Lowwwww Geared Sub Posi Rear? (and lousy MPG too!) - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Hunter · 11-22-2004, 11:08 PM

I bought a rusted out 71 Sub and put the posi rear in my C10 shorty truck. The Sub was a 4X2 1/2 ton 350/350 auto.
My motor is a mild 350 with a 350 auto and a little Carter square bore. I was amazed at how low this thing is geared. Its into 3rd gear by 25 mph. Plus, I get like 9-10 mpg.

At 70 its screaming. Did the subs come with THAT low a gearing? I thought with the auto it would be higher.... Whussup? How do I fix it? H.

*rage'nrat638* · 11-22-2004, 11:18 PM

sounds like 4/11 gears to me....man it must scream.......mileage sounds about right.....and rpm's.....it feel's good to drive a man's truck......

just think about the fag in the toyota next to you getting 28 mile's per gal....
but has no peddal under his foot.....plus how many thumb's up do you think he get's ....none.....

Longhorn Man · 11-22-2004, 11:22 PM

onless chainged out, or had an optional gear, it shoulda had 3.07 gears.
Do you have a tach?

Robert1970C20rstbukt · 11-23-2004, 12:14 AM

I'm running a 350/TH400/4:11's in mine and when I go 70 the engine is turning about 3200RPMs, mine shifts into third at about 40MPH. If yours is screaming like you say it is it's gotta be 4:57's. Best thing to do is check the code on the axle or pull the cover and count the teeth on the ring gear.

Hunter · 11-23-2004, 01:11 AM

No tach but I'll spend some time underneath crawling around see if I can find a tag. Thanks for the replies. Yes, I do get the thumbs up, both on the highway and from the local gas station owner! H.

curbyk · 11-23-2004, 08:10 AM

I had an 86 6.2 Diesel Burb with the heavy duty tow package and at 70 MPH it was wound pretty tight. I thought the gearing was 4:56 but don't remember, would sure pull tho.

1969 GMC · 11-23-2004, 08:40 AM

Quote:

Originally Posted by rage'nrat638

sounds like 4/11 gears to me....man it must scream.......mileage sounds about right.....and rpm's.....it feel's good to drive a man's truck......

just think about the fag in the toyota next to you getting 28 mile's per gal....
but has no peddal under his foot.....plus how many thumb's up do you think he get's ....none.....

that just brought tears to my eyes, definitely going in my sig! thanks ragenrat

GreyHoundSteve · 11-23-2004, 11:05 AM

i have a 350/400 w 4.11's and its not "screaming" at 70 mph.

**FRENCHBLUE72** · 11-23-2004, 08:23 PM

Damn RAGE and I though driving a toyota was cool.... It has 4:10 gear's and a mildly built 350, haul's ass!! Yep I even get thumb's up in it.... The only thing I can't pass is a gas station LOL...

Cethern · 11-23-2004, 09:42 PM

Frenchblue72
That is a cool Yota...... But still a Toyota...lol

I had a FJ-40 that I droped in a 400 with FI it was locked up front and rear. When I did the swap I put a Bowtie from a 72 K5 on the front grill. I would get asked by ppl " Hey I didn't know Chevy made those" It was funny... Even better was the Clue-less parts guy....

vtblazer · 11-24-2004, 08:19 AM

Your asking how do I fix it?
You could drop in a 700r but if that's not an option your looking at putting in another set of gears. If you want better highway RPM's go with 3:08's.
The "screaming" you mention, could it be your fan? Are you running a clutch fan or a flex fan? That will make a difference also.
Gotta consider your usage of the truck too, do you haul with it or just cruise?

Hunter · 11-24-2004, 10:09 AM

I do indeed wanna do the 700R4, but I'm waiting 'till I can score a TBI 350 donor vehicle (which would have the seats etc...). Still gotta jack it up and see what the ratio is. I know that changing out the ring gear is expensive, 'cause I'm an electrician so I'd have to pay somebody to do it.
I haul stuff in it all the time. Matter of fact there's a huge pile of crap in it right now destined for the dump this afternoon. It gets really crappy fuel economy and at 2 bucks a gallon, I don't drive it as much as I'd like. On the highway its as if you're driving a small tank and 70 feels like its rapping out. Yep, have the stock fan clutch and it does indeed roar.
I did a TBI 350 / 700R4 into a Toy FJ60 and it went from 9 to 14 MPG (and actually had some POWER). Thanks for the input, I'm gonna lift it this afternoon and see if I can figure out the rear ratio. H.

Longhorn Man · 11-24-2004, 10:42 PM

Did you figure the gear ratio?

Hunter · 11-25-2004, 02:58 PM

Well, I put the truck up on a floor jack and slowly turned the wheel forward. For one turn of the wheel, the driveshaft turns about 4 turns. Guess that makes it a 4.11 rear, no? That would explain the high R's on the highway.
THANKS to all for the input and advice. Happy Thanksgiving, Hunter

11-22-2004, 11:08 PM	#1
Hunter Lovin' Arizona Since 1961 Join Date: Jul 2004 Location: Camp Verde, Arizona Posts: 173	Lowwwww Geared Sub Posi Rear? (and lousy MPG too!) I bought a rusted out 71 Sub and put the posi rear in my C10 shorty truck. The Sub was a 4X2 1/2 ton 350/350 auto. My motor is a mild 350 with a 350 auto and a little Carter square bore. I was amazed at how low this thing is geared. Its into 3rd gear by 25 mph. Plus, I get like 9-10 mpg. At 70 its screaming. Did the subs come with THAT low a gearing? I thought with the auto it would be higher.... Whussup? How do I fix it? H. __________________ "Religion is like a toothbrush: Everybody should have one, but I don't want yours" - Will Rogers

11-23-2004, 12:14 AM	#4
Robert1970C20rstbukt Registered User Join Date: Jun 2004 Location: Beeville, Texas Posts: 1,943	I'm running a 350/TH400/4:11's in mine and when I go 70 the engine is turning about 3200RPMs, mine shifts into third at about 40MPH. If yours is screaming like you say it is it's gotta be 4:57's. Best thing to do is check the code on the axle or pull the cover and count the teeth on the ring gear. __________________ RJD 1984 Chevrolet C30 welding truck 454/SM465 1977 GMC C15 pickup 350/TH350 1970 Chevrolet C20 pickup 350/TH400 1970 Chevrolet K20 Suburban 454/TH350 1969 GMC 8500 478M V6/5 spd winch truck

11-23-2004, 01:11 AM	#5
Hunter Lovin' Arizona Since 1961 Join Date: Jul 2004 Location: Camp Verde, Arizona Posts: 173	No tach but I'll spend some time underneath crawling around see if I can find a tag. Thanks for the replies. Yes, I do get the thumbs up, both on the highway and from the local gas station owner! H. __________________ "Religion is like a toothbrush: Everybody should have one, but I don't want yours" - Will Rogers

11-23-2004, 08:10 AM	#6
curbyk 69 C-10 project Join Date: Sep 2004 Location: Eastern, NC Posts: 184	I had an 86 6.2 Diesel Burb with the heavy duty tow package and at 70 MPH it was wound pretty tight. I thought the gearing was 4:56 but don't remember, would sure pull tho. __________________ 69 C-10 Work is complete

11-23-2004, 08:23 PM	#9
FRENCHBLUE72 PROJECT 7DEUCE Join Date: Dec 2002 Location: GRANTS PASS OR Posts: 21,600	Damn RAGE and I though driving a toyota was cool.... It has 4:10 gear's and a mildly built 350, haul's ass!! Yep I even get thumb's up in it.... The only thing I can't pass is a gas station LOL... Attached Images __________________ GO BIG GREEN GO DUCKS MEMBER #6377 72 k-5 daily driver 6'' lift 35'' 350-350-205 slowly getting rust free. Project "7DEUCE" check out my build http://67-72chevytrucks.com/vboard/s...d.php?t=267665 Tim Powell..R.I.P EastSideLowlife..... R.I.P..

11-22-2004, 11:18 PM	#2
rage'nrat638 Account Suspended Join Date: Mar 2003 Location: THE FALL GUY CHICAGO IL Posts: 5,883	sounds like 4/11 gears to me....man it must scream.......mileage sounds about right.....and rpm's.....it feel's good to drive a man's truck...... just think about the fag in the toyota next to you getting 28 mile's per gal.... but has no peddal under his foot.....plus how many thumb's up do you think he get's ....none.....

11-22-2004, 11:22 PM	#3
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	onless chainged out, or had an optional gear, it shoulda had 3.07 gears. Do you have a tach?

11-23-2004, 11:05 AM	#8
GreyHoundSteve Registered User Join Date: Oct 2004 Location: DFW TX Posts: 769	i have a 350/400 w 4.11's and its not "screaming" at 70 mph.

11-23-2004, 09:42 PM	#10
Cethern What was I thinking?? Join Date: Dec 2003 Location: Commerce City Co Posts: 637	Frenchblue72 That is a cool Yota...... But still a Toyota...lol I had a FJ-40 that I droped in a 400 with FI it was locked up front and rear. When I did the swap I put a Bowtie from a 72 K5 on the front grill. I would get asked by ppl " Hey I didn't know Chevy made those" It was funny... Even better was the Clue-less parts guy.... __________________ 72 K5 that needs lots of parts.. 71 GMC 3/4 4x4 That doesn't need much which is a good thing I don't think I could aford another money pit...

11-24-2004, 08:19 AM	#11
vtblazer Registered User Join Date: Oct 2003 Location: Central Vermont Posts: 8,538	Gearing Your asking how do I fix it? You could drop in a 700r but if that's not an option your looking at putting in another set of gears. If you want better highway RPM's go with 3:08's. The "screaming" you mention, could it be your fan? Are you running a clutch fan or a flex fan? That will make a difference also. Gotta consider your usage of the truck too, do you haul with it or just cruise?

11-24-2004, 10:09 AM	#12
Hunter Lovin' Arizona Since 1961 Join Date: Jul 2004 Location: Camp Verde, Arizona Posts: 173	I do indeed wanna do the 700R4, but I'm waiting 'till I can score a TBI 350 donor vehicle (which would have the seats etc...). Still gotta jack it up and see what the ratio is. I know that changing out the ring gear is expensive, 'cause I'm an electrician so I'd have to pay somebody to do it. I haul stuff in it all the time. Matter of fact there's a huge pile of crap in it right now destined for the dump this afternoon. It gets really crappy fuel economy and at 2 bucks a gallon, I don't drive it as much as I'd like. On the highway its as if you're driving a small tank and 70 feels like its rapping out. Yep, have the stock fan clutch and it does indeed roar. I did a TBI 350 / 700R4 into a Toy FJ60 and it went from 9 to 14 MPG (and actually had some POWER). Thanks for the input, I'm gonna lift it this afternoon and see if I can figure out the rear ratio. H. __________________ "Religion is like a toothbrush: Everybody should have one, but I don't want yours" - Will Rogers

11-24-2004, 10:42 PM	#13
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Did you figure the gear ratio?

11-25-2004, 02:58 PM	#14
Hunter Lovin' Arizona Since 1961 Join Date: Jul 2004 Location: Camp Verde, Arizona Posts: 173	Well, I put the truck up on a floor jack and slowly turned the wheel forward. For one turn of the wheel, the driveshaft turns about 4 turns. Guess that makes it a 4.11 rear, no? That would explain the high R's on the highway. THANKS to all for the input and advice. Happy Thanksgiving, Hunter __________________ "Religion is like a toothbrush: Everybody should have one, but I don't want yours" - Will Rogers



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