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Thats easy, The one thats been tuned to.Ok, sorry. I couldnt resist.
I have successfully tuned both of those to get into the mid to high 30mpgsUSg., on a 360 of closer to 400 hp.I dont recommend you try it. Just saying it can be done.If mpg is important to you, the rest of the combo is very important , as well as powertrain choices and chassis set-up.
Being limited to these two carb choices, I would choose the carb with the metering rods, and purchase the tuning kit on the same orderform.
The Holley is popular because its quick and easy to make WOT stumble-free power, on almost any combo that it has been correctly sized for.Theyre not known for tune-ability in the circuits that affect fuel-ecomomy.
The metering rod carbs, take a bit more time/effort to tune for WOT power. But they have the distinct advantage of being adjustable in the circuits that relate to low speed and fuel economy.Edebrocks are a fine example, and are well-supported in the tuning department. If you take the time to learn and understand them, and the rest of your combo is set up to advantage them, you will not be disappointed.
I always preferred the Carter AVSs with the adjustable air valve secondaries, until the TQs came out. Unfortunately the TQ is no longer supported. And the AVSs were a short run effort, and three-step rods for them are hard to find.Choose the engine/chassis build carefully, and you will be rewarded with an excellent very driveable vehicle.And those are often the most fun.