C-20 Mileage.

[Longhorn321]

Originally Posted by Longhorn Man

not all eatons were no spin units. The no spin is actually uncommon.

Very interesting. I prolly should have noted that my knowledge is limited to one truck with this diff.

[Longhorn Man]

lol... it's all good. Before I found this board and was corrected, I always said my truck had a Dana rear, becouse *in my head* all 3/4 ton and one ton trucks had a dana.

[spinning wheels]

My 72 3/4 ton GMC with the 350/350 setup gets 8-10 mpg with an Edelbrock 1406 carb (600 cfm). Towing a car trailer dropped it down to 7mpg. Then again, I found that my fuel line going from the fuel pump to the carb was rotting apart. More gas was being dumped on the ground than in the carb.

The rear end gears are 4:10s. In '72 three different rear axles were offered in the 3/4 ton. I can't remember all of the names, its been a while. When I rebuilt the rear brakes, all I know is that I have the largest rear offered that year. Which is a good thing, cause it meant I got the biggest drum brakes.

My planned upgrade is to install a 700r4, cause at 70 mph, this truck screams on the highway, running on 31 inch tall tires. I picked up the trans at a wrecking yard for 80 bucks (half price day). Just need a stock torque converter and the TV linkage for my Edelbrock 1406.

Dtech... check and make sure your brakes aren't dragging. This could be a problem on older vehicles, as brake parts rust and corrode.

[Longhorn321]

Originally Posted by Longhorn Man

lol... it's all good. Before I found this board and was corrected, I always said my truck had a Dana rear, becouse *in my head* all 3/4 ton and one ton trucks had a dana.

Definitely always a chance to learn...I shoulda known that there was more than one possibility for a diff.. It's easy to assume but unless you have done the research, it's also easy to be wrong. The internet and this site are awesome things...to be able to share information is totally cool. Even with all of that tho, these things are 40 years old +or- a few years so to get it right 100% of the time is a task few would master. Kudos to those who get it right most of the time.

[Mayo]

Originally Posted by special-K

Your`s could be a 3.54 ratio with the Dana60.Back then it jumped from 4.10 to 3.54.That`s what my white`72 GMC has and I get 12.5 with 12.50/33s.

One of my dad's friends told me it had a 3.54 in it but my dad thought it was a 4.56. My brother and I have always assumed it was 3.73 or 4.10 for some reason. Is there any way to tell what gears are in it without tearing it apart?

[ed2552]

Yes, you can mark the tires, and mark the drive shaft, and see how many turns of the drive shaft it takes to spin the tires once. Jack it up, obviously. That's a good way to approximate it.

[spinning wheels]

Dtech, thought of one more item you should check.

Make sure your alignment is true. Bad alignment can really cut your fuel milage down. Not to mention low inflated tires.

[long67]

Mine is a 350/400 combo with 4.11 gears and gets 11 mpg. 3400 rpm at 70mph, it does not do that very often.

[ERASER5]

Originally Posted by ed2552

Yes, you can mark the tires, and mark the drive shaft, and see how many turns of the drive shaft it takes to spin the tires once. Jack it up, obviously. That's a good way to approximate it.

Not exactly. If you have a open diff, jack up one tire and rotate the tire 2X while counting the drive shaft turns. The spider gears take up half of the rotation, thus the need for 2 tire rotations.

If you have a posi of any kind, jack up both rear wheels and rotate the tire 1X while counting the drive shaft turns.

Take whatever drive shaft turns and compare that to the actual available gear set. For instance, if you count just over 3 turns, and you have a 12 bolt, then you have a 3.08

[Skirkpat]

Oh, one tip I have not seen yet is, drive it easy, like you have an egg between your foot and the pedal (yes I know I sound like a Grandpa) and no I do not drive like this, but it will drasticaly improve mileage...

[onlyifihadachevy]

you guys are getting around 11mpg with around stock size tires and no lift. Im going to hate the gas mileage with 410s a 6in lift 36s and california traffic

[C20-67_N_MO]

Is it me or does the exhaust look a bit restrictive?


Looks like one of the pipes are very narrow or its just the camera angle.

[Longhorn Man]

by the time the exhaust gets clear back there and through the mufflers, an over the axle pipe isn't going to restrict it much at all.

[67shortie]

Try bumping your initial timing to 12-15*, but keep your total timing to around 36* all in by 2500-3000rpm. If it detonates back it off until it doesn't and keep it there.

Also, I don't know if this needs to be said or not,but make sure you have the vacuum advance hooked up and working (not when settting your initial timing though, you will be way off if you do) and have it hooked up to a ported source on your carb. On a Holley this will be the port on the upper part of the pass side metering block.

Doing this will bring your idle speed down and keep your throttle blades out of the transition slots at idle keeping extra fuel from being pulled in without need.

Quadrajets ARE good carbs , but as mentioned can be finiky if not set up with the right factory combo. Jets, metering rods, power piston springs, main feed well resrictor sizing and float settings has allot to do with the carb being happy on any engine. There are quite a few of different setups from the factory, and as Longhornman said mass rebuilders don't care to much about this and put in whatever they have laying around and those carbs have more of a chance of running badly right out of the box than good....

Every little bit will help, pay attention to every detail and you will be rewarded.