Precision calculators for tires, gears & fuel.
Every number is computed by a tested formula engine — and the formula is printed right next to it. No black boxes, no sign-up, nothing leaves your browser.
Tire Size
Enter a metric tire size to get its true overall diameter, sidewall height, circumference, and revs per mile — the numbers your speedometer and odometer actually respond to.
26.16 in · 771 revs/mile TireTire Comparison
Will the new size work? Compare diameter, width and circumference, see how far off the speedometer lands, and get a drivetrain-aware verdict — not just a bare percentage.
+2.24% · GREEN TireTire Height
"Tire height" is the marketing word for overall diameter. Get it in inches and millimetres for any metric size, and compare against the common sizes in the chart.
26.16 in = 664 mm DrivetrainGear Ratio
Calculate cruising engine RPM at highway speeds for any axle gear ratio and tire diameter — and see what bigger tires really did to your effective gearing.
2,714 rpm @ 65 mph Fuel & MPGFuel Cost
How much will that drive cost? Enter the distance, what your car actually gets, and the pump price.
$35.00 / 300 mi Fuel & MPGGas Trip Cost
Total gas money for the trip, what each person owes, and how many tanks you will burn — with fill-up bookkeeping for the return math.
$19.55 per person Fuel & MPGMPG Calculator
The dashboard lies a little. Hand-calculate your true MPG from the odometer and the pump, then compare it against what the car claims.
28.5 mpg hand-calculatedTested formula engine
Every calculator is a pure function with an assertion net behind it — unit conversions, tolerance bands, and rounding pinned by tests.
Formulas on the page
The exact math sits next to every result, so you can audit it against your shop manual or your spreadsheet.
Sources & assumptions listed
Thresholds like the 3% rule, the 0.75 in AWD line, and the 336 constant cite where they come from — on the page that uses them.
Tire Size Difference Safety: The 3% vs 5% Rule
Is a 2% or 4% tire size difference safe? The community splits between 3% and 5% — here is both schools, the math, and how to decide.
AWD Mismatched Tires: How Much Difference Is Too Much?
Why full-time AWD destroys itself over mismatched tires — the 0.5" shudder and 0.75" destruction thresholds, with a 6.7% case study.
Mismatched Gear Ratios: How 2% Destroys a 4x4
Front and rear axle ratios differing by ~2% bind 4WD, throw faults, and destroy drivetrains. How to check before buying, and repair costs.
Speedometer Accuracy: How Wrong Is It, and What You Can Do
Speedometers read high from the factory and drift further with tire size. Legal limits, the +2–3 mph factory bias, GPS checking, and fixes.
Hand-Calculated MPG: The Number the Pump Actually Knows
Dashboard MPG estimates run 3–8% optimistic. The fill-up method for your true fuel economy, how to convert to L/100km, and when apps like Fuelly help.
Twelve Plus-Size Swaps, Computed: What Changing Tire Size Actually Does to Your Speedometer
We ran 12 common OE-to-plus-size tire swaps through the comparison engine: only 8 of 12 stay under the 3% line, and the worst misreads your speed by 3 mph.
3.73 vs 4.10 vs 4.56: Cruising RPM Across Six Tire Sizes, Computed
A 3.73-to-4.10 regear raises 70-mph cruise RPM by 9.9% on any tire; going 265/70R17 to 35-inch tires quietly undoes a 4.10 as if you had installed a 3.71.