In the Carcols there is something that has <sequencerBpm value="500"/>
50 0is the number you can change and it equals the beats per minute. you can see below that it is on the bottom. Just CTRL+F for sequencerBpm but you have to make sure you cahnge the correct ID number for what is in your carvariations. Hope this helps!
<Item>
<id value="1"/>
<name>police/sheriff</name>
<timeMultiplier value="1.00000000"/>
<lightFalloffMax value="10.00000000"/>
<lightFalloffExponent value="10.00000000"/>
<lightInnerConeAngle value="2.29061000"/>
<lightOuterConeAngle value="70.00000000"/>
<lightOffset value="0.00000000"/>
<textureName>VehicleLight_sirenlight</textureName>
<sequencerBpm value="500"/>