transmission rearend question

[peeweecollins]

I have a 81 dodge pickup I wanted more power so I took the slant six out with the manual tranny and put a 318 with the 727 torqueflight and now I can't keep gas in it, I was told that the slant six had taller gears for the power I might need to change the gears in the rear end but I know nothing about gear ratios. any solution might help thanks!

[acton mike]

This may seem like a dumb question - but is the auto transmission shifting into top gear

[bherder]

How much 'can't keep gas in it' is what, exactly?

You're never going to get the milage from a V8/Auto that you did from a Slant-6/Manual .... So what kind of milage drop are you talking about?

[sgillett]

Do you have a tachometer?
What is the RPM at 60?

[mikehopkinsjr]

My 78' w-150 has a 318-727 auto, my dad said it may have a shift kit in it, anyways 3.55 gears, 33 inch tires. It sort of shakes sometimes leading up into what I think is second gear. Gets 10-12 mpg and will only get up to around 70-80 on a very long run (stops shifting after 50-60 or at least it isnt noticeable at all if it is).

[Speed Dragon]

Damn, a V8 gets worse mileage than a 6-banger?!? And here I thought I had a hole in my tank or something.

[B-300]

The /six will out run a V-8 in miles. The smaller bore is more efficient in the surface to volume ratio... This is why most newer engines are closer to the same bore and stroke. Also there is less parasitic loss from 6 cylinder and fewer bearings, along with the fact the V-8 is loafing were the /6 is breathing deeper and more efficiently.
(The internal combustion engine is most efficient at/near full load.)

[Speed Dragon]

Actually the Slant 6 has 7 mains, IIRC. Still, it also has the most efficient oiling system of any of the Chrysler engines, good to 7500 stock I've heard.

[TOWTRUCKING]

You Went From A 6 Cyl To A V-8, You Are Experencing "gas Shock" You Are Having To Put Gas In More Frequently. I Went Through That When I Went From 6 Cyl 3spd Od To 360 Auto.

[farmguy]

Geez guys,,,,,,,,,, my `87 D150 is bone stock yet, 318 w/ 904 tranny, kicking around here i get around 15 -16, on highway if i keep it under 65, I can squeeze almost 18... I recently replaced the choke pull off, and the mileage picked up again, and the sooty black tail pipe went away.......SOmething to keep a check on.....

[dr. bob]

Back to the original question...the slant six and v-8 equiped models both came with a variety of rear ends, with 3.21, 3.55, or 3.90 ratio gears. You get the best acceleration with the 3.90, and the best fuel mileage with 3.21. So, if you currently have the 3.90 or 3.55, you would see an increase in MPG with the 3.21 gears. But realistically, 12-15 MPG is a decent figure to get for this truck, so it might not be worth the effort and cost to change it out. For the novice mechanic it is far easier to simply swap the entire rear end than it is to try and change the gears.
So the first step is to find out what gear set you have. Check the rear end for a brass tag under one of the cover bolts.

[B-300]

The 225 slant 6 I took out of my '78 van had 4 main bearings, not 7. It ran completly out of breath just under 5000rpm... I wouldn't rev it higher than 4500 with that 4.+" stroke on only 4 mains.