You should not downsize your winter tires.
First, let’s start with the under-sizing, a very popular practice since the 90s. But why exactly? Simply because it's easy! The winter tire market is very small, snow tires were one of the manufacturer’s last worry because, as we know, a vehicle always comes equipped with summer tires or 4 seasons tires.
We also know that the original tires have greatly magnified in recent years. Just remember the 2004 Toyota Camry and the 2004 Mitsubishi Lancer Ralliart, both mounted in 16-inch versus 18-inch in 2012. Those dimensions are not always available in winter tires, it had to find a solution and that was the under-sizing. Everyone has already changed from 205/55R16 to 195/65R15. Obviously, the overall diameter remains the same. This transition will not affect in any way your speedometer. By cons, did we already think about the contact patch?
Vehicles are now equipped with several technologies that did not exist in 1995! ABS brakes, ESP and "traction control" are now standards in 95% of car manufacturers. These systems are calibrated with the contact patch and not the overall diameter. This means that by removing a portion of the contact surface of the tire, the vehicle has a shortfall. Support systems will work unnecessarily and will lose some performance during your trips.
The newest technology, and I kept it for last. The EPS systems are becoming really popular. Electric steering request an hyper accurate adjustment. So when you change any part that affects the direction, the vehicle doesn’t work properly.
I speak from experience, during my last tour in more than 20 Toyota dealers in Quebec, I found that there were many complaints about the stability of vehicles in winter. After several consultations, I noticed that the problems were mainly on the Venza and Corolla, both equipped with an EPS system. Complaints about stability were resolved by simply reinstalling the original rims with the winter tires and replace the steel wheels that are one or two inches lower than the originals.
Moral of the story: be aware of the vehicle when you practice down-sizing.
Did you know that the limit of difference to the overall diameter is not +3% / -3% to +2% but / -3%.
autoExpert.ca & Kevin Brisebois