Build Impact Calculator: Tire, Lift & Gear Ratio

Going from a 245/75R17 (about 31.5 in tall) to a 285/75R17 (about 33.8 in) makes the tire about 2.4 inches taller, so with 3.73 axle gears the effective ratio drops to about 3.47 and a speedometer reading 60 mph means you are really doing about 64.5 mph. This free calculator does that math for your own sizes: real tire diameters, speedometer error, effective gear ratio, the axle ratio that restores stock feel, crawl ratio, clearance gained and how far the new tire moves toward the suspension or the fender.

How it works

• Diameter. For metric sizes (285/70R17): diameter = 2 × width × aspect ratio ÷ 100 ÷ 25.4 + rim diameter, in inches. For flotation sizes (35x12.50R17) the first number is the overall diameter.
• Speedometer. True speed = indicated speed × new diameter ÷ current diameter (assumes the speedometer was accurate with the current tire).
• Effective gear ratio. Axle ratio × current diameter ÷ new diameter. The ratio that restores stock feel = axle ratio × new diameter ÷ current diameter.
• Crawl ratio. 1st gear × transfer case low × axle ratio; the tire-adjusted figure divides by the diameter ratio.
• Clearance. Axle/diff clearance rises by half the diameter increase; frame/body clearance rises by that plus lift height.
• Rub check. The tire is assumed centred on the wheel. Inboard edge = offset + half tire width; outboard edge = half tire width − offset. The tool shows the change from your current setup.

Limits: the worked example above uses the formulas shown here; actual tire diameters vary by brand and wear, and measured section width changes with rim width, so measure a mounted tire for a final answer. Lift height and tire size do not change what clearance a specific vehicle has at full flex; test-fit before buying. Your saved rig stays in this browser (localStorage) and is never sent anywhere. Vehicle presets are not included yet because every value must come from a sourced data page.

Sources: tire code format and flotation size meaning, Wikipedia: Tire code (opened Nov 10, 2026); gear and crawl ratio relationships as used in our crawl ratio calculator and regear calculator. Last updated: November 10, 2026.