Be careful going to too-tall gearing, either with a high rear end gear or an overdrive. Once the intake manifold vacuum drops due to lugging, the power valve in the carburetor opens up and gas mileage drops precipitously. Letting the engine wind up a little and keeping the manifold vacuum high is the best way to get good fuel economy, coupled with proper ignition timing. One degree too much timing advance drops as much power as 5 degrees retarded from the best power setting. I've proved that repeatedly on dyno runs.
There are two possibilities for good gas mileage from a 292:
1. Slim
2. None
Jerry
