'75 360 Rebuild, Cam?

Bad mileage. I don’t think so. You could loose some mileage. It is possible. It’s a catch 22. The stock cylinder heads will only breath in so much. The most they’ll breath in is only as much as they flow and the amount of air and fuel it breaths in is limited to the duration and lift of the cam. Increaseing breathing ability will consume more air and fuel. But only as much as the gas pedal is pushed & the cam opens the valve in duration and lift. Just a slight bit more.

This increases efficiency of the cam and the power it makes with the available cam specs. Or in other words, more power without using a bigger cam which for sure will consume more fuel.

Mileage will hardly be effected. I think you’ll suffer a mileage loss due to stepping on the throttle harder more often. The throttle opening amount will not change a lot. You’ll make more power at a lower rpm which is a slightly more closed throttle position because it is more efficient. Because your making more power at a lower rpm, you should still get good mileage. Though a cam of this size, mileage will suffer a little. The wife reports about 16 on the average. I don’t have any straight answers on exactly what the car is getting due to several factors starting with inaccurate speedometer & a inaccurate wife with inaccurate mileage divided by fuel pumped in at the pump. She drives the Hwy. mostly.

The bad gas mileage most cars suffer from are due to big cams, poorly balanced combinations, high stall and gears, I’ll timed and poor ignition, I’ll tuned carburation. In your particular combo, one of the best bang for the buck mods you can afford yourself is an overdrive transmission. There are other tricks to pull as well. A small carb is good for mileage but bad for top end. The use of a spreadbore carb like a TQ or QJ, or fuel injection, multiple spark ignitions, wider spark plug gaps, variable duration lifters.

I don’t know what you would consider bad mileage or what is acceptable for you. Dropping the cam duration 10*’s or more would be a help. So would changing the rear end ratio to 2.76. But then I say, where’s the fun? With those two changes alone, you might as well just slide in a 2bbl carb and stock engine.

As her car sits now, I wouldn’t have a problem driving a few hundred miles or hours.

I’ll report back what she tachs at Hwy. speeds. IIRC, it’s 3K.