Or toe out. The shimmy caused by toe problems happens because asymmetrical forces (say, a bump or dirt clod) pull one wheel off center. On a street car, these unequal impacts are minor. Toe in means that if the tire is pulled off course, the 1/8"or whatever the toe is means the tire is now pointed straight ahead. If the force pulls the tire past that toe amount, the opposite tire is going to tend to turn more sharply, and the vehicle tends to turn more. Having toe out means that opposite tire is now pointing straight, and will tend to hold a straight course.
At positions close to center, toe in or toe out can cause a self amplifying oscillation. Sometimes one way works better than the other.
When I was fabricating off road race cars we often found the cars handled large bumps better with extra toe out; the large amounts of suspension travel (approaching two FEET) made that combination work
better too.
Having said that, every manual I have seen calls for toe in. Check your steering linkage, bearings, pivots, etc and I bet you can get it driving
true
again. The larger tire probably increased the caster angle a bit too, and the slight extra width may have been just enough to put it over the edge. Let us know what you find. It is also possible that the old tires were "pointier" on the center rib, making them effectively narrower. Narrow tires track straighter, as a rule.
