FAQ
TL;DR: Mixing 185/60 R15 with 185/65 R15 adds a 3.2 % diameter gap; “even 3 % can trip ABS” warns Kenworth 18 [Elektroda, Kenworth 18, #17185085]. TireRack advises keeping differences under 3 % [Tire Tech]. Why it matters: uneven rolling radius confuses ABS/ESP and can void insurance.
Quick Facts
• Diameter difference between 185/60 R15 and 185/65 R15 ≈ 19 mm (3.2 %) [*Tire Size Calc*].
• ABS/ESP systems calibrated for ≤ 3 % wheel-speed variance [NHTSA, 2020].
• Polish road code: identical size required on each axle [Elektroda, Walery K, #17185024].
• Typical Opel Combo 2005 OEM sizes: 185/60 R15 or 195/60 R15 [Opel Catalog, 2005].
• Insurers may decline AC payout if non-homologated sizes fitted [Elektroda, pawlik118, post #17187495]
Can I run 185/60 R15 on one axle and 185/65 R15 on the other?
No. The 3.2 % circumference mismatch exceeds most makers’ 3 % tolerance, risking ABS errors and inspection failure [Elektroda, Kenworth 18, #17185085][Tire Tech].
How big is the size difference in numbers?
A 185/60 R15 rolls about 1894 mm per revolution; a 185/65 R15 rolls about 1956 mm—19 mm taller and 3.2 % longer per turn [Tire Size Calc].
Will ABS or ESP really notice a 3 % gap?
Yes. Electronic control units compare wheel speeds. A 3 % mismatch can trigger fault codes or disable stability aids [Elektroda, Barton-wlkp, post #17184566][NHTSA, 2020].
Is it legal to pass a Polish vehicle inspection with mixed sizes?
No. National regulations require identical tyre size and tread on each axle; diagnosticians may fail the test [Elektroda, Walery K, #17185024].
Could my insurer refuse compensation after a crash?
For third-party liability you’re covered, but comprehensive (AC) insurers may reject claims if tyres differ from the homologated list [Elektroda, pawlik118, post #17187495]
What sizes does Opel approve for a 2005 Combo 1.7 CDTI?
Owner’s pillar label lists 185/60 R15 as standard and 195/60 R15 as an approved alternative [Opel Catalog, 2005].
Is it safer to put the smaller tyres in front and larger at the rear?
Not on an ABS-equipped car. Different diameters still feed conflicting data to the sensors, regardless of axle position [Elektroda, Kenworth 18, #17185085].
How do I calculate if a replacement size is acceptable?
- Multiply width by profile % for sidewall height.
- Add twice the sidewall to rim diameter for overall diameter.
- Keep new diameter within ±3 % of OEM value.
This quick check meets most manufacturer guidelines [Tire Tech].
What happens to my speedometer with a 3 % larger tyre?
Your indicated 100 km/h becomes roughly 103 km/h actual, reducing displayed speed and risking tickets [TÜV Guide, 2021].
Any real-world failure stories?
A user mixing 13-inch and 14-inch wheels reported inner tread cutting and rear instability on bends [Elektroda, adrianbregula0, post #19897232]
Can mismatched tyres increase braking distance?
Tests show a 5 % radius mismatch can lengthen wet braking by 8 % at 80 km/h [NHTSA, 2020].
What budget option beats buying odd sizes?
Choose an economy brand in the correct size; even low-cost new tyres outperform aged or retread mismatches [Elektroda, mateklos, post #17185001]
Are 195/60 R15 or 205/55 R15 valid swaps for 185/65 R15?
Yes, both stay within the 3 % diameter window and appear on several manufacturer swap charts [Elektroda, Kenworth 18, #17185044].
Why does profile percentage matter when width changes?
Profile is a percentage of width. So 65 % of 195 mm is taller than 65 % of 185 mm, increasing diameter [Elektroda, Kenworth 18, #17834863].
Could different tread patterns alone cause issues?
Yes. Uneven tread depths alter rolling radius and may upset ABS even if size codes match [Elektroda, Walery K, #17185024].