system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Which transmission gives more mileage and why?

I drive a manual transmission car and I find my mileage to be comparable if not lower to an automatic transmission car. While activating cruise control, my mileage rises by a few kms because of this I have always wondered which transmission for my next car would be the best, considering mileage to be an important factor.
gpt
The blunt truth: Driver skill/style is 10x more important than transmission. 

If fuel consumption is a priority:

1) Drive at the speed limit. EVERY car (other than sports cars perhaps) will be a LOT more fuel efficient at 65mph or less. This can easily save 10-15% on fuel consumption on highways.

2) Accelerate gently and anticipate slowing down and coasting to more gentle stops. In stop and start traffic, this again easily saves 5-10% on fuel consumption.

3) Maintain tire pressures, tire alignment and maintenance of engine.

The differences in fuel consumption between a modern automatic transmission and a manual are small, if any, if a driver practices the above.

This is a very easy thing to test. Drive slower and less aggressively and monitor mileage. Do the comparison.