rear gear ratio and posi suggestions - The 1947 - Present Chevrolet & GMC Truck Message Board Network

kevncar · 04-28-2007, 02:13 PM

Here's the low down. I'm looking for advise on what gear ratio to use and which posi carrier you guys suggest. I am running a 454 with a th400 and am going to put a used gear vendors overdrive behind it. It has a .78:1 overdrive. I'll be running a 295/50/20 in the rear.

I used a speed this speed analyzer...
http://www.keislerauto.com/tools/gm_analyzer_6sp.html
which has a .8 overdrive. So my speeds should be just a hair higher.

I'd like to have a pretty nasty bottom end to have a little fun with...but I want to be able to take the wife two hours away to IKEA and pickup the couch of the month also.

Any and all suggestions would be appreciated.

**special-K** · 04-28-2007, 05:33 PM

3.73?I have a truck with o/d and4.10s and that`s great.But,the tires are 255/85x16,about 33" tall and 9.5" wide.It`s also a diesel.I like4.10 w/od.But,3.73 would keep a nice bottom in your truck and help a bit with consumption of the liquid gold.

KFreddy · 04-28-2007, 08:03 PM

3.73 would be real nice for your application. I have close to the same set up as you sans the gear vendors

ZZ454, Th400 and a currie 3.08 posi. Yes it will spin the tires and yes it seems to be getting 14 to 17 miles per gallon, and turns 2100rpm at 60 mph, this is with a 29" tire. However it is a compromise, daily driveability vs outright performance. It can spin the tires "well done" however it is certainly missing the low end snap that only a 3.73 or above can provide. It goes a bit over 60mph in 1st gear and I'm yet to hold it all the way through second, I would imagine 105 to 120 in second.

With a gear vendors I would be all over a different ring and pinion, most likely a 3.73...

hgs_notes · 04-28-2007, 10:04 PM

If your truck came original with the 3.73's, like many did, you can just keep them and the carrier and install a power trax lock-right or traction system. A friend of mine has them front and back in his lifted 4x4 with 35"s and good HP. He's a mechanic by trade and mudder for fun. He swears by them.

HG

bigblockorange · 04-29-2007, 02:00 AM

I'm using a ford 9" in my truck I like the fact that I can change chucks out in 45 min to an hour. I have a 2:73 and a 3:50 both posi's, also a 4:10 open just in case my tires get too big and I need the power. Much cheaper than the factory 12 bolt stuff.

mnunn454 · 04-29-2007, 09:17 AM

I have a 454/TH400/373:1 now so am following this thread closely.

This is a tough question because there are so many options ahead of you.

If you don't have a rear axle now, then with all your power, it'll be hard to beat the Ford 9" for strength and flexibility.

Assuming you already have a 12-bolt now, my next question is what carrier do you have, and is it posi? 3.73 and up take one carrier, 3.42 and down take a different one. New Eaton sets can be had for about $350 give or take. Also what cam and torque converter stall are you running?

My Elky has a 454/TH700/3.55:1 and I love the ratios all the way around. The TH700 has a 0.7:1 overdrive. I turn about 2200 revs at 60 and 2400 at 70 as I recall. It has Performer RPM induction so it's just getting started at 2200 revs, and a 2200 stall (not an issue after converter locks up). Assuming your rear tires and mine are about the same diameter, your GVO's .78 OD would spin the engine a little faster (8% to be exact) than mine for a given speed. Also, since your converter doesn't lockup, there'll be another 3% to add in for converter slippage give or take.

That said, I'd opt for the 3.42 posi unless your engine's cam or the torque converter require the higher revs.

Hope it helps.

**Tom Vogel** · 04-29-2007, 12:01 PM

Im pretty sure Gear Vendors will tell you 3:50s. As for posi setups, if your going to be doing burnouts, opt for a locker (youll cook a posi). If you want a setup for open track, go with a posi.
Tom
PS I have a dtroit locker.... big engine+big torque= Burnout=toasted posi.

brake · 04-29-2007, 11:39 PM

Try this calculator: http://www.wallaceracing.com/gear-speed.php There are several others on that site that may help you.
That it is a pretty tall tire. I'd opt for at least 3.73's. That would put you in the low 2k RPM range at highway speeds. What is the stall speed on your converter? What is the range & cruise RPM of your cam?

04-28-2007, 02:13 PM	#1
kevncar Active Member Join Date: Dec 2002 Location: Atwater, CA Posts: 259	rear gear ratio and posi suggestions Here's the low down. I'm looking for advise on what gear ratio to use and which posi carrier you guys suggest. I am running a 454 with a th400 and am going to put a used gear vendors overdrive behind it. It has a .78:1 overdrive. I'll be running a 295/50/20 in the rear. I used a speed this speed analyzer... http://www.keislerauto.com/tools/gm_analyzer_6sp.html which has a .8 overdrive. So my speeds should be just a hair higher. I'd like to have a pretty nasty bottom end to have a little fun with...but I want to be able to take the wife two hours away to IKEA and pickup the couch of the month also. Any and all suggestions would be appreciated. __________________ Kevin 72 Chevelle 67 C10...454/TH400 Build Thread:http://67-72chevytrucks.com/vboard/s...d.php?t=193961

04-28-2007, 05:33 PM	#2
special-K Special Order Join Date: Nov 2004 Location: Mt Airy, MD Posts: 85,851	Re: rear gear ratio and posi suggestions 3.73?I have a truck with o/d and4.10s and that`s great.But,the tires are 255/85x16,about 33" tall and 9.5" wide.It`s also a diesel.I like4.10 w/od.But,3.73 would keep a nice bottom in your truck and help a bit with consumption of the liquid gold. __________________ "BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~

04-28-2007, 08:03 PM	#3
KFreddy Active Member Join Date: Apr 2005 Location: Miami, Florida Posts: 286	Re: rear gear ratio and posi suggestions 3.73 would be real nice for your application. I have close to the same set up as you sans the gear vendors ZZ454, Th400 and a currie 3.08 posi. Yes it will spin the tires and yes it seems to be getting 14 to 17 miles per gallon, and turns 2100rpm at 60 mph, this is with a 29" tire. However it is a compromise, daily driveability vs outright performance. It can spin the tires "well done" however it is certainly missing the low end snap that only a 3.73 or above can provide. It goes a bit over 60mph in 1st gear and I'm yet to hold it all the way through second, I would imagine 105 to 120 in second. With a gear vendors I would be all over a different ring and pinion, most likely a 3.73... __________________ "The Great State of Florida"

04-28-2007, 10:04 PM	#4
hgs_notes GEARHEAD Join Date: Aug 2003 Location: MN Posts: 6,126	Re: rear gear ratio and posi suggestions If your truck came original with the 3.73's, like many did, you can just keep them and the carrier and install a power trax lock-right or traction system. A friend of mine has them front and back in his lifted 4x4 with 35"s and good HP. He's a mechanic by trade and mudder for fun. He swears by them. HG __________________ If no one knows what you're doing, they can't tell you you're doing it wrong HG's Dream Car Build - Shelby Cobra Kit Build your own adjustable track bar 71 Long Fleet C/10 72 Jimmy 4x4 (Junkyard Jimmy) HG's Plow Truck '78 K35 Dually Bigass Farm Truck HG's thread of miscellaneous stuff

04-29-2007, 02:00 AM	#5
bigblockorange Senior Member Join Date: Jan 2007 Location: central pa Posts: 428	Re: rear gear ratio and posi suggestions I'm using a ford 9" in my truck I like the fact that I can change chucks out in 45 min to an hour. I have a 2:73 and a 3:50 both posi's, also a 4:10 open just in case my tires get too big and I need the power. Much cheaper than the factory 12 bolt stuff. __________________ I guess you could say I’m not really lazy I’m just real good at doing nothing!!!!

04-29-2007, 09:17 AM	#6
mnunn454 Registered User Join Date: Dec 2006 Location: Rhode Island Posts: 602	Re: rear gear ratio and posi suggestions I have a 454/TH400/373:1 now so am following this thread closely. This is a tough question because there are so many options ahead of you. If you don't have a rear axle now, then with all your power, it'll be hard to beat the Ford 9" for strength and flexibility. Assuming you already have a 12-bolt now, my next question is what carrier do you have, and is it posi? 3.73 and up take one carrier, 3.42 and down take a different one. New Eaton sets can be had for about $350 give or take. Also what cam and torque converter stall are you running? My Elky has a 454/TH700/3.55:1 and I love the ratios all the way around. The TH700 has a 0.7:1 overdrive. I turn about 2200 revs at 60 and 2400 at 70 as I recall. It has Performer RPM induction so it's just getting started at 2200 revs, and a 2200 stall (not an issue after converter locks up). Assuming your rear tires and mine are about the same diameter, your GVO's .78 OD would spin the engine a little faster (8% to be exact) than mine for a given speed. Also, since your converter doesn't lockup, there'll be another 3% to add in for converter slippage give or take. That said, I'd opt for the 3.42 posi unless your engine's cam or the torque converter require the higher revs. Hope it helps. __________________ 69 C-10 SWB Fleetside 454/TH400 (For Sale) http://67-72chevytrucks.com/vboard/c...ct=1299&cat=12 70 El Camino 454 TH700 29 Model A 4-DR Sedan Deluxe Last edited by mnunn454; 04-29-2007 at 09:18 AM.

04-29-2007, 12:01 PM	#7
Tom Vogel 2WD Jimmy Join Date: Mar 2005 Location: Davis County, Utah Posts: 2,565	Re: rear gear ratio and posi suggestions Im pretty sure Gear Vendors will tell you 3:50s. As for posi setups, if your going to be doing burnouts, opt for a locker (youll cook a posi). If you want a setup for open track, go with a posi. Tom PS I have a dtroit locker.... big engine+big torque= Burnout=toasted posi.

04-29-2007, 11:39 PM	#8
brake Registered User Join Date: Aug 2002 Location: Pittsburgh, Pa Posts: 18	Re: rear gear ratio and posi suggestions Try this calculator: http://www.wallaceracing.com/gear-speed.php There are several others on that site that may help you. That it is a pretty tall tire. I'd opt for at least 3.73's. That would put you in the low 2k RPM range at highway speeds. What is the stall speed on your converter? What is the range & cruise RPM of your cam?



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