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07-03-2010, 11:14 PM
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#1 |
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Registered User
 Join Date: Jul 2009
Location: Houston
Posts: 7
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Better MPG?
Ok guys and gals, help me justify purchasing a '67 GMC 3/4 ton LWB Stepside that has a 305 that needs to be replaced. The truck needs total resto but I can get it for $600 or maybe less. But of course I need some rationalizations to help convince me that I need a project truck.
Since I will be replacing or rebuilding the 305 V-6 I was thinking I would install a FI engine and change the rear end gearing. If I were to do so, does anyone have any idea of what kind of MPG I might hope for. Any suggestions on gear ratio & cam? The truck would be a weekend cruiser and backyard project hauler. I am not interested in the fastest 1/4 mile or pulling a 5k lb tow. Although a boat may be in the future. |
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07-04-2010, 01:47 AM
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#2 |
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Registered User
Join Date: Apr 2005
Location: Chula Vista, California
Posts: 495
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Re: Better MPG?
If it were me I would rebuild the V-6 and install a Holley 2-barrel and a Buick V-6 HEI conversion distributor on it. Split the exhaust into duals using small diameter tube,(1 5/8 to 1 3/4") and change the rear gears to 3:54. Run the tallest, narrowest tires you can, aired up to about 60 psi. and you may squeak 14-15 MPG out of it.
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rwgregory 67 GMC 2500 ,2w/d wide box 305 V-6, 4-speed |
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07-05-2010, 01:54 PM
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#3 |
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Registered User
Join Date: Mar 2002
Location: Dallas, TX
Posts: 555
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Re: Better MPG?
I second that. Keep the Big 305 V6. But if you decide to remove it, let me know. I would hate to see it go to scrap. I designed the Buick HEI conversion and it works great. The Holley is well worth the money too.
Erl
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1968 GMC 305 V6 3/4 4x4 Stepside 06 Corvette Daily Driver Dallas TX |
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07-05-2010, 02:09 PM
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#4 |
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Between Trucks...
Join Date: Mar 2002
Location: Los Angeles, CA.
Posts: 3,830
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Re: Better MPG?
Assuming an Eaton rearend, best gears you can hope for are 4:10. There were 3:73 aftermarket gears for them back in the day, but they are non existent now.
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Beat it to fit, Paint it to match... 
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07-05-2010, 03:12 PM
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#5 |
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Registered User
Join Date: May 2010
Location: Austin Texas
Posts: 84
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Re: Better MPG?
If you swap it over to an LS motor with OD trans you could get pretty decent mileage. For the rear I'd look for 3:40ish or 3:08 gears to get the best mileage. I read somewhere that a gen III 5.3 could make around 20mpg on the hwy if it's tuned and geared right.
Someone said that 3:73 gears are rare,I had not heard this. Seems like summit racing sells them, unless the 12 bolt parts I was looking at were the wrong parts. |
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07-05-2010, 03:15 PM
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#6 |
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Between Trucks...
Join Date: Mar 2002
Location: Los Angeles, CA.
Posts: 3,830
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Re: Better MPG?
He has a 3/4 ton truck. Assuming it has an Eaton and not a Dana, gearsets aren't made for them anymore.
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Beat it to fit, Paint it to match...
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07-05-2010, 03:45 PM
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#7 |
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Registered User
Join Date: May 2010
Location: Austin Texas
Posts: 84
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Re: Better MPG?
Oh got ya. Didn't realize that. Could always swap to a half ton rear I would think.
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07-06-2010, 12:37 AM
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#8 |
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Registered User
Join Date: Mar 2003
Location: San Francisco, CA USA
Posts: 2,454
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Re: Better MPG?
Being that the truck is a GMC, my money is on it having a Dana.
Ray |
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07-06-2010, 12:59 PM
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#9 |
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Registered User
Join Date: Nov 2006
Location: Portland
Posts: 382
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Re: Better MPG?
I would keep the 305 V6 as they are uncommon now. As for MPG, going with 3.08 gears, 2 barrel carb, and the HEI, I wouldn't expect anything better than around 15 MPG.
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