How to make cars slow down when they're driving off-road?

Hi.

I'm currently making a script which makes cars slower by 30% when they drive on dirt, snow etc. The script I wrote works fine, the problem is that it activates when only 1 wheel is touching, and I'd like the script to work only when all 4 are touching the off-road surface. Despite my attempts at solving the problem, I end up with similar results. Any help would be appreciated.

local Wheels = script.Parent.Parent.Wheels:GetChildren()

function Slowing(RoadType)
    if RoadType.Material ~= Enum.Material.Concrete then
        local NormalCarSpeed = script.Parent.Parent:FindFirstChild("NormalCarSpeed")
        NormalCarSpeed = NormalCarSpeed.Value / 100 * 70
        script.Parent.MaxSpeed = NormalCarSpeed
    else
        script.Parent.MaxSpeed = script.Parent.Parent.NormalCarSpeed.Value
    end
end

for i,v in pairs(Wheels) do
    Wheels[i].Touched:Connect(function(TouchPart)
        local RoadTouch = v:GetTouchingParts()
        for i,v in pairs(RoadTouch) do  
            if v.Material ~= Enum.Material.Concrete then
                Slowing(TouchPart)
            end 
        end
    end)
end