Free drivetrain tool
Gear Ratio Calculator — RPM, Speed & Ratio
Calculate RPM at speed, find the right gear ratio for bigger tires, and compare axle ratios side by side.
Gear Ratio & RPM Calculator
Two modes: RPM at speed, or ratio for new tires.
| Ratio | RPM @ 70 mph | Character |
|---|
What Is Gear Ratio?
A 4.10 axle ratio means the driveshaft spins 4.10 times per wheel revolution. Higher numbers = more torque and acceleration, but higher highway RPM and worse MPG. Lower numbers = relaxed cruising, slower off the line.
Why Regear for Bigger Tires?
- Bigger tires act like a taller gear — the engine lugs, transmission hunts, MPG drops.
- Rule: new ratio = old ratio × (new tire ÷ old tire). Going 31″ → 35″ with 3.73s wants ~4.20s — 4.10 or 4.56 are the common picks.
- Regearing both axles on a 4×4 typically costs $1,500–$3,000 installed.
Common Axle Ratios
- 3.21–3.55: economy cruising, light trucks.
- 3.73: the all-rounder — towing and daily.
- 4.10–4.56: big tires, heavy towing, off-road.
Frequently Asked Questions
What gear ratio do I need for 35 inch tires?
Typically 4.10–4.56, depending on your stock ratio and use. Use ratio-finder mode above with your old and new tire diameters.
Do higher gear ratios hurt MPG?
On the highway, yes — higher RPM at cruise. But with oversized tires, regearing restores MPG by un-lugging the engine.
How do I find my current gear ratio?
Check the door-jamb axle code, RPO sticker (GM), or VIN decode. Or jack up the rear, mark the driveshaft, and count rotations per tire turn.
What RPM should I cruise at?
1,800–2,200 RPM is the sweet spot for most gas engines — enough torque reserve without wasting fuel.
Is regearing worth it?
With 2″+ bigger tires, yes — it restores acceleration, stops transmission hunting, and often pays back in MPG and drivability.
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