Switching Rear-Ends - The 1947 - Present Chevrolet & GMC Truck Message Board Network

supermanox · 07-12-2005, 07:35 PM

I have a question. I have a 72 GMC and a 72 Chevy. The GMC has a 3:08 gear and the Chevy has a 3:73. The Chevy has a full cam and is really hitting a lick.
They both have 350 turbo's but the GMC has one hell of a shift kit in it. I'm thinking of switching the entire rear-ends, putting the 3:73 in the GMC and the 3:08 in the Chevy. Will the performance or mileage be significantly altered in either truck by this switch? Thanks for any help you guys can give me.

JimKshortstep4x4 · 07-12-2005, 08:04 PM

You will see a significant difference in both trucks. Your GMC will be a lot quicker off of the line (if you have traction) and the Chevy will be slower. Gas mileage will go up on the Chevy and down on the GMC.

Jim

supermanox · 07-12-2005, 08:53 PM

Any idea as to what the mileage will go up with the Chevy by, or down on the GMC by. Right now the Chevy is only getting 8 miles to the gallon. I'd like the chevy to be a little more drivable, I know getting rid of the full cam would help, but I love the noise. Any other comments on what the effect on performance would be? BTW, here is a link to pics of the chevy.

http://www.freewebs.com/supermanox/truck.htm

**Palf70Step** · 07-12-2005, 09:03 PM

With your engine mods in the Chevy, I think you'd be better off the way they are. The 3.08 will lower your RPMs, but if you have a heavy cam, it is probably going to want the revs and not lug at low RPMs. You might even see a loss in your gas milage. BTW, just so you'll know changing out the rear end gears would throw both vehicle speedos way out of wack. probably in the neighbor hood of 10-15 mph off.

To me if anything, I think I would switch tranny's. put all the mods in one truck, and use the GMC as a daily, stock driver.

supermanox · 07-12-2005, 10:10 PM

The chevy's speedo is way off now, from a previous gear change before I bought the truck. I didn't think that the chevy could get worse mileage with a higher gear due to the cam. Does anyone else think that a 3.08 gear couldn't handle a full cam or know of any sites that may have more information about this.

07-12-2005, 07:35 PM	#1
supermanox Registered User Join Date: Jun 2003 Location: Asheville, NC Posts: 36	Switching Rear-Ends I have a question. I have a 72 GMC and a 72 Chevy. The GMC has a 3:08 gear and the Chevy has a 3:73. The Chevy has a full cam and is really hitting a lick. They both have 350 turbo's but the GMC has one hell of a shift kit in it. I'm thinking of switching the entire rear-ends, putting the 3:73 in the GMC and the 3:08 in the Chevy. Will the performance or mileage be significantly altered in either truck by this switch? Thanks for any help you guys can give me. __________________ -Mike Cooper Asheville NC 1972 GMC Sierra Grande (Stock) 1972 Chevy (Not so much stock)

07-12-2005, 08:53 PM	#3
supermanox Registered User Join Date: Jun 2003 Location: Asheville, NC Posts: 36	Any idea as to what the mileage will go up with the Chevy by, or down on the GMC by. Right now the Chevy is only getting 8 miles to the gallon. I'd like the chevy to be a little more drivable, I know getting rid of the full cam would help, but I love the noise. Any other comments on what the effect on performance would be? BTW, here is a link to pics of the chevy. http://www.freewebs.com/supermanox/truck.htm __________________ -Mike Cooper Asheville NC 1972 GMC Sierra Grande (Stock) 1972 Chevy (Not so much stock)

07-12-2005, 09:03 PM	#4
Palf70Step State of Confusion! Join Date: Jul 2002 Location: Gulfport, MS USA Posts: 47,216	With your engine mods in the Chevy, I think you'd be better off the way they are. The 3.08 will lower your RPMs, but if you have a heavy cam, it is probably going to want the revs and not lug at low RPMs. You might even see a loss in your gas milage. BTW, just so you'll know changing out the rear end gears would throw both vehicle speedos way out of wack. probably in the neighbor hood of 10-15 mph off. To me if anything, I think I would switch tranny's. put all the mods in one truck, and use the GMC as a daily, stock driver. __________________ Bill 1970 Chevy Custom/10 LWB Fleetside 2010 Toyota Tacoma PreRunner SR5 Double Cab - DD Member of Louisiana Classic Truck Club (LCTC) Bill's Gallery Life isn't tied with a bow, but it's still a gift. Live simply. Love generously. Care deeply. Speak kindly. Leave the rest to God! Last edited by Palf70Step; 07-12-2005 at 09:05 PM.

07-12-2005, 10:10 PM	#5
supermanox Registered User Join Date: Jun 2003 Location: Asheville, NC Posts: 36	The chevy's speedo is way off now, from a previous gear change before I bought the truck. I didn't think that the chevy could get worse mileage with a higher gear due to the cam. Does anyone else think that a 3.08 gear couldn't handle a full cam or know of any sites that may have more information about this. __________________ -Mike Cooper Asheville NC 1972 GMC Sierra Grande (Stock) 1972 Chevy (Not so much stock)

07-12-2005, 08:04 PM	#2
JimKshortstep4x4 Senior Member Join Date: Jul 2001 Location: Muskegon,MI,USA Posts: 6,026	You will see a significant difference in both trucks. Your GMC will be a lot quicker off of the line (if you have traction) and the Chevy will be slower. Gas mileage will go up on the Chevy and down on the GMC. Jim



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