Gas Mileage

[Clarkwb100]

My "87 El Camino sat for several years until renovation. The 305 was replaced with a new Goodwrench crate 350/300hp motor from Jeg's with an Edelbrock manifold, Holley Avenger (650?) carb, complete 2-1/2" dual exhaust. The radiator, water pump, and flex a lite fan are new. Also running a helper electric fan. DIstributor is Jeg's HEI. WIres are Excel

The 200r four speed auto has been gone thru and works great.

Only the cast iron exhaust manifolds from the 305 remain.

STarts right up with fast idle at about 1100 rpm, after warming drops to about 800rpm.

Running 255r60-15 tires on Chevy Rally wheels, did not recalibrate the speedometer or odometer

No smell of fuel when running, either under hood or at exhaust

Car is responsive with no carb lag and seemingly runs great (although it won't get enough RPM to lay rubber in first)

AC runs full time here in Texas

I'm consistently getting 9, yep, nine, MPG. Shouldn't this be much higher? Please offer your thoughts.

Thanks -- Bill

 

[565bbchevy]

I'm consistently getting 9, yep, nine, MPG. Shouldn't this be much higher? Please offer your thoughts.

Did you say you were running a 565 BBC with no overdrive, sounds like my car but I expect it.
What is your rear gear?

 

[olds307 and 403]

You have a fairly big cam, 222 duration and lower compression. It shows 10 to 12 inches of vacuum, the carb needs to be tuned accordingly. You want a 2500 stall lock up converter and at least 3.73 gears. Your efficient cruising rpm won't be at 1500 rpm, more like 2000+ and may never be good. This is why people go LS. Your motor lacks bottom end just like a LS but won't even have the horsepower or fuel mileage of a stock 5.3 with basic bolt ons.

 

[DRIVEN]

I'm curious about gearing as well.

 

[Clarkwb100]

I believe the rear gear us 2.56

 

[TURNA]

Gas mileage is irrelevant!!!!!!!!!!

 

[olds307 and 403]

I believe the rear gear us 2.56

As said, go to a 3.42 gear minimum with that cam. With a taller tire 3.73 will be no issue either.

 

[olds307 and 403]

Use the calculator, 2004R .67 OD and lock converter already calculated in. You should turn 1300 rpm at 60 mph!
OldsGmail.com Reference Manual

OldsGmail.com Reference Manual

 

[airboatgreg]

Use the calculator, 2004R .67 OD and lock converter already calculated in. You should turn 1300 rpm at 60 mph!
OldsGmail.com Reference Manual

OldsGmail.com Reference Manual

Possible lugging at that RPM and allowing power circuit in carb to engage causing poor economy also

 

[lilbowtie]

I do better than that with my V-10 motor home.