Axle Ratios & Tire Size: Get the Right RPM for Towing

Bigger tires and stock gears often wreck tow manners—unlocking, hunting, and heat. Here’s the math, examples, and a simple process to pick the right ratio so your truck tows cooler and stays in lockup.

The Core Formula

Engine RPM ≈ (MPH × Transmission Gear × Axle Ratio × 336) ÷ Tire Diameter (in)

Overall Gear = transmission gear × axle ratio. Measure your tire’s loaded diameter—real numbers matter.

Example: Mild Tow Rig, 6-Speed Auto

Stock: 32″ tire, 3.73 axle, 6th = 0.67, 65 mph → RPM ≈ (65×(0.67×3.73)×336)/32 ≈ 1,706

Now 35″ tires: Same gearing → RPM ≈ 1,559 (about 150 lower). Downshifts/unlocks more often and runs hotter.

Re-gear to 4.10: RPM ≈ (65×(0.67×4.10)×336)/35 ≈ 1,623; far better lockup behavior and EGT control.

Effective Ratio Shortcut

Going from tire D₁ to D₂, your effective axle ratio ≈ Axle × (D₁ / D₂). From 32″→35″ on 3.73: 3.73×(32/35) ≈ 3.41. That’s why it feels lazy.

When to Re-Gear

• Converter unlocks on gentle grades or headwinds at your normal cruise speed.
• Frequent hunting between top gears; rising EGT/trans temps.
• Tire upsize ≥10% and you lost the sweet spot.

6-, 8-, 10-Speed Notes

More gears help, but lockup strategy is still king. If the engine runs below peak torque in your chosen gear, the ECU will unlock/flare to make power—this is where heat comes from. Re-gearing restores a usable locked ratio.

Step-By-Step Re-Gear Planner

1. Measure real tire diameter (loaded radius × 2).
2. Compute your cruise RPM at 60–70 mph in the gear that locks on flats.
3. Pick an axle ratio that lands ~1,800–2,100 rpm at your preferred tow speed.
4. Recalibrate the speedometer/shift schedule afterward.