I've looked at this in the past. As you probably know, both JPG and PNG files already have heavy compressions. After I applied aggressive zlib compression, I got 16% compression on PNG based maps and 3% for JPG based maps. And this was for compressing the whole GPSFS file and NOT individual tiles. If I did individual tile compression, the result was less than half of this due to per-tile compression table overheads.
The CPU load on the PSP for decompressing (inflating) was significant, thus battery life could be reduced as much as 30~40 percent! I don't think it's worth it...
Cheers,
MIB.42
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