twisted her hard coming off the line or banged on the gears, but rolling on hard after I get her moving, when the engine gets to about 2300 rpm she'll break the non-posi loose.

As far as top end goes, at about 90 my testicles disconnect!

Around town driving civil I get 10 to 12, and I have broke 16 mpg on the highway.

I put together an Excel spreadsheet that pretty much calculates anything you want to know about RPM / tire sizes / tranny info, and I'm working on speedo-gears. PM me if you'd like to see it. . .

[Longhorn Man]

a low end torque monster can pull that off. Any 455 or caddy big block could do it to, but not your typical stock/stock-ish small block

[stich626]

Originally Posted by 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.

hahahahaha,, sorry,
my '86 monty SS 3.73 and 200r4 locked up is 2200rpm at 65
same tire size

[stich626]

to the o/p
I'd install that 2.73 rear end..
with a 400 cid it pull those gears just fine..
then down the road, you can install a 700r4 with it's 3.00 first gear..
and all that talk about bog'n and lag'n gears will be all but history..
you might be happy with the th400 and 273's
alot of 400cid trans am's had 2.43 rear gears and are almost as heavy as these trucks..
you said the rear is cheap.. if you don't like it, your out short money and a days work..
I say go for it..

[oldgold70c10]

Originally Posted by 7T2

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.)

Unless your running some tiny tires, for 3.73 gears, your tach is way off. When I had 3.73's in my c10, 65 was right at 3100 rpm. I switched to 3.08's and 65 is now about 2400-2500 rpm with TH350. Still drives plenty nice and I can break both wheels loose from a stop.

[7T2]

I was going to do it, but I waited too long and the seller scrapped the rear end. Didn't occur to me at the time, but I would have been going from a 10 bolt to a 12 bolt, so I probably would have had to change the yoke as well.

Sounds like a 3.08 is what I should be looking for. I might just swap gears. I've been reading up on it, and while it is a pretty precise procedure it doesn't seem too complicated if I take my time.

[Super73]

Originally Posted by 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?

What tire do you run?


Some simple math:

Tire ------- Gear -- 3rd -- verter slip
275/60/15 + 3.73 + 1:1 + 15% = 65mph @ 3,500rpm
275/50/15 + 3.73 + 1:1 + 15% = 65mph @ 3,800rpm (Roughly same as 275/40/17)
225/50/15 + 3.73 + 1:1 + 15% = 65mph @ 4,100rpm

275/60/15 + 3.73 + 1:1 + 20% = 65mph @ 3,700rpm
275/50/15 + 3.73 + 1:1 + 20% = 65mph @ 4,000rpm (Roughly same as 275/40/17)
225/50/15 + 3.73 + 1:1 + 20% = 65mph @ 4,300rpm



So either you have a very inificiant verter, a short tire, different than 3.73 gear or you are doing more than 65mph.

[benoit454]

Originally Posted by Winola73

1200 is "lower" than stock, stall speed for my truck is 1600.

You obviously don't comprehend what I'm building and I'm not going to try to explain it. I've done a similar build before and I'm building it not having it built. It works, there's no "maybe" to it and it suits my purposes just fine.

good for you, you act like I said something to you like you're dumb or something and I never said a word. So what if YOU are building it, I BUILD allof my own stuff too, good for you, most don't, I do, nothing big or special there.

As for building a highway gear rear end, on anything, I personally hate it, if you like it and it works for you good. It sure as heck doesn't work for me and I hate it.

[7T2]

Originally Posted by Super73

What tire do you run?

I don't remember off the top of my head, but the wheel/tire combo is 29" tall. I think I am running 255/45/20's in the rear.

[Super73]

29" tire + 3.73 + 4,00rpm@65mph = 30% converter slip. If that number is accurate, it is horrible!!!

Is it a stock converter? If not, who makes it and what is the stall rating?

You sure that you have 3.73's in the rear?
Jack up the rear, mark the tire and driveshaft. Rotate the driveshaft and count the turns it takes to make the tires go one full revolution. If it's 3 3/4 turns of the drive shaft then you have 3.73's. If it's 4 1/8 it's a 4.10. If it's 4 1/4 turns a 4.30 and 4 1/2 a 4.56..

Do you have a trans temp guage? If so what is your freeway cruise temp?


It is possible your tach is off like you have been suspect of.
Who makes the tach?
Is there a 4cyl/6cyl/8cyl mode? If so is it in the correct mode?

[Birdman6165]

I'm trying to decide the same thing. I was told a big block 400 with over 300hp should still have the get up and go I would like as well as good highway cruising. I was going to put in 2.73.

[7T2]

Ok, put another tach on it today and ran her up the road and back - new tach is showing 3100 rpm @ 65 mph, old one is still showing 4500 rpm @ 65 mph. 3100 is a little more sane I think. I'm guessing if I get some 3.08's in there I'll be a happy boy.

FYI, gunning the engine with the old tach showed an approximate redline of 7500-8000 rpm! Hello NASCAR, watch out for the big blue-green ambulance...

[Super73]

Current.. 3,100 rpm + 65mph + 3.73 + 29" tire = 10% verter slip

With your new gears.. 10% slip + 65mph + 3.08 + 29" = 2,575 rpm

[andyh1956]

I would add this to every ones already good advice. I have installed lots of gears in Chevys, and TWICE in my lifetime I found going from 3.73s to a faster gear, one was 2.73 & another was 2.56 the pinion was physically too large to fit into the rear axle housing. Only saw this happen twice. On one truck we got another axle housing & gears dropped right in. On the other the chosen ratio was ditched for another.
In hindsight I think it had to do with the year of the housing. It was prolly one that was made before 2.73s were available & so no provision was made for them.
Never had a problem with 3.08s.
Just something else to keep in mind, it COULD happen!

[68Gold/white]

I would not touch what you've got!!!

Sound like your speedo and tach could both be a touch off., maybe. Not trying to sound like a Beavis here, just help...

I had a 67 Bel Air (my first car, Had from 1975-1997) was a heavy car, a;though not quite as heavy as a pickup. Had a 2.73 gear in it, MPG was 18-20 with a 283. Power was OK, but I will never do that again.
My life learned lesson is to compromise on the side of power, not MPG.
These old pickups are very heavy. The $$ spent to make one get better MPG is not worth it, my opinion...Just drive it 55-60 MPH on the highway, since you don't do that very often...

[Tom]

I ran 3.07's with overdrive. No overdrive with 2.73's is no problem what so ever. I had a stock vortec 350 in the truck [a 79]. 3.07's with no overdrive was 3k at 80mph, which to me still isen't a freeway gear. 2.73's will still be howling at that speed but more reasonable. Do it.

Originally Posted by 68Gold/white

Just drive it 55-60 MPH on the highway, since you don't do that very often...

So far right lane with hazard flashers on everywhere? That doesn't make any sense. Most people want to at least drive the speed limit.