Guides

Plain-English explanations of the questions behind our calculators, with worked examples you can open in the tools and check for yourself.

• How to Read a Tire SizeWhat every part of 225/45R17 94W means, and how to turn it into real dimensions. Updated 8 November 2026.
• Wheel Offset and Backspacing ExplainedET, positive and negative offset, backspacing, and how a new wheel moves in and out. Updated 8 November 2026.
• Bigger Tires: Speedometer, Odometer and MPGWhy taller tires make the speedometer read low and your MPG look worse than it is. Updated 8 November 2026.
• Plus Sizing: Bigger Wheels Without Changing the Overall SizeHow to go up a rim size and keep the overall diameter, and what changes when you do. Updated 8 November 2026.
• US MPG, UK MPG and L/100 kmWhy the same car has two different MPG figures, and NEDC vs WLTP vs EPA. Updated 8 November 2026.
• Why Your Real MPG Differs From the Official FigureHow official figures are measured, what changes them on the road, and how to measure yours. Updated 8 November 2026.