The 6 cylinder engines were made by IH until the switch in 69 to the AMC engines. In a D series (square) pickup or travelall, it's an AMC 6. In 68 or earlier, it's a 220, 240, or 265 IH, and those are 650 lb engines. They get about 10-20% better fuel economy in my experience, have a very flat torrque curve, and will only turn up to 3400 rpm continuous, so you lose some top end. I have a 68 travelall that gets 17 when properly tuned and the speed is kept below 55. Without the right carb, it drops dramatically.
Originally Posted by damnesia
In your truck, speed is the key. It's not the rpms, but rather the wind resistance that makes the difference. If you have a running V8 already, keep it. For slightly better economy, fuel injection would be easier and cheaper.
A properly tuned 304 can get a consistent 14-15 mpg without much effort at 50 mph. At 65, figure 11-12 mpg. That's my personal experience.
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