I just took a drive with the AC running and it seems to only cool down 10 degrees compared with the outside temp with full fan running at the vents. However with the lowest fan position, the air coming out was over 18 degrees cooler than the outside. Why is that? How cold should the center vents be with a properly charged system at full fan?

I have not recharged as I am debating on R12 or R134.

This is a 89 300E.

Thanks.