fInitialDriveMaxFlatVel

The file holds km/h. At load the game divides by 3.6 and multiplies by 1.2 to get fDriveMaxFlatVel in m/s. Each gear's redline speed is that number divided by the gear ratio, and the top gear ratio is always 0.9.

Redline in top gear

fInitialDriveMaxFlatVel × 1.2 ÷ 0.9 = value × 1.333 km/h

Gear n redline

value × 1.2 ÷ ratio[n]

gtamods rule

TopGearMaxSpeedKph × 0.75, the same thing backwards

×1.32 rule

an upper bound, never a prediction

Measured cars

1.24 to 1.29 × value, fully upgraded

Why do measured cars fall short of the redline? Above 80 % rpm the engine only delivers 55 % of its torque, and drag at that speed already eats most of it. The car settles where the two balance, typically 93 to 97 % of the redline figure.

Set a Sultan to 300, expect 400 km/h from the rule of thumb, and get 227. That is not a bug. The gears now cover such a range that the car tops out in third, where drag balances the wheel force long before any redline. The gear ladder above shows it: the bars race ahead, the green line follows at a crawl.

The launch does not suffer in our model. First gear keeps its 3.33 ratio, it only lasts longer, and fewer shifts before 100 km/h even help: the Sultan goes from 5.9 to 5.3 s at 170. What you lose is the feel. Each gear spans a huge speed range, the revs barely move, and a stock-looking car that pulls 227 km/h in third is easy to spot on a roleplay server.

To actually go faster: raise fInitialDriveForce so the force line crosses drag later, or lower fInitialDragCoeff. Raise this field only until the redline sits a little above the speed you want.

The gear ratios and the high gear live in the vehicle, not in the handling entry. They are computed once from nInitialDriveGears and this value when the vehicle is created. Changing the field on a live car does nothing until it spawns again.

FiveM offers a way round for tests: set the ratios or the top speed modifier on the live vehicle instead.

-- change the ratios of the spawned car (gear 1..n)
SetVehicleGearRatio(veh, 6, 0.85)
SetVehicleHighGear(veh, 6)

-- or scale the top speed without touching gears
ModifyVehicleTopSpeed(veh, 15.0)   -- percent