head to head
Neither is meaningfully harder - they pose close to the same test. The widest gap is variance, and it is a single point.

Execution · Practiced Mastery
A gentle licensed racer testing basic driving lines against scripted opponents.

Execution · Practiced Mastery
A grittier crime story where weighty driving and gunplay carry the experience.
Cars leans Execution (Practiced Mastery); Grand Theft Auto IV leans Execution (Practiced Mastery).
Same score on execution - which means the number cannot separate them. The difference is in what each one asks of you.
Cars
Forgiving kart-style driving with modest racing line demands
Grand Theft Auto IV
Heavier driving and cover shooting reward control.
Same score on reads - which means the number cannot separate them. The difference is in what each one asks of you.
Cars
AI racers follow rubber-band scripts; split-screen rivals barely adapt
Grand Theft Auto IV
Scripted AI, no adaptive opponent modeling.
Grand Theft Auto IV leans a little harder on variance, though the gap is small.
Cars
Tracks and events are fixed once seen
Grand Theft Auto IV
Minor mission variance.
Same score on optimization - which means the number cannot separate them. The difference is in what each one asks of you.
Cars
Learning racing lines and shortcuts gives most of the edge
Grand Theft Auto IV
Mission routing and weapon picks learnable.
A matching score does not mean a matching test. Two games can both sit at 10 on execution and demand completely different things of you - which is why the reasoning above matters more than the number beside it.