Tire size comparison
275/40R20 vs 305/35R20
305/35R20 is −0.9% shorter than 275/40R20: 28.4 in vs 28.7 in (−0.26 in), and 1.2 in wider. With 305/35R20 on a vehicle set up for 275/40R20, the speedometer shows 60 mph at an actual 59.5 mph. That is within the ±3% difference many shops use as a rule of thumb (not a manufacturer or legal limit).
Set up for: checking a replacement size (305/35R20 instead of 275/40R20) · Different situation?
Your current size
e.g. 265/70R17, 2657017 or 33x12.50R20
Where do I find my size?
It's on the tire sidewall, e.g. P265/70R17 115T, and on the tire placard sticker on the driver's door jamb (also in the owner's manual). The placard shows the size the vehicle came with; the sidewall shows what is fitted now. If they differ, use the placard as your starting point. Digits are enough here: 2657017 or 33125020.
Size you're considering
275/40R20 vs 305/35R20: full comparison
| 275/40R20 | 305/35R20 | Difference | |
|---|---|---|---|
| Overall diameter | 28.66 in | 28.41 in | −0.26 in (−0.9%) |
| Width | 10.83 in | 12.01 in | +1.18 in (+10.9%) |
| Sidewall height | 4.33 in | 4.20 in | −0.13 in |
| Circumference | 90.0 in | 89.2 in | −0.8 in |
| Revs per mile (nominal) | 704 | 710 | +6 |
| Wheel diameter | 20 in | 20 in | ±0 in |
| Diameter in mm | 728 | 722 | −6 mm |
Speedometer difference
Your speedometer is calibrated for 275/40R20. Fitted with 305/35R20, the actual speed is the reading × 28.41 ÷ 28.66.
| Speedometer shows | Actual speed | Difference |
|---|---|---|
| 20 mph | 19.8 mph | −0.2 mph |
| 30 mph | 29.7 mph | −0.3 mph |
| 45 mph | 44.6 mph | −0.4 mph |
| 55 mph | 54.5 mph | −0.5 mph |
| 60 mph | 59.5 mph | −0.5 mph |
| 70 mph | 69.4 mph | −0.6 mph |
| 80 mph | 79.3 mph | −0.7 mph |
Real tires: published diameters
275/40R20: 28.4–29 in published by 6 makers; 305/35R20: 28.1–28.5 in published by 5 makers. Calculated figures assume a nominal tire; the exact difference between two specific tires depends on their models.
More about each size: 275/40R20 · 305/35R20
FAQ
Which is bigger, 275/40R20 or 305/35R20?
305/35R20 is −0.9% shorter than 275/40R20: 28.4 in vs 28.7 in (−0.26 in).
How much will my speedometer be off with 305/35R20 instead of 275/40R20?
The speedometer is calibrated for 275/40R20. With 305/35R20 it shows 60 mph when you are doing 59.5 mph, and 70 mph at 69.4 mph (−0.9%). Your speedometer also has its own factory tolerance.
Can I replace 275/40R20 with 305/35R20?
The diameter difference is −0.9%. Whether it fits depends on things this calculation can't see: wheel width and offset, clearance at full lock and over bumps, load index and speed rating, and your vehicle maker's approval. Check your door-jamb placard and owner's manual and ask a tire professional.