Since channel 40 is worse than channel 1, the antenna is still too long for the setup. Keep trimming until it gets better. The goal is to get channels 1 and 40 as low as possible, and 20 should be a little lower than the extremities. However, the short antennas are not as wide-banded as longer antennas, so they don't work as well (except for the fact that they are much more forgiving of low obstacles such as garage roofs, which is why I use short antenna myself when needed.)
My magnet-mount K30 antenna had to be trimmed from its 35-inch starting point to 32.5 inches. I stopped there when the SWRs got to: Ch 1 =1.1, Ch 20 =1.2, Ch 40 = 2.9. I could have tried a little more cutting to get 40 a bit lower, but all of my usage is on channel 19 for traffic reports, so I left it as it is. With such a short length, I knew the little shorty was probably not flexible enough to get too much better anyway,
My antenna is mounted on the roof of my sedan. I can also mount it on the trunk, but the SWRs are not quite as good there because the little trunk is not as good a ground plane as the big roof.
As I said, the little short fellas are not as wide-banded as the tall guys, but they sure are handy when the ceiling is low!
I also have a couple of Wilson antennas, the 500 and 1000 mag mounts. I've had a lot of luck with these new-era Wilsons no matter where they are made.
I specialize in mag mounts for the ultimate in no-new-holes-needed flexibility.
Addendum: The principle to antenna tuning is, if the SWRs are lower on Channel 1 than on Channel 40, the antenna is too long. If the SWRs are lower on Channel 40 than on Channel 1, the antenna is too short. There's not much you can do if the antenna is too short except getting a longer one, but you can trim one that is too long.