How To Get Your Suspension And Fitment Right

When you change tyres or springs on a 4WD, small numbers make big differences; Superior Engineering’s online tools remove the guesswork by converting tyre data and estimating spring rates to match your real-world load.

Why These Two Tools Matter?

Tyre Size Calculator
The Tyre Size Calculator converts metric and imperial tyre codes to overall diameter and width, compares candidate tyres to your factory fitment, and highlights percentage diameter changes that affect gearing, speedo accuracy and electronic systems such as ABS and traction control. Use it to check clearance, legal limits and the likely effect on driveline behaviour before you buy.

Spring Rate Calculator
The Spring Rate Calculator estimates the spring rate you should be running based on your measured or estimated constant vehicle load (bullbar, winch, canopies, drawer systems, batteries etc.). It is built around the principle that springs should be selected for the weight permanently carried by the vehicle, not for occasional load. This prevents springs that are too soft (sag, poor travel) or too stiff (harsh ride).

How The Tyre Size Calculator Works?

Metric tyre sizes (for example 285/75R16) encode width, aspect ratio and rim diameter. The calculator applies the standard geometric formula to compute overall tyre diameter:

Overall diameter (mm) = (tyre width × aspect ratio / 100) × 2 + (rim diameter in inches × 25.4)

Here is an example comparing 265/70R16 to 285/75R16

265/70R16 overall diameter = (265 × 0.70) × 2 + (16 × 25.4) = 777.4 mm (30.59 in)
285/75R16 overall diameter = (285 × 0.75) × 2 + (16 × 25.4) = 833.9 mm (32.83 in)

That change is approximately a 7.27% increase in diameter, which means your speedometer and gearing will be affected by the same percentage; electronics that use wheel speed for control can also be impacted. Use the calculator to see those numbers instantly and assess whether speedo correction, gearing changes or suspension modifications are required.

How The Spring Rate Calculator Works?

The spring tool asks you to build a representation of your vehicle’s permanent load by selecting common accessories and fitments (bullbars, winches, roof tents, fridges, drawers, secondary batteries etc.). It then aggregates those loads and produces an estimated spring rate appropriate for the front and rear (or ute/wagon specific load cases). Key technical points the tool enforces: measure real axle loads where possible (weighbridge or corner scales); design springs for constant load rather than occasional items; and treat the calculator’s result as a guide to select a spring with the correct dynamic behaviour.

Guidance For Interpretation And Specification

Treat the Tyre Size Calculator result as a fitment and systems-impact guide; if the diameter change is more than about 3–4%, plan to address speedo recalibration and check local compliance rules. For larger changes, check guard clearance, steering lock and wheel arch geometry.

Use the Spring Rate Calculator outputs as a starting point; whenever possible validate with axle/corner weights in your vehicle’s loaded configuration. If you have measured axle loads, call Superior Engineering with those numbers for a bespoke confirmation and product match.

Remember: spring rate affects ride height, wheel travel, and dynamic load transfer. Selecting the correct rate avoids premature wear, poor ride quality and unstable handling.

Typical User Workflow To Minimise Risk And Maximise Performance

1. Run the Tyre Size Calculator to compare the new tyre’s diameter and width to the factory fitment; note percentage diameter change. If >3–4% plan for speedo correction and a fitment check.

2. Complete the Spring Rate Calculator using the gear that lives on the vehicle full time; export the results and use those estimated rates when selecting coils, spacers or coilover springs.

3. If you’re upsizing tyres and adding permanent weight, re-check corner weights after the changes and confirm spring selection. Contact Superior Engineering for product recommendations and compatible spring part numbers.

Compliance, Safety And Best Practice

Both tools include disclaimers and guidance to consult qualified fitters and engineers prior to committing to modifications; the calculators are education and planning tools and should be complemented by measured weights and professional fitment checks. Superior Engineering provides speedo correction solutions and suspension components that integrate with these decisions.

TRY THESE TOOLS TODAY!