it depends on your wheels, standard st's you can get away with the 20mm but my wheels are a wider offset
im lowered 45mm at the back and 40mm at the front with no problems, on pro race 3 alloys
i have had the front arches rolled though
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Red Escort 1.6LX
Then - Red Escort 1.4
Then - Silver Fiesta ST 170BHP
Then - White Fiesta ST MR200+
Now - Mitsubishi Evo IV Forged 2.3 500BHP
and Mondeo Mk3 2.0TDCI