Multiclass and endurance racing in iRacing

Multiclass means two or more speed tiers sharing one track at the same time. The whole skill is reading closing speed, and almost every wreck you see is one car misjudging it. Get the blue-flag rule right and you avoid the single most common rookie mistake.

The class hierarchy and speed gap

The IMSA iRacing Series stacks three classes in one race. GTP (LMDh hybrid prototypes — 5 cars, top speed plus huge downforce) is fastest. LMP2 (the single Dallara P217, no hybrid) sits in the middle. GT3/GTD (10 manufacturers, all with ABS and traction control) is the slowest of the three. Lower-license multiclass uses slower pairings — LMP3 with GT4, or GT4 with the MX-5 — and Le Mans context adds GTE.

Prototypes lap GT3 cars by several seconds a lap, and on a straight the closing speed is large enough that the GT3 driver often can’t judge it from the mirror. One quirk helps: prototypes (GTP, LMP2, LMP3) run the left or right of the road, while GT3, GTE, GT4 and Cup cars run the crown in the center. Knowing where the faster class wants to be tells you where not to go.

Blue flags are information, not a command

iRacing is not F1, and getting this wrong fills the protest queue. From Sporting Code 7.4:

7.4.1. A blue flag with a diagonal yellow stripe indicates faster cars are approaching. This flag is informational only. 7.4.2. In all cases, it is the responsibility of the faster car to safely overtake the slower car. It is the responsibility of the slower car to maintain a consistent line.

The blue flag does not order you to move over. It tells you traffic is coming. Be predictable, not polite.

If you’re the slower car

Hold your line. Do not dive off-line to “make room” — that puts you exactly where the faster car already committed to pass, which is how the slower car ends up wrecking the leader of another class. Check mirrors, lift on the straights where you can’t lose track position, and let them through cleanly. Consistency is what they need from you.

If you’re the faster car

The pass is your responsibility, full stop. Going side by side through a corner costs less time than waiting for a clean run, so commit rather than hover. Pick low-risk zones — long straights and big braking areas — and assume the GT3 ahead can’t see you until you’re alongside.

Lap 1 and cold tires

The first two laps are the highest-incident window in any endurance race. Cold tires arrived for GTP first, then LMP2, GT3, and now GT4. Brake roughly 20 m early on lap 1 and 10 m early on lap 2 until the rubber comes in. Watch for cars rejoining the racing line after an off — they’re slow and cold and not where you expect them.

Driver changes

A driver swap needs the car stationary in its own pit stall. A 30-second swap timer starts when the car stops and runs concurrently with fueling, tires and repairs, so it usually costs no extra time. The outgoing driver holds the exit button (the same one you use to leave the car in practice) or the incoming driver removes them. Teams need a minimum of 2 drivers and a maximum of 16. The setup is loaded once and locked for the entire car through every swap — mid-race you can only change what the in-car black box exposes.

The classic trap: leaving the driver-swap toggle ON in the pit menu during a solo team session swaps you out for an AI driver. Turn it off when you’re running the team car alone, or your laps may not count.

Stint and fuel planning

Grid with max fuel. Taking less is basically a self-imposed penalty with no upside. Under the current tire model, tires double-stint fine if you don’t overdrive them — the old “new tires every stop” advice is outdated, and a set is still in good shape at the end of a 20-minute race.

When you do change, take all 4. A four-tire stop is about 9 seconds, covered by roughly 18 L of fuel, and the pace gain is usually worth it even when the fuel pumped doesn’t fully cover the tire time. Don’t overfill beyond that — excess fuel adds pump time and weight. Set the auto-fuel margin to about 1 lap to guarantee you finish without carrying dead weight. To save fuel, lift about 1 second before braking, back up your braking points and run slightly tighter lines; raise the fuel-save target in strong-draft weeks like Watkins Glen, where the tow lets you back off the throttle and still hold pace. The race strategy page owns the full fuel math and stint-length numbers. See pedals and racecraft for the braking consistency this depends on.

The iRacing endurance calendar

The big multiclass team events are the 24 Hours of Daytona, the 12 Hours of Sebring, Petit Le Mans, the Bathurst 12 Hour, the Nurburgring 24, the 24 Hours of Le Mans (Global Endurance Tour), and the 6 Hours of the Glen and Spa. Team Driving must be enabled per event, and your split is assigned by combined team iRating, so the top splits hold the fastest teams in the world. Startlight shows what iRacing session is running now, what’s next, and time-to-green, so you know exactly when your stint and the green flag land.