The 07's were the first year of the Mustangs to have the TPM system, and the your new rims and tires do not have the TPM module in them, you will have the low tire pressure warning,,,, If they did have the modules,,, you have to get the TPM monitoring system retrained to the new modules.
I have an 07 V6, and a month ago got a deal on some brand new 09 GT wheels, so my TPM flashes at me occasionally. Until i get the TPM system retrained, I'll be doing frequent pressure checks.
2007 4.0 V6 Mustang, Victim of Black Ice
1989 2.3 Fastback, traded for:
2011 3.7 Pony Package, Traded for:
2018 GT 5.0 Premium