Checked that, indeed. Some cars already have value OFFROAD_ABILITY1 or 2.
Offroad wheels won't affect them.
Here list:
only OFFROAD_ABILITY1 - Offroad wheels won't matter:
Barracks
Bobcat
FBI Rancher
Landstalker
Picador
Rancher
Rancher Lure
Sadler
Sadshit
Towtruck
Yosemite
only OFFROAD_ABILITY2: - THESE WON'T MATTER AT ALL. MOREOVER, THIER OFFROAD SURFACE GRIP IS BETTER THAN ANY CAR WITH OFFROAD WHEEL
Alpha
Banshee
Buffalo
Bullet
Cheetah
Coach
Phoenix
Police LS
Police LV
Police SF
Premier
Super GT
Taxi
both OFFROAD_ABILITY1 and OFFROAD_ABILITY2:
Bandito
BF Injection
Dozer
Dumper
Dune
Kart
Monster
Monster A
Monster B
Mower
Patriot
Police Ranger
Sandking
Sweeper
Tractor
Tug
All other vehicles get OFFROAD_ABILITY1 when put offroad wheels