3mm is the correct size for custom exhausts and intakes. 2-2.5mm is great for custom headers and radiator hoses. Also available in 1mm thickness of which would be too thick for fuel lines in 1/24 scale, but would work well for larger scales.
Easy to bend and cut. Huge roll should last years: 16 ft. The 1mm rolls are 32 ft.
Available in plain aluminum, gold and anodized black, red and blue. Other colors are available by request.