To everyone having problems with tuning, do the following:
Download this mod: https://www.gta5-mods.com/tools/modkitlimit-extender

Change the numbers in Carcols.meta and Carvariations.meta if it still doesn't work, something like "1726".

Carcols.meta:
<kitName>909_26grgt_modkit</kitName>
<id value="909" />

Carvariations.meta
<kits>
<Item>909_26grgt_modkit</Item>

Now it should work

@vy6x: Regarding handling, you can use this.

<HandlingData>
<Item type="CHandlingData">
<handlingName>26grgt</handlingName>
<fMass value="1225.000000"/>
<fInitialDragCoeff value="5.749996"/>
<fDownforceModifier value="0.750000"/>
<fPercentSubmerged value="88.000000"/>
<vecCentreOfMassOffset x="0.000000" y="0.000000" z="0.000000"/>
<vecInertiaMultiplier x="1.250000" y="1.280000" z="1.750000"/>
<fDriveBiasFront value="0.070000"/>
<nInitialDriveGears value="8"/>
<fInitialDriveForce value="0.385000"/>
<fDriveInertia value="1.180000"/>
<fClutchChangeRateScaleUpShift value="5.750038"/>
<fClutchChangeRateScaleDownShift value="5.320028"/>
<fInitialDriveMaxFlatVel value="238.501648"/>
<fBrakeForce value="1.149999"/>
<fBrakeBiasFront value="0.560000"/>
<fHandBrakeForce value="1.050000"/>
<fSteeringLock value="38.800018"/>
<fTractionCurveMax value="2.119999"/>
<fTractionCurveMin value="1.749999"/>
<fTractionCurveLateral value="21.580212"/>
<fTractionSpringDeltaMax value="0.150000"/>
<fLowSpeedTractionLossMult value="0.200000"/>
<fCamberStiffnesss value="0.050000"/>
<fTractionBiasFront value="0.470000"/>
<fTractionLossMult value="1.000000"/>
<fSuspensionForce value="2.580000"/>
<fSuspensionCompDamp value="1.560001"/>
<fSuspensionReboundDamp value="2.320001"/>
<fSuspensionUpperLimit value="0.070000"/>
<fSuspensionLowerLimit value="-0.070000"/>
<fSuspensionRaise value="-0.010000"/>
<fSuspensionBiasFront value="0.480000"/>
<fAntiRollBarForce value="1.100000"/>
<fAntiRollBarBiasFront value="0.550000"/>
<fRollCentreHeightFront value="0.270000"/>
<fRollCentreHeightRear value="0.270000"/>
<fCollisionDamageMult value="0.880000"/>
<fWeaponDamageMult value="1.150000"/>
<fDeformationDamageMult value="0.880000"/>
<fEngineDamageMult value="1.500000"/>
<fPetrolTankVolume value="62.749538"/>
<fOilVolume value="6.750051"/>
<fSeatOffsetDistX value="0.000000"/>
<fSeatOffsetDistY value="-0.050000"/>
<fSeatOffsetDistZ value="0.080000"/>
<nMonetaryValue value="15000"/>
<fRocketBoostCapacity value="1.250000"/>
<fBoostMaxSpeed value="72.250481"/>
<strModelFlags>00440010</strModelFlags>
<strHandlingFlags>00020002</strHandlingFlags>
<strDamageFlags>00000020</strDamageFlags>
<AIHandling>SPORTS_CAR</AIHandling>
<SubHandlingData>
<Item type="CCarHandlingData">
<fBackEndPopUpCarImpulseMult value="0.180000"/>
<fBackEndPopUpBuildingImpulseMult value="0.220000"/>
<fBackEndPopUpMaxDeltaSpeed value="0.400000"/>
<fToeFront value="0.000000"/>
<fToeRear value="-0.010000"/>
<fCamberFront value="-0.020000"/>
<fCamberRear value="-0.020000"/>
<fCastor value="-0.050000"/>
<fEngineResistance value="0.000000"/>
<fMaxDriveBiasTransfer value="-1.000000"/>
<fJumpForceScale value="1.000000"/>
<fIncreasedRammingForceScale value="1.000000"/>
<strAdvancedFlags>10044000</strAdvancedFlags>
</Item>
</SubHandlingData>
</Item>
</HandlingData>
</CHandlingDataMgr>