Hi guys, newbie here with rear end question - The 1947 - Present Chevrolet & GMC Truck Message Board Network

bgood · 05-14-2004, 07:41 PM

Guys, I've never felt this mechanicaly inept, I've read all the posts on rearend gear ratio calculations and still can't get it..Here's what i got. I have just bought a 67 chevy short wheel base step side, it has a transplanted 350 eng. w\ a turbo 350 trans. I guess its rer end is still stock with 12 bolt pattern, tire size is 275/60r/15. I'm trying to reduce rpm at cruising speed no tachometer yet but am guessing close to 2800 at 50mph (gps verified). I tried jacking the rear end up and rotating the wheel and counting the turns of drive shaft to tire rotation.
Well starting with the tires valve stem at 12 o'clock I turn it only until stem reaches about 8 o'clock and the drive shaft has turned one complete turn. What gear ratio does this indicate?

thanks, you folks have a great board here... lots of info for us challenged newbies, Bill

walker · 05-14-2004, 07:51 PM

First off, you have to have both wheels turning in the same direction to do the calculation that way. NEED to have a tach to properly compute RPM. Best method for rear gear calc is to pull the rear cover. It will cost you a few buck in oil and a gasket, most likely needs it anyway. Then you will know your rear ratio, condition of gears, and if you have a Posi or not.
Good luck.

crazy longhorn · 05-14-2004, 08:08 PM

http://www.smokemup.com/auto_math/mph_range2.php my guess is that if you had a 6cly /stick truck, the rear ratio is most likely 3.73. there were a few trucks that had 4.11 gears, but they were a 3 spd stick with overdrive(i cant help on yrs or option codes), but the 3.73 was a pretty base gear in the I6 trucks. the cheapest way to slow the revs is to swap the rear diff for a complete unit with 3.07 gears......overdrive is more fun tho! If you opt for a 2004r, or a 700r4 you can still have all the "seat of the pants"out of the hole, & cruise easy on the tack. good luck,crazyL

NPilot1975 · 05-14-2004, 10:04 PM

The most accurate way to find what gears you have is to take off the rear diff cover and count the number of teeth on both ring and pinion gears. Then you simply divide the number of ring gear teeth by the number of pinion gear teeth. It will also give you a chance to inspect everything and add new gear oil. The only things you will need is a gasket and gear oil.

Bill72BB · 05-14-2004, 10:04 PM

If you know the RPM,Speed and tire Dia. plug the # in here.http://richmondgear.com/streetcalculators.html
if not do what Chevy67Tk said.

Welcome to the board from SoCal.

*dtlilly* · 05-14-2004, 10:45 PM

I second all the previous advice, and wanted to welcome you to the board

**67sss** · 05-15-2004, 01:01 AM

crawl under the truck and get the #'s off the rearend. They are on the top side of the rearend housing on the drivers side.I have the codes to decifer the #'s if you need more help. Welcome to the board, another 67 shorty stepside........

Chevy67Tk · 05-14-2004, 08:00 PM

You need to turn your wheel 1 full revolution and count the number of turns the drive shaft makes this will give you the ratio

Example: for one turn of the wheel if the drive shaft turns 4.11 turns you have a set of 4.11 gears

05-14-2004, 07:41 PM	#1
bgood Registered User Join Date: May 2004 Posts: 80	Hi guys, newbie here with rear end question Guys, I've never felt this mechanicaly inept, I've read all the posts on rearend gear ratio calculations and still can't get it..Here's what i got. I have just bought a 67 chevy short wheel base step side, it has a transplanted 350 eng. w\ a turbo 350 trans. I guess its rer end is still stock with 12 bolt pattern, tire size is 275/60r/15. I'm trying to reduce rpm at cruising speed no tachometer yet but am guessing close to 2800 at 50mph (gps verified). I tried jacking the rear end up and rotating the wheel and counting the turns of drive shaft to tire rotation. Well starting with the tires valve stem at 12 o'clock I turn it only until stem reaches about 8 o'clock and the drive shaft has turned one complete turn. What gear ratio does this indicate? thanks, you folks have a great board here... lots of info for us challenged newbies, Bill

05-14-2004, 07:51 PM	#2
walker Registered User Join Date: Jul 2003 Location: north of Phx AZ about 30 miles Posts: 698	First off, you have to have both wheels turning in the same direction to do the calculation that way. NEED to have a tach to properly compute RPM. Best method for rear gear calc is to pull the rear cover. It will cost you a few buck in oil and a gasket, most likely needs it anyway. Then you will know your rear ratio, condition of gears, and if you have a Posi or not. Good luck. __________________ Andy,Phx AZ '67 C-10 (Ahhh, done at last. Well there is that disk front end I want to put in and...) "23 C-Cab-sold '48 Ford 8N tractor(still working) '67 Scout(Now on the road) '70 MG B.-sold

05-14-2004, 08:08 PM	#3
crazy longhorn Fabricate till you "puke" Join Date: Jun 2002 Location: Ill Posts: 9,403	http://www.smokemup.com/auto_math/mph_range2.php my guess is that if you had a 6cly /stick truck, the rear ratio is most likely 3.73. there were a few trucks that had 4.11 gears, but they were a 3 spd stick with overdrive(i cant help on yrs or option codes), but the 3.73 was a pretty base gear in the I6 trucks. the cheapest way to slow the revs is to swap the rear diff for a complete unit with 3.07 gears......overdrive is more fun tho! If you opt for a 2004r, or a 700r4 you can still have all the "seat of the pants"out of the hole, & cruise easy on the tack. good luck,crazyL __________________ 69 longhorn,4" chop,3/5 drop, 1/2 ton suspension/disc brakes,1 1/2" body drop,steel tilt clip, 5.3/Edelbrock rpm intake/600 carb, Hooker streetrod shorties,2 1/2" exhaust/ H pipe/50's Flows , 6 spd Richmond trans,12 bolt/ 3.40 gears....

05-14-2004, 10:04 PM	#4
NPilot1975 Senior Member Join Date: Aug 2002 Location: Plymouth, MA Posts: 891	The most accurate way to find what gears you have is to take off the rear diff cover and count the number of teeth on both ring and pinion gears. Then you simply divide the number of ring gear teeth by the number of pinion gear teeth. It will also give you a chance to inspect everything and add new gear oil. The only things you will need is a gasket and gear oil. __________________ 1972 Blazer body...need to get a thread started for the rest of the story

05-14-2004, 10:04 PM	#5
Bill72BB Thats the Hillbilly way Join Date: Jul 2001 Location: Torrance,Ca Posts: 5,980	If you know the RPM,Speed and tire Dia. plug the # in here.http://richmondgear.com/streetcalculators.html if not do what Chevy67Tk said. Welcome to the board from SoCal. __________________ BILL Torrance Calif 90504 Member DUF Club Member Old Farts Club 72 3/4ton Custom Camper BB402/400 trans Dynomax Cerama-coat headers , Hei distributor ,Edelbrock 1411 air,ps,pb,leafsprings ,98 chevy 60/40 seat,S10 shoulder belts tilt,3.54 Dana 60 rear. 16" Eagle wheels with LT265/75 16 tires,Gaylord Lid,H4 headlights with 100W relay conversion. " If I knew I was going to live this long I would have taken better care of myself " Marriage changes passion...suddenly you're in bed with a relative. A GUY'S PRAYER.... Lord, I pray for a nympho with huge boobs who owns a liquor store and likes to fish and hunt and has a Harley .Amen ,

05-14-2004, 10:45 PM	#6
dtlilly Account Suspended Join Date: Jan 2003 Location: Shadow of Lambeau Field Posts: 5,597	I second all the previous advice, and wanted to welcome you to the board

05-15-2004, 01:01 AM	#7
67sss Registered User Join Date: Nov 2002 Location: Mt. Vernon, WA.. Posts: 2,322	rearend crawl under the truck and get the #'s off the rearend. They are on the top side of the rearend housing on the drivers side.I have the codes to decifer the #'s if you need more help. Welcome to the board, another 67 shorty stepside........

05-14-2004, 08:00 PM	#8
Chevy67Tk Registered User Join Date: Mar 2003 Location: PA Posts: 349	You need to turn your wheel 1 full revolution and count the number of turns the drive shaft makes this will give you the ratio Example: for one turn of the wheel if the drive shaft turns 4.11 turns you have a set of 4.11 gears __________________



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