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Big Wheels - How they affect your performance and mileage
Calibration
The wheel/tire customizing trend for sport compacts features massive wheels and low profile, or pencil-thin, tires. The aesthetic impact is significant, yet the overall diameter of the custom wheel/tire package doesn't necessarily vary from the stock tire diameter and neither do the speedometer/odometer readings.
Muscle car restorers appreciate the look of a wheel well filled out with larger-than-stock rubber. As long as the increased diameter stays within a couple of percentage points, however, the impact on the performance is negligible. For instance, upping the diameter from 29- to 30-inches represents only a 3 percent increase and will not cause a noticeable change in acceleration or overall handling and performance. However, most states require a re-calibration of a vehicle's speedometer and odometer when the tire diameter or gearing has been altered.
The mileage metering is based on the rotation of stock tires—rather than the actual distance traveled and speed of those tires. When the diameter is larger, the vehicle goes farther and faster than the metering reads. The result could be a lot of indefensible speeding tickets and, over time, a misleading account of the total miles the vehicle has traveled.
One re-calibration option is an aftermarket computer module or programmer. Many of the re-programmers on the market also include an automatic transmission adjustment feature. Like the metering, the automatic transmission is programmed to shift under specific conditions—speed, load on the engine, etc. Since all this is stored in the computer, the system does not acknowledge the different tire diameter. The result can be an automatic that keeps jumping around on steep grades or a dreaded lugging engine. The aftermarket modules or programmers change the shift points to accurately reflect the true conditions.
Over the Top
Over sized wheel/tire combos on pick-ups are another matter. The negative impact on the performance and handling of the vehicle can be significant. The engine may lug, with significant decreases in acceleration and fuel economy.
Upgrading the running gears alleviates this problem, but at a heavy cost to the driver's wallet. The stock gears are set up to provide torque and horsepower to stock tire diameters. Going up in tire size, may mean that the gear ratio is simply too high to adequately move the new tire diameter. The important thing is to understand just what those handling differences are.
Balancing Act
The added clearance from over sized wheel/tire packages, obviously, raises the vehicle's center of gravity and impacts how the vehicle handles. When a lift kit has been installed to make room for all that rubber, even if the steering geometry has been trued and thicker sway bars added, the driver must still compensate for top-heavy maneuvers. That once sprightly ride may assume the characteristics of a tugboat through turns and any quick diversionary tactic, especially at high speeds, may risk a rollover.
In addition, the added, unsprung weight of big, non-stock wheels and tires increases the distance needed to come to a full stop, a situation that, again, can be alleviated with correspondingly big brake upgrades. Unsprung weight, as opposed to a full capacity load of cargo in the truck bed, is not supported by the suspension and, therefore, has a greater negative effect on overall handling and performance. The manufacturers of 20-inch and up wheels for sport compacts can use lighter materials, or design the wheel with elements that decrease their weight.
This is not a recommendation to stay away from larger-than-stock wheel/tire combinations. Just know all the implications to performance and handling and take precautions.
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