Cruise rpm for your axle ratio and tires, and what happens when you change tire size: how far off your speedometer will read and what ratio gets your performance back.
| mph | Top gear | 1.00 gear |
|---|
336 combines the unit conversions: miles to inches, hours to minutes, and diameter to circumference. Metric tire sizes are converted to diameter as (width × aspect ratio ÷ 100 × 2 ÷ 25.4) + wheel diameter.
A taller tire covers more ground per turn, so the engine turns slower at the same speed. That acts like a numerically lower axle ratio: less acceleration and towing pull, and a speedometer that reads slow. The effective axle ratio is your ratio × old diameter ÷ new diameter.
To get the old feel back, regear to your ratio × new diameter ÷ old diameter. That exact number rarely matches a gear you can buy, so the calculator shows the common ratios on either side. Going to the higher one restores more acceleration and towing; the lower one keeps cruise rpm down.
Measured tire heights often run a little shorter than the size on the sidewall, especially on a loaded tire. For the most accurate rpm and speedometer numbers, measure your tire's actual height and use the diameter box.
Standard formula, as explained in Restyling & Customizing News, "Calculating mph/rpm/Gear Ratio/Tire Size".