Worst gear ratio EVER!!!!

[ElGracho]

As you may or may not know, I recently swapped a Goodwrench 350 and TH700R4 into my 1980 GMC Indy Hauler. I've only driven the truck about 300 miles since the first of June or so when I got it done and I've been annoyed since my speedo has been off.

I put a new drive and driven gear in the tranny to match up with my 255/70R15 tires (within .05" of the stock 235/75R15's, or probably closer than one brand to another of that same size...) and what I assumed were 3.07 gears. Ah yes, I used the assume word, and it bit me.

I've checked my speedo gear math but always used 3.07 for the rear end ratio since I knew it was higher than 3.42s but didn't think GM was using 2.73s much in 1980 (even though my 84 4WD and 86 2WD trucks BOTH had 2.73s...)

Anyhow, I know the only way to know my gear ratio for sure is to pull the cover and read the numbers off the ring gear, so tonight I did that.

I was shocked and very dissapointed to see a part number followed by:
16 41 3 80 which tells us my ratio is 2.56 to 1!!!

Holy crap! I was shocked when I pulled down 16mpg on my way back from California with a worn out 350 when i picked this thing up, but I gave most of the credit to the TH350C that used to be in the truck. I guess it was the gears. I'm sure they are original gears since the date code matches and there are no signs that the carrier has ever been out of the axle. GM sure made some bad choices in the late 70's and early 80's.

With those gears, at 70 mph in OD on the TH700, I'd be running 1450 RPM, thats pretty close to the converter stall speed and if it isn't locked up, it could even be slipping. No thanks, the 700R4 needs all the help it can get to keep from grenading and those terrible gears aren't the ticket.

Anyhow, I guess I'm looking for some 3.23 or 3.55 gears now!

[ocbaud]

i run 3000rpms at 70mph

[TIMSPEED]

Originally Posted by ocbaud

i run 3000rpms at 70mph

then you don't have 2.73's...I run 3000rpm @ 70mph, with 3.42's and 28" tires.

[ocbaud]

Originally Posted by TIMSPEED

then you don't have 2.73's...I run 3000rpm @ 70mph, with 3.42's and 28" tires.

my rear tires arent exactly 28" anymore

[AirMale]

Originally Posted by ocbaud

my rear tires arent exactly 28" anymore

NICE! smoke em' up

[N2TRUX]

Originally Posted by ElGracho

......Anyhow, I guess I'm looking for some 3.23 or 3.55 gears now!

Joe what size tire are you running? You may need to go that low if your tires are tall. If not you might consider 3.08's. With the low first gear in the 700r4 it gives you a 21% reduction or the equevilant to 3.73's on take off.

I talked to a mechanic that does a lot of transmission work and has done a ton of 350 to 700r4 conversions. He said the 3.08 was the best combo to run on our trucks. He built a 4wd for his daughter with this combo that gets good mileage and has decent power with 31" tires....

[Greenlee]

I have a 350 with a 700r4 too. I was told by the previous owner, who also changed out the rear end, that I have 2.96 gears. Does that sound plausible? I don't remember ever hearing of that ratio.

[Beausoleil Racing]

My 1979 Chevy 1/2 ton 4x4 came with 2.56s and some nut put 36" tires on it... Once rolling, things were okay... Getting up there was the hard part!

[bigblock73]

Originally Posted by N2TRUX

He said the 3.08 was the best combo to run on our trucks. He built a 4wd for his daughter with this combo that gets good mileage and has decent power with 31" tires....

Ken, like you eluded to, tire size plays a role...as does the converter stall speed. It is possible to undergear a overdrive equipped truck.

With a properly equipped truck (matchng gears, tires, and converter), 3.08 gears would be great for highway cruising and mileage...but not soo good for acceleration...if that was what you were after.

I am running 4.10's...I will have a helluva time leaving the line w/o spinning, my first gear ratio will be more like 4.56 with the new 700. But my truck is being built for go-fast duties and occasional drag strip fun.

[ElGracho]

On this truck, I'm going to stick with 255 70R15s they are 29.06" tall which is very very close to what the 235 75 15's were stock. 3.08's would probably be ok too. One thing you have to keep in mind is the steep drop from 1st to 2nd on a 700R4. 3.55's or 3.73s will help keep the revs up closer to the power band on that shift.

That said, I'm not going to be racing this truck so some kind of compromise in gear ratio will be made for economy, especially since the days of $2+ gas seem to be here to stay.

[RockHQ]

I'd send you a set of 2.78 gears for a 12 bolt for free but, shipping them to the USA would cost more than they're worth

[ElGracho]

Originally Posted by RockHQ

I'd send you a set of 2.78 gears for a 12 bolt for free but, shipping them to the USA would cost more than they're worth

Ughhh, yeah, I think I'll find something a little closer to home. Thanks for the offer though....

[Destructo]

3.08s or 3.42s would be the ticket I think.

I have a 700r4, 350 and 33" tall tires (more like 32" probably) and I think if I had 31" tires it would be perfect.

So I would say 3.08s or 3.42s otherwise you will have too high an rpm running 3rd gear while towing. Right now with the 33's and 3.42 gears I turn about 1800-1900 and a couple more hundred would help out some.

[Tom]

Originally Posted by ElGracho

GM sure made some bad choices in the late 70's and early 80's.

With those gears, at 70 mph in OD on the TH700, I'd be running 1450 RPM, thats pretty close to the converter stall speed and if it isn't locked up, it could even be slipping. No thanks, the 700R4 needs all the help it can get to keep from grenading and those terrible gears aren't the ticket.

Not when it comes to gear ratio's. Besides the fact the owner could opt for whatever they wanted, those are perfect freeway gears with a th350c. Next time I need a automatic tranny it will be your exact combo, 2.-- somthing rear gears and a th350c. I have a 2004R and 3.07 gears and those suck, too little RPM in overdrive and too much RPM out of it. My lockup went out on my convertor once, had to drive to work for 2wks in 3rd instead of overdrive. Thats 3000rpm for 2hrs a day compared to the normal 2000rpm in overdrive. I got better milage unlocked and in 3rd then lugging the motor around at 80mph and 2000rpm.

BTW- why are you not running the lockup?

[ElGracho]

Originally Posted by Tom

Not when it comes to gear ratio's. Besides the fact the owner could opt for whatever they wanted, those are perfect freeway gears with a th350c. ...
...I got better milage unlocked and in 3rd then lugging the motor around at 80mph and 2000rpm.

BTW- why are you not running the lockup?

First off, I agree with you that those are good gears for running down the highway, if that's all you're doing, and the highway is flat.

The rest of the time you are trading acceleration and probably non-highway mileage. You get the best of both worlds by using steeper gears and an OD trans. The biggest flaw with 700R4s (aside from early reliability issues) is the big gap between 1st and 2nd that was mentioned earlier. If you don't want to lug the engine in 4th locked up, you need steeper gears, just like I will be getting.

Oh yeah, I am running the lockup. Even then, it will unlock under high load when manifold vacuum goes away. That will happen when pulling a grade or passing someone without forcing a downshift. Again, by running steeper gears, I will raise my cruise RPMs closer to the efficient power band and will stay away from the stall speed of my converter.

[8t5Silverado]

Destructo,

Are you saying that 31s with a 700R4 and 3.08s in the rear is a good combo?

I am afraid it wouldn't lock up on the highway...

[Destructo]

Originally Posted by 8t5Silverado

Destructo,

Are you saying that 31s with a 700R4 and 3.08s in the rear is a good combo?

I am afraid it wouldn't lock up on the highway...

I would run 3.42s and 31s, and 3.73 and 33" etc.

With 33's and 3.42s i'm about the same as 3.08s and 31's. The engine does chug and downshift to third on hills though, so some 3.73s and 33's would work best I think and still turn around 2000 rpm on the highway. But 4.10s may be better.

I couldn't run overdrive with my bad carb but not with a good carb I can run OD on flat land, and my 700r4 locks up right when it shifts into 4th regardless of the rpm. I have a bowtie overdrives lockup kit, its not factory.

[RockHQ]

Well, my 79 C-10, Olds 350, TH400 with a diesel converter, 2.78 gears+two barrel carb got around 7 mpg doing 65mph. Did i mention it was SLOOOW. Yeah. REAL slow.

So today i ordered 4.10 gears+a complete overhaul kit for the 12 bolt from Superior Axle, and topped it off with a Powertrax No-Slip. Should make up for the crappy first and second gears of the TH400. I'll be running 325/50/15's in the rear, as soon as i can drop my 468 BB in.

[Tom]

Originally Posted by ElGracho

If you don't want to lug the engine in 4th locked up, you need steeper gears, just like I will be getting.

Yeah I know. 3.42's would be perfect, I only need about 300-400rpm more at cruise to be in the good part of the powerband [2300-4000rpm]

Oh yeah, I am running the lockup. Even then, it will unlock under high load when manifold vacuum goes away. That will happen when pulling a grade or passing someone without forcing a downshift.

My 2004R dosen't gauge lockup by anything other then if its in 4th gear or if I flip the manual lockup switch. I didn't know 700R4's had a vaccum lockup? Thats cool though, wish mine had one.

All I was trying to say was that with a non-overdrive tranny, those 2.xx rear gears aren't bad at all. The 3.07's are too much gear for a non-overdrive IMO even not at 80mph. With my motor I can easly take off from a start as fast as my traction will let me, so anything bigger just dosen't make sence. Of course my 78 is getting at least 4.10's

[ElGracho]

Originally Posted by Tom

...
My 2004R dosen't gauge lockup by anything other then if its in 4th gear or if I flip the manual lockup switch. I didn't know 700R4's had a vaccum lockup? Thats cool though, wish mine had one.

Not so much a vacuum lockup as that's one of the things (stock) that will prevent it from locking up. It looks like the ESC vacuum switch but its near the brake booster instead of the distributor. Basically, the power to the lockup solenoid goes: switched 12v - brake switch (normally closed, opens when braking to interrupt 12v) - vacuum switch (opens with low vacuum) - pressure switches in trans valve body (I'm only using the 4th gear switch, it closes when in 4th) - lockup solenoid (power here causes converter lockup).

Originally Posted by Tom

All I was trying to say was that with a non-overdrive tranny, those 2.xx rear gears aren't bad at all. The 3.07's are too much gear for a non-overdrive IMO even not at 80mph. With my motor I can easly take off from a start as fast as my traction will let me, so anything bigger just dosen't make sence. Of course my 78 is getting at least 4.10's

I agree with you on that, except with stock low power motors that tends to lug the engine in town. My Goodwrench 350 is actually a bit better than the stock one (headers, true duals, performer intake) but I think the 700R4 with some 3.42s will be a better combo than stock.

I'm really glad to hear other people's experience with this!

Oh yeah, I'll get some pics up on my page Chris, give me a day or so. Unfortunately it doesn't look much different than when I bought it except under the hood.

[loblazer]

I am running 28 inch tire with 305 and 3.08 gears. Not bad mileage. I am going to a 700R4 also to get better mileage. I was going to go 350 c.i. but may not.

ElGracho....I would love $2.00 gallon gas. Today it is 85.5 cents per litre here in Ontario. That converts to $3.21 a gallon.

Keep blastin anyway!!!!!!!!!

Doug

[cliffsta]

hmmm this thread is very informative. ill throw in my .02 on the comment about GM making bad gear choices.
I'm running my 80 c10 with 305 v8, TH350 tranny, and stock wheels and stock size tires. no idea what my gear ratio is. but its JUST FINE i like having my truck that idles low and climbs up into third real easy as long as i ain't dogging on it. whoever said "its easier on the motor to turn 2000 rpms rather than 3000 rpms", i love you!!! my buddy has a toyota tercel thats 14 years younger than ol red with less miles, but i bet he blows the thing up the way he dogs on his AUTOMATIC by jumping into low 1 and low 2 to rev it up.

[Captkaos]

Not to throw a fork in the road on this, but were are the pics of the Indy!!!!

[86 Red_Bowtie]

If you take a combination of 3.73 gears in a 12 bolt posi rear end, with a set of tires that are 26.5" tall, and a 700 R4 tranny (without lockup) which is getting an RPM of 2500 at 70 Miles per hour. The power curve for a 350 Chevy engine is good from approximately 2100 to 2800. The optimum place is 2300 to 2500 for good acceleration and response. The cruise controls and the transmission are working well at these speeds and RPMs, so that they don't have to shift down on hills or climbs. This in itself will increase dependability and gas mileage. We had a truck with 3:08 gears and a 700R4 that was only getting a solid 12 MPG, after changing the rear end gears to the 3.73 ratio the mileage increased to 14.5 MPG. You guys are talking about running 3000 RPM at 70 MPH, then you definitely don't have an overdrive tranny or your gear ratio is way low. At 3000 RPM, our trucks are pushing 92 MPH; so there is a definite difference in tranny output or rear end gears. (just ask the guys with the Mustangs that can't lose us) The 3.08 combo of gears is good if you don't have an overdrive, but if you do, the RPMs will be to low to get a good RPM for daily driving and your engine will work itself to death. We were getting 1900 RPMs at 70 MPH and we could not get the cruise to work smoothly and not shift down on every incline. The 3.42 gears are not bad, but the RPM is at the bottom end of the 350 power band and the cruise control will still be somewhat inefficient, especially with a tall tire. I drove on 3.42 gears for a long time and the change to 3.73s was worth the money and time. You can do what you want, but I can run with the best of them and still get a reasonable gas mileage (for a 4000 lb truck with a 400+ Hp 383 engine) (Kick A-- and take no names) I understand the thought of $2.00 and higher gas, but if you do anything but drive the highway and haul NOTHING, then you will need a truck. Want gas mileage then buy a small Toyota or Nissan, there is a reason that they exist. I was in California for a period and I do realize that life out there is a struggle of how to get to work. I live 15 minutes from my work place and I drive a truck everyday. My 91 chevy does have a 350 motor and 3.42 gears and with the fuel injection it is not bad on gas. My 84 GMC has a 383 stroker motor and 3.73 gears, but it is carbureted and it gets 15 MPG plus. The difference is the computer in the truck. The 84 lost it's computer long ago to the garbage gods, In California you are stuck with what the "CARB" (California Air Resources Board) tells you to do.

[Grande Rojo]

I am running a 700r4 behind a 355 sbc,12bolt,4.10 gears and 275/60/17's (31" tall") tires
It like this combo well, great low end and great hiway rpms.


just my .02