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GMC V6 and V12 Engines Engine repair and rebuilding

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  #1  
Old September 21st, 2014, 11:14 PM
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Quigley Quigley is online now
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Default Fuel Mileage

Just got home from a 250 mile ride with my 66 C-1000.It has 305E with a Powerglide and Dana44 with 3:07 gears. I have a 500 Holley on it with Pertronix ignition,plenty of advance and the plugs look perfect. Unleaded gas with No alcohol. 11.5 MPG,what do you think,is that about what I should expect? Thanks, Dave
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  #2  
Old September 22nd, 2014, 12:11 AM
Clyde Clyde is offline
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Default Re: Fuel Mileage

My 1966 GMC 3/4T, 400 Auto, 351 V6, Gear Vendor OD, 4.10 rear gear. Holly 500 re-jetted leaner and HEI, I average mixed driving 12 MPG.
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Clyde McKaba
Abbeville,SC
1961 GMC Suburban 4WD 305 V6, 4spd
1966 GMC 3/4T P/U 351 V6, Turbo 400,Gear Vender OD
1969 GMC Suburban 1/2T
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  #3  
Old September 22nd, 2014, 04:47 PM
Jim A Jim A is offline
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Default Re: Fuel Mileage

Over 44 years now on my '60 3/4 T, 4 speed, all stock with 1 bbl Holley.
On 1000 mile freeway trips, 2 ton gravel hauls and all day granny-gear firewood excursions: 10 mpg.
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Old September 22nd, 2014, 05:24 PM
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aphaynes aphaynes is offline
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Default Re: Fuel Mileage

Only a few months with my '65 1000, 4 speed, Pertronix ignition and coil, new plugs and wires, 305E still with Stromberg carb. First official mpg check of mixed (but mostly city) driving, 10.5 mpg. Have not bothered with hwy much since it seems to be winding out at 65 mph. I have not verified my rear gears yet.
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  #5  
Old September 23rd, 2014, 04:03 AM
BarryGMC BarryGMC is offline
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Default Re: Fuel Mileage

Quigly. The 3.07 1/2 tons I have had all got about 11-12. All stock with 700.16 tires. The worst have been the 4x4 trucks. 8-10 average. Compared to my current 72 k20 that I have pulled some 14-15 avg. My best v6 mileage is my 60. Its overdrive watsson 2021 and 3.54 gears with 235-85.16 tires have given me a 14 mpg average at 65 for over a 100 mile road trip. Barry
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  #6  
Old September 23rd, 2014, 04:10 AM
BarryGMC BarryGMC is offline
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Default Re: Fuel Mileage

I should add the 60 is a 305a with the one barrel Holley and a hydramatic with PS and pb and points ignition. My 72 is a 350 with a 4l80e and 4.10 gears and tall 285-85-16 tires.
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  #7  
Old November 16th, 2014, 08:48 PM
tommr tommr is offline
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Default Re: Fuel Mileage

401M in our 47 Flxible Clipper has gotten into the 8s when we are rolling light on flatter highways. if we are heavy and/or in hills we get 6-7mpg. Stromberg 2bbl, points ignition and Spicer 4 speed 1:1 direct drive.
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  #8  
Old November 17th, 2014, 12:32 AM
Clarke Clarke is offline
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Default Re: Fuel Mileage

I have the stock 305D/4 sp trans/carb/ignition with 238-85.16 tires. When I was running 4.56 rear end, I was consistently getting 9.0 mpg with some in-town driving, but mostly highway (not exceeding 65 mph). Then after I swapped the 4.56 for a 3.54, I consistently get 11.3 mpg driving the same route (to/from work), except I run 70-75 mph.

Towing my 3,000 lbs camper (with or without my canoe), I'm getting 7.0 mpg running 55-65 mph in the flat lands.
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  #9  
Old May 2nd, 2017, 02:06 AM
James James is online now
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Default Re: Fuel Mileage

Yesterday I finally got to drive my truck around just to check my mileage. I travel 58 miles round trip. My mileage came out to be 12.9 mpg. Half of the driving was at 50 mph and the rest was at 70 mph (just purring). My 1964 1500 truck has the stock 2 barrel carburetor, points ignition (engine is stock, I have not updated anything on it yet), 31.7" diameter tires, the SM420 transmission, and a 3.54:1 rear axle ratio. At 70 I calculate about 2650 rpm (I need to get my tachometer connected.).
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  #10  
Old May 2nd, 2017, 03:11 AM
BillT BillT is online now
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Default Re: Fuel Mileage

Before about a 500 mile road trip, I "super-tuned" my '66 GMC 1000, 1/2 Ton Short Bed with a 305E, 3-Speed on the column. Not sure of the rear gears, but they were the typical stock type gears of those years (It was revving at 60, lol).

The engine was 100% pure stock. I filled the tank to the top to get the most accurate reading and I got 15 1/2 MPG on that trip.
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Drove:
-'60 GMC 2500 Dump, 305 V-6
-'62 GMC 6500 Flatbed Dump, 478 V-6
-'67 GMC 1500 Pick-up, 305 V-6
-'71 GMC 9500 Flatbed Dump, 637 V-8

Owned:
-'62 GMC 4000 Flatbed, 305 V-6
-'64 GMC 1500 Pick-up, 305 V-6
-'66 GMC 1000 Pick-up, 305 V-6
-'67 GMC 7500 Box, 6V-53 Detroit
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