For many car enthusiasts, one of the first mods they make on their cars is to replace the wheels. Wheel preferences vary, and it’s not anyone’s place to judge other people’s choice of rims. However, in terms of wheel sizing, our government—more particularly, the Land Transportation Office (LTO)—has recently been at the center of some online backlash because of the enforcement of a law that limits what we can do to our vehicles. You can read our coverage here.
In any case, now is a good time to talk about the technical aspects of changing wheels in light of this recent squabble between government regulators and car enthusiasts.
Most people’s basis for changing their wheels is to give their ride a sportier look and improve handling. But bear in mind that though big wheels may look good, a bigger rim size may carry disadvantages like added weight and a harsher ride.
In many cases, the bigger a wheel-and-tire combination is, the heavier it gets. And although bigger might look better, heavier is definitely a disadvantage—the most significant disadvantage being a less comfortable ride. Also, upsizing to a larger wheel may lead you to buy tires with a lower profile. A lower-profile tire also has less cushioning effect and will again contribute to a harsher ride. If you value ride comfort, don’t get wheels that are too heavy or a tire profile that’s too low.
Despite the ongoing LTO debacle, I can hazard an educated guess that there is a tolerable range when it comes to changing wheel sizes. I surmise we can still change rim sizes so long as the rolling circumference we end up with is within 2% to 3% of the OEM specifications of the car. Such a variance from OEM specs can be tolerated by the car’s design and will not have any adverse effects on the vehicles systems like speedometer calibration, ABS, traction control, and stability controls.
Thus, when upsizing or downsizing wheel diameters, you must check the end result of the change by consulting the tire dealer. Better yet, do your own research by consulting online tire calculators and checking the size and resulting rolling circumference of the wheel/tire package you’re about to buy in relation to your car’s OEM specifications. To be safe that you’re still within the law, make sure you’re within 2% to 3% of these spec. As such, it can still be argued that you haven’t effectively changed your wheel size.
Wheel offset is the distance between the wheel’s mounting hub and its centerline. The OEM spec of your wheel’s offset is often marked on the wheel itself, and is prefixed by the letters ‘ET’ followed by a number expressed in millimeters, like ‘ET+45.’
Offset determines how a wheel mounts in the car’s wheel well, and how far the inner lip of the wheel will extends toward the suspension. Getting the offset wrong has repercussions. But the most important concepts about offset to keep in mind are the following: Going more into the positive side of your OEM offset spec means your wheel goes farther into your fender and and closer to rubbing against your suspension. Going toward the negative direction of your wheel’s specified offset will push the wheel out of the wheel well. The farther out you go, the more stress it will put on your wheel bearings, and as your tire exceeds the confines of your fenders, the dirtier your car gets from the dirt kicked up by your tires.
To be on the safe side, you want your new wheel’s offset to be within 5mm of the OEM specified offset.
The foregoing are the simplest ways that I can explain the effects of variances in OEM wheel diameter and offset. It is by no means an exhaustive discussion. And the advice I have given is very conservative and not everyone will agree. But it should be able to help you safely determine the specs of your new wheel and stay within the confines of the law. I hope it wasn’t too technical, and I hope it helps when you go out shopping for new wheels.