I assume
you would be using the +1, +2, or +3 system. You can't really get 20" wheels in 65 aspect ratios because the overall tire diameter will be HUGE and the market just does not make then that way. Fancy rims, low profile.
So to get large wheels, you need to decrease tire aspect ratio--sidewall height. And mnost people also increase the width as well.
If you have the 17" wheels and go to 18", then you sidewall height or aspect ratio should go from 265/65/70 to 275/55/18, Using 20" wheels you are literally going to +4 since there is no +3 19" wheels...
So you would be using something like 285/45/20 or 285/35/20, or whatever is the closest fit.
When you ride on 35-45 aspect ratios, you have just about 2-3 inches of sidewall. You will expereince several changes:
1. Yes, it will have quicker steering. So quick in fact it may get squirrelly as there is not enough flex in the sidewall to accomodate tiny changes in road conditions to the steering mechanism.
2. Rougher ride. The sidewall works as a part of the suspension and cusions minor imperfections in the road. 35 aspect ratios are fine on a smooth track that was recently swept and cleaned but real road conditions are like that. Pot holes and running into curbs that would not hurt normal tires can pinch and dent expensive wheels. Slamming into a pothole not only dents your wheel, but transmits more shock and stress to the suspension system.
3. Heavier wheel/tire package. The added mass will wear your system faster as more weight is slammned up and down.
4. Tires can be very pricey and needs to be special ordered.
5. Tires tend to be high speed rated and tend to pick up nails and screws like a magnet.
6. High speed tires wear out much faster becuase of the softer rubber compounds.
7. High speed tires tend to wear out faster from the thinner rubber tread.
8. More flats from the thinner treads.
9. No true mud and snow rated tires at taht size. It may say M+S on the sidewall, but the tread pattern is anything but M+S.
10. more chance of tipping over. Now a 4Runner is pretty low profile compared to Tahoes and Excursions, but the low profile high performance tire is supposed to have grip and in a fast emergency manuever, there is much less forgiveness for the vehicle to slide so the top heavy weight has nowhere to go but tip over. I had to do a very fast crank-to-crank maneuver at about 35-40 mph to avoid a deer running in front of me. (Running stock size 265/75/16) The Tahoe rolled to one side then whipped to the other side with even more force as the weight transfered over and felt like it was going past 30 degrees from perpendicular before righting itself. I felt the tires roll on the sidewall before the Tahoe righted like a ship in a storm but we settled back down. No damage and no harm. I checked the tires afterwards and saw scuff marks almost to the middle of hte sidewall. THe tires worked just as it should, rolling over to the sidewall before sliding before tipping over. Something to really consider.
