FiveM handling editor

Goes in your resource's data folder, with a data_file 'HANDLING_FILE' line in fxmanifest.lua pointing at it. This is the version you ship.

<?xml version="1.0" encoding="UTF-8"?>

<CHandlingDataMgr>
  <HandlingData>
    <Item type="CHandlingData">
      <handlingName>ADDER</handlingName>
      <fMass value="1800.000000" />
      <fInitialDragCoeff value="7.800000" />
      <fPercentSubmerged value="85.000000" />
      <vecCentreOfMassOffset x="0.000000" y="0.000000" z="0.000000" />
      <vecInertiaMultiplier x="1.000000" y="1.300000" z="1.500000" />
      <fDriveBiasFront value="0.200000" />
      <nInitialDriveGears value="6" />
      <fInitialDriveForce value="0.320000" />
      <fDriveInertia value="1.000000" />
      <fClutchChangeRateScaleUpShift value="3.000000" />
      <fClutchChangeRateScaleDownShift value="3.000000" />
      <fInitialDriveMaxFlatVel value="160.000000" />
      <fBrakeForce value="1.000000" />
      <fBrakeBiasFront value="0.450000" />
      <fHandBrakeForce value="0.700000" />
      <fSteeringLock value="42.000000" />
      <fTractionCurveMax value="2.500000" />
      <fTractionCurveMin value="2.380000" />
      <fTractionCurveLateral value="22.500000" />
      <fTractionSpringDeltaMax value="0.150000" />
      <fLowSpeedTractionLossMult value="1.500000" />
      <fCamberStiffnesss value="0.000000" />
      <fTractionBiasFront value="0.485000" />
      <fTractionLossMult value="1.000000" />
      <fSuspensionForce value="2.400000" />
      <fSuspensionCompDamp value="1.400000" />
      <fSuspensionReboundDamp value="2.100000" />
      <fSuspensionUpperLimit value="0.120000" />
      <fSuspensionLowerLimit value="-0.100000" />
      <fSuspensionRaise value="0.000000" />
      <fSuspensionBiasFront value="0.500000" />
      <fAntiRollBarForce value="0.900000" />
      <fAntiRollBarBiasFront value="0.600000" />
      <fRollCentreHeightFront value="0.410000" />
      <fRollCentreHeightRear value="0.410000" />
      <fCollisionDamageMult value="0.700000" />
      <fWeaponDamageMult value="1.000000" />
      <fDeformationDamageMult value="0.700000" />
      <fEngineDamageMult value="1.500000" />
      <fPetrolTankVolume value="65.000000" />
      <fOilVolume value="5.000000" />
      <fSeatOffsetDistX value="0.000000" />
      <fSeatOffsetDistY value="0.000000" />
      <fSeatOffsetDistZ value="0.000000" />
      <nMonetaryValue value="80000" />
      <strModelFlags>440010</strModelFlags>
      <strHandlingFlags>0</strHandlingFlags>
      <strDamageFlags>0</strDamageFlags>
      <AIHandling>AVERAGE</AIHandling>
      <SubHandlingData>
        <Item type="CCarHandlingData">
          <fBackEndPopUpCarImpulseMult value="0.075000" />
          <fBackEndPopUpBuildingImpulseMult value="0.030000" />
          <fBackEndPopUpMaxDeltaSpeed value="0.250000" />
        </Item>
        <Item type="NULL" />
        <Item type="NULL" />
      </SubHandlingData>
    </Item>
  </HandlingData>
</CHandlingDataMgr>

A client script. Get in the car, type /handling in the F8 console, and the values are on the vehicle you are sitting in — no restart, no cache. The float and vector setters are documented; pushing the three flag words through SetVehicleHandlingInt is community practice rather than something the native reference states, so treat those three lines as the part to verify yourself.

local MODEL = 'adder'

local FLOATS = {
    fMass = 1800.0,
    fInitialDragCoeff = 7.8,
    fPercentSubmerged = 85.0,
    fDriveBiasFront = 0.2,
    fInitialDriveForce = 0.32,
    fDriveInertia = 1.0,
    fClutchChangeRateScaleUpShift = 3.0,
    fClutchChangeRateScaleDownShift = 3.0,
    fInitialDriveMaxFlatVel = 160.0,
    fBrakeForce = 1.0,
    fBrakeBiasFront = 0.45,
    fHandBrakeForce = 0.7,
    fSteeringLock = 42.0,
    fTractionCurveMax = 2.5,
    fTractionCurveMin = 2.38,
    fTractionCurveLateral = 22.5,
    fTractionSpringDeltaMax = 0.15,
    fLowSpeedTractionLossMult = 1.5,
    fCamberStiffnesss = 0.0,
    fTractionBiasFront = 0.485,
    fTractionLossMult = 1.0,
    fSuspensionForce = 2.4,
    fSuspensionCompDamp = 1.4,
    fSuspensionReboundDamp = 2.1,
    fSuspensionUpperLimit = 0.12,
    fSuspensionLowerLimit = -0.1,
    fSuspensionRaise = 0.0,
    fSuspensionBiasFront = 0.5,
    fAntiRollBarForce = 0.9,
    fAntiRollBarBiasFront = 0.6,
    fRollCentreHeightFront = 0.41,
    fRollCentreHeightRear = 0.41,
    fCollisionDamageMult = 0.7,
    fWeaponDamageMult = 1.0,
    fDeformationDamageMult = 0.7,
    fEngineDamageMult = 1.5,
    fPetrolTankVolume = 65.0,
    fOilVolume = 5.0,
    fSeatOffsetDistX = 0.0,
    fSeatOffsetDistY = 0.0,
    fSeatOffsetDistZ = 0.0,
}

local INTS = {
    nInitialDriveGears = 6,
    nMonetaryValue = 80000,
    strHandlingFlags = 0x0,
    strModelFlags = 0x440010,
    strDamageFlags = 0x0,
}

local VECTORS = {
    vecCentreOfMassOffset = vector3(0.0, 0.0, 0.0),
    vecInertiaMultiplier = vector3(1.0, 1.3, 1.5),
}

local function applyTo(veh)
    if not DoesEntityExist(veh) then return false end
    for key, value in pairs(FLOATS) do
        SetVehicleHandlingFloat(veh, 'CHandlingData', key, value)
    end
    for key, value in pairs(INTS) do
        SetVehicleHandlingInt(veh, 'CHandlingData', key, value)
    end
    for key, value in pairs(VECTORS) do
        SetVehicleHandlingVector(veh, 'CHandlingData', key, value)
    end
    return true
end

RegisterCommand('handling', function()
    local veh = GetVehiclePedIsIn(PlayerPedId(), false)
    if veh == 0 then
        print('^1Not in a vehicle.^7')
        return
    end
    if GetEntityModel(veh) ~= GetHashKey(MODEL) then
        print('^3This is not a ' .. MODEL .. '. Applying anyway.^7')
    end
    applyTo(veh)
    print('^2Handling applied to the vehicle you are in.^7')
end, false)