- How much does a larger tire throw off my speedometer?
- Every millimetre of extra outer diameter matters. Going from a stock 265/70R17 (802.5 mm OD) to a 285/70R17 (830.6 mm OD) is about a 3.5% increase — so when your dash reads 60 mph you're actually doing 62.1 mph. This calculator shows the exact discrepancy at four common highway speeds (40, 60, 80, 100 mph) so you know how much to compensate — or budget for an SCT/DiabloSport tune that re-calibrates your PCM.
- What is wheel ET offset and why does it affect clearance?
- ET (from German Einpresstiefe, "press depth") is the distance in millimetres from the wheel's mounting face to its centre plane. A high positive ET pushes the tyre inward (toward the suspension); a low or negative ET pushes it outward toward the fender lip. When you fit aftermarket wheels with a lower ET than stock, the tyre moves outward by the difference — directly reducing your available fender-lip clearance on bump. This calculator treats the ET change as a lateral shift and flags it in the verdict. A spacer has the same outward-shifting effect as an equivalent ET reduction.
- Does lift height fully compensate for a taller tyre?
- Only for vertical clearance at the fender well. A 3-inch (76 mm) lift adds 76 mm of vertical room, but a 285/75R17 instead of 265/70R17 adds roughly 40 mm to the outer diameter — that's a 20 mm increase in radius per side. So you have about 56 mm of net vertical margin, which is generally fine. The hidden risks are: (1) the tyre being wider pushes sidewall toward the inner fender liner and control arms, and (2) a lower-ET wheel shifts the tyre outward at the same time. This tool models both dimensions so you see the full picture, not just the height delta.
- What are common lifted-truck tire size upgrades?
- A 2–3 inch leveling kit typically fits 265–275 wide tires on a 33-inch (838 mm) OD rim with no trimming. A 3.5–4 inch lift on a half-ton (F-150, Silverado 1500, Ram 1500) comfortably clears 285/70R17 or 295/65R20. A 6-inch lift opens up 305/70R18 or 35×12.50R18 territory. Full-size 1-ton trucks (F-250, Ram 2500) with a 6-inch lift can usually run 37-inch tires (315/70R17 ≈ 934 mm OD). Always verify inner fender liner and upper control arm clearance physically — this tool gives you a pre-check, not a substitute for a visual inspection at full droop and lock-to-lock steering.