[E4ODnut]

Most factory EFI systems will hold the A/F ratio to about 14.7:1 in cruise conditions. Some will allow open loop idle at slightly richer. Most will go into open loop full throttle at about 13:1 or richer.

I've spent a lot of time and effort tuning my Ford 300 CID 6 for lean cruise. This is a low tech, low compression engine, but I find I can run lightly loaded as lean as about 18:1 before I get a lean stumble. In theory that should yield about a 22% improvement in economy. Real world is different. At this ratio the power output decreases significantly, so you have to open the throttle more allowing more mix into the engine to compensate. You are still ahead to run lean, but not nearly to the extent that one might hope for. The other thing is that with the lack of power with lean mixtures, the engine will spend less of it's time in this range, so the benefits are limited by this factor as well.

The 1L Metro engine spends quite a bit of it's time with relatively large throttle openings which reduces pumping losses to begin with. This will make it harder to get gains in economy from leaning out because the engine is already sized right for the job.

I'm not saying it can't be done, but the gains will be small and it will take a lot of effort to get them. If you're into it though, the experience can be lot of fun.