yomam wrote: |
How significantly can winter tires of a different size affect things - I believe they are 175-65-R14 (I don't want to go into the garage anymore). |
Hi.
175/65 tires are slightly larger than 185/60 tires.
With
standardwheel circumference, they are approximately 1 km/h faster at 130 km/h compared to the 185 models, meaning the speedometer error is reduced by 1 km/h.
However, this is overlaid with a permissible size tolerance of

.5% / -2.5%, meaning that this 4% uncertainty translates to approximately 5 km/h at a speed of 130 km/h.
So, if your speedometer, which is calibrated to display 185 km/h accurately, shows 136 km/h instead of 130 km/h, then with tires of size 175, the actual speed within the tolerance range would be approximately 128 to 133 km/h.
However, typically, a few percentage points of measurement tolerance are subtracted from the measured result, in favor of the driver.