Relative Fuel Economy LS3 stock vs built

[srode]

I'm planning on using an LS3 in my 69 Camaro build as the powerplant and debating on choices between blown or naturally aspirated and would like get to something in the 600+ HP range. One of the benefits I would like to see is much better fuel economy than I get on my other 69 which is a old school 400 sbc roller cam motor around 450hp/500# torque that is a fuel sucking pig. So I'm curious what people are seeing for fuel economy out of LS3 builds both stock and built. Fully understand there's lots of variables like gearing, body weight, how a person drives so to narrow it a little would like to hear highway economy 65 to 70mph and brief description of the drive train and body. Thanks in advance!



On edit - to be clear the concern for fuel economy is not so much about the cost of the fuel as it is about the inconvenience of stopping for it frequently on longer trips.

[130fe]

I have an LS3/T56 close ratio magnum in my car with 3.89 gears and 25.6” tires. Gas mpg is relative to your left foot �� In town I get around 17 and on the highway at 75 I get around 20. I had an LS T56 before with the same combo (.5 6th gear) and got about 3 more mpg on the highway.

[dhutton]

Around 20 if you drive easy.

Don

[srode]

I have an LS3/T56 close ratio magnum in my car with 3.89 gears and 25.6” tires. Gas mpg is relative to your left foot �� In town I get around 17 and on the highway at 75 I get around 20. I had an LS T56 before with the same combo (.5 6th gear) and got about 3 more mpg on the highway.

Thanks, good information. Would you describe your LS3 as fairly stock or built? Lopey/choppy idle?

[r67nova]

I have a LS3 with a 2.9L Whipple Supercharger and a close ratio T-56 magnum in my 1967 Chevy II. I have no problem getting in the 20's on the highway. 3.55 rear gear. You will get better overall fuel economy with a mild built boosted LS3 than a N/A wicked built LS3 to achieve close power levels. With a supercharger or turbos you only use additional fuel when you are in boost. The rest of the time you are in "economy" mode. If you build a high compression big cubic inch LS it will need more fueling all the time.