Here is a tire chart comparing the various AFX Mega-G+ rear tires that are available at BRS Hobbies. A smaller diameter tire will bring the traction magnets closer to the track rails which is why the car will have more grip. Another consideration is when upgrading to Viper level 42 or level 52 magnets, it's best to use a medium, firm or rubber rear tire. This helps the tire to maintain shape better as a soft tire may compress too much on the track which could cause inconsistent handling. For this reason, The Viper AFX Mega-G+ soft compound rear tires are best used with the stock traction magnets.
The tire chart can be used to equalize drivers of different skill levels. For example, a really good driver can use the stock rubber compound tires and a beginner can use a Mega-G+ HO slot car with grippier Bulldog 7808M medium compound rear tires. Also, in some cases having too much rear grip can make a car harder to drive since it may not be as forgiving to drive in the curves by giving little to no warning when it de-slots. Ideally, an HO slot car should have a small amount of rear end slide when pushed to the limit which gives a good warning to slow down some before the car actually comes off the track.