I have a small problem with the handling editor, when I try to load my modified handling.dat (Which was modified with an older version of your handling editor) it gives me an error saying "The selected file doesn't appear to be the handling.dat from GTAIV.". What do I need to edit with notepad++ so I can load it in the newer version of your handling editor? Does it have a problem with handling extra handling lines? Cause I've added a few and those worked fine to edit with the older version but that's what I'll try first to change and see if it loads up then.
EDIT: Nope, deleting the extra handling lines did not fix the problem.
I have a similar problem. I'm using a realistic driving mod, which is just custom handling.dat files and makes the cars handle much better. It also has increased damage, but I find it's still too forgiving. I used to use the stand-alone handling editor to bump up the collision multiplier to make things more challenging. I recently re-installed GTA IV and EFLC and installed the realistic driving mod. When searching for the handling editor, I found the newest version of it is part of this new IV Needle tool. However it won't load the handling.dat files from the EFLC/TLaD and EFLC/TBoGT directories. I get an error saying: "The selected file doesn't appear to be the handling.dat from GTAIV." It loads the one in EFLC/common just fine though.
I managed to find a download of v1.4 of the handling editor, and it will load the above files just fine. I wonder if CoMPMStR is still active and can offer a fix.