How to Read a Tire Size
A tire size like 265/70R17 breaks down into three numbers:
- 265 — Section width in millimeters (from sidewall to sidewall)
- 70 — Aspect ratio: sidewall height as a percentage of width (70% of 265mm = 185.5mm)
- R17 — Radial construction, 17-inch wheel diameter
Overall diameter = (2 × sidewall height) + wheel diameter. That's what this calculator does for both tires and compares them.
Why Tire Size Matters for Trucks
Going to a bigger tire affects more than just looks. Here's what changes:
- Speedometer error — Bigger tires cover more ground per revolution, so your speedometer reads slower than you're actually going. A 3% diameter increase means at 60 mph displayed, you're really doing ~62 mph.
- Revs per mile — Fewer revs means your odometer under-reads mileage (affects maintenance intervals and resale).
- Gearing feel — Bigger tires effectively raise your final drive ratio. You'll feel it off the line, especially with stock gears. Many guys go from 3.55 to 3.73 or 4.10 when jumping to 35s.
- Fuel economy — Bigger, heavier tires + more rolling resistance = worse MPG. Expect 1–3 mpg drop going from 33s to 35s on a half-ton.
- Clearance — The big one. Measure before you buy. You need clearance at the upper control arm, the fender flare, and the frame rail when turning full lock.
Quick Clearance Check
With the truck on level ground, turn the wheel full lock and measure from the tire's widest point to:
- The fender flare inner lip (top)
- The upper control arm or ball joint (inboard)
- The frame rail or splash guard (front of tire)
You want at least 1 inch of clearance at all three points with the suspension fully compressed. If you're close, a 1–1.5 inch body lift or trimming the plastic splash guard usually solves it.
Common Tire Size Upgrades
| Stock Size | Popular Upgrade | Diameter Change | Notes |
|---|---|---|---|
| 265/70R17 (31.6") | 285/70R17 (32.7") | +1.1" | No lift needed on most half-tons |
| 265/70R17 (31.6") | 285/75R16 (32.8") | +1.2" | Switch to 16" wheel for more tire sidewall |
| 275/65R18 (32.1") | 295/70R18 (34.2") | +2.1" | May need 2" level or lift |
| 275/65R20 (34.1") | 305/55R20 (33.2") | -0.9" | Wider but shorter — fills wheel well |
Do I Need to Recalibrate My Speedometer?
If your tire diameter changes by more than 3%, you should recalibrate. Options:
- Dealer reflash — Most dealers can update the PCM tire size parameter for $50–$100
- Handheld tuner — Devices like HP Tuners, Bully Dog, or Edge let you set tire size and gear ratio. Worth it if you've already got one.
- Aftermarket speedometer calibrator — Standalone units that plug into the speedometer signal wire. Common on older trucks without CAN bus.
Ignoring it won't hurt the truck mechanically, but you'll be speeding without knowing it — and your odometer will under-report miles, which matters for warranty and resale.