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Tool Belt Calc

Automotive Calculators

Wheels and tyres, drivetrain gearing, engine geometry, running costs and towing. The physics behind these has not changed in a century, so the arithmetic is exact — what varies is whether the figures you feed it came off the car or off a forum.

12 tools in this category

Overall diameter is the number that matters

Almost every tyre question reduces to overall diameter. It sets your speedometer accuracy, your effective final drive, your odometer reading and whether the wheel fits the arch. Section width and aspect ratio are only inputs to it — which is why a wider tyre on a shorter sidewall can come out exactly the same height, and why that is the entire basis of plus-sizing.

Three per cent is the working tolerance. Beyond it the speedometer reads meaningfully wrong, the ABS and traction systems receive wheel-speed data they were not calibrated for, and on some vehicles the transmission shift points move.

Gearing, tyres and revs are one system

Axle ratio, transmission ratio and tyre diameter combine into a single relationship between road speed and engine speed. Change any one and the other two have to be reconsidered — which is why fitting substantially larger tyres without re-gearing is the most common cause of a truck that suddenly feels gutless and hunts between gears.

The rpm-at-speed calculator is the one to reach for before committing to any of those changes. Cruising revs that land outside the engine's efficient band cost fuel whichever direction they miss by.

Where these stop being reliable

The quarter mile estimator is empirical rather than physical — derived from decades of timeslips, and assuming a competent launch on a prepared surface. Traction, gearing, driver and air density all move the real number.

Towing is the one place where being wrong is a safety matter rather than a cost one. Real capacity is limited by whichever of several ratings you hit first, and the lowest governs. The placard on your door pillar and the figures in your handbook are the authority — not this page, and not a forum.

Frequently asked questions

How much can I change tyre size without problems?

Keep overall diameter within about three per cent of the original. Beyond that the speedometer and odometer read meaningfully wrong, and the wheel-speed data feeding ABS and traction control drifts from what those systems were calibrated against.

Why does my speedometer read wrong after fitting bigger tyres?

A larger tyre covers more ground per revolution, so the speedometer under-reads and you are travelling faster than it shows. The odometer is wrong by exactly the same proportion, which quietly affects service intervals and recorded mileage.

What is the difference between offset and backspacing?

They describe the same geometry from different reference points — offset in millimetres from the wheel's centreline, backspacing in inches from the inner lip. Converting between them requires adding an inch to the nominal width for the flanges, which is the step most people miss.

Can I trust the towing figure?

Treat it as an estimate from one rating. Real capacity is governed by the lowest of GCWR, GVWR, rear axle rating, hitch rating and tyre load rating. Your door placard and handbook are the authority.

Is anything I enter sent to a server?

No. Everything runs in your browser. There is no back end, no analytics and no account.