Here is a gear/speed/rpm chart for the Metro with its current transmission (4.390 differential ratio). I say current transmission because my 4th gear grinds and if I find another one at a good price I will probably nab it up. Who knows what ratios it may be, but this is the highest (aka worst for mileage) transmission that came with the Metro. All the other ones are geared taller.
For comparison, here is the tallest trans (3.524 differential ratio) you can put in the Metro.
The difference between the two at 45 mph (my typical cruising speed) is pretty huge. The 4.39 gives me 2219 rpm, and the 3.524 gives me 1779 rpm, a 440 rpm difference.
I'm not looking to design a header that is perfectly designed for 2200 rpm @ 45 mph. I'm just not that good, plus I think it would really choke the high end like Darcane said. I'm really looking to design something that gives better fuel economy than stock without absolutely murdering the max power output. I'm fine with sacrificing some high end.
For future reference here are the gear ratios:
95-01 1.0L hatch and convertible
3.420
1.890
1.280
0.910
0.760
4.390
89-94 base, lsi, gti
3.420
1.890
1.280
0.910
0.760
4.110
XFI & 1.3l
3.580
1.890
1.280
0.910
0.760
3.790
89-94 swift 1.3l
3.580
1.890
1.280
0.910
0.760
3.524
Tire diameter is 22.76"