Why JPEG at 40%?

The simple answer is that I have limited disk space, and JPEG is scalable while GIF is not. I know JPEG is not designed for images with large areas of solid color, but the JPEG's are 3 times smaller than the equivalent GIFs. I find the image quality acceptable, especially if you have a good quantizing algorithm in your viewer (XV's 8-bit "slow 24->8" give good results for me) or better yet have a 24-bit display.