Tire Size Guide: How Plus-Sizing Affects Speed, Fuel and Handling
Blog › Conversions · 9 min read · Published 2026-05-07
What 225/65R17 means, how to safely change tire size, and why your speedometer lies after you do.
Decoding the Sidewall
Take 225/65R17. 225 is the tread width in millimetres. 65 is the aspect ratio — sidewall height as a percentage of width. R means radial construction. 17 is the rim diameter in inches. The Tire Size Calculator takes any two sizes and shows the diameter and speedo difference.
The ±3% Rule
Vehicle electronics — speedometer, ABS, traction control, transmission shift logic — are calibrated to original tire diameter. Stay within ±3% of original and recalibration isn't required. Beyond 3%, speedo error and gearing changes get noticeable.
How Speedometer Error Works
Speedos count wheel revolutions. A larger tire travels farther per revolution, so the dash reads slower than your true speed. A 5% larger tire means your dash showing 60 mph means you're actually doing 63 mph — and your odometer is undercounting miles.
Plus-Sizing Done Right
Plus-sizing means going up in rim diameter while reducing sidewall to keep total tire diameter constant. Plus 1: 17→18 with proportionally lower aspect ratio. Plus 2: 17→19. Benefits: better handling, sportier look. Costs: harsher ride, more wheel/tire damage from potholes, often worse mpg.
Wider Tires — The Trade-Off
Wider tires improve dry grip but increase rolling resistance, weight and road noise. A 245-wide tire vs 225 typically costs 1–3 mpg. Decide whether you'd rather have grip or fuel economy.
Off-Road Oversize
Going up 2+ inches in tire diameter for ground clearance demands re-gearing the differential to restore acceleration and proper engine RPM at highway speed. Otherwise you'll burn the transmission and lose fuel economy badly.
Speed Rating and Load Index
The numbers after the size (e.g. 95H) are load index and speed rating. Don't downgrade either — match or exceed OEM. A lower speed rating can void insurance after an accident.
Bottom Line
Tire size is more than aesthetics — it changes performance, safety and your dashboard's honesty. Run the numbers before you buy.