The complete guide to upsizing safely, from Plus-1 to Plus-3 and everything in between
Plus-sizing is the practice of fitting larger-diameter wheels than factory spec, keeping the overall tire diameter roughly the same by running a shorter-sidewall tire. A factory 17-inch wheel with a 225/45R17 tire might become an 18-inch wheel (Plus-1) with a 215/40R18 tire. The appeal is obvious: bigger wheels look more aggressive, and they're relatively simple to install. But plus-sizing involves real tradeoffs in ride quality, speedometer accuracy, and fitment. Understanding the math helps you make an informed decision.
When done correctly, plus-sizing maintains speedometer accuracy and overall tire diameter so your car drives essentially the same. When done wrong, it costs you in comfort, fuel economy, and accuracy. The key is understanding the numbers.
The naming system is straightforward: Plus-1 means one size up from factory, Plus-2 is two sizes up, and so on. A factory 18-inch wheel becomes 19-inch (Plus-1), 20-inch (Plus-2), or 21-inch (Plus-3).
The challenge is that as wheel diameter increases, tire sidewall height must decrease to maintain the same overall diameter. A 225/45R17 tire has a sidewall height of about 3.1 inches. Upsizing to 18-inch wheels while keeping diameter the same requires something closer to a 215/40R18 tire, with a sidewall of about 2.7 inches.
Here's a real-world example:
The math isn't always perfect, but good plus-sizing keeps the difference under 2–3%, which is imperceptible in driving and speedometer error.
This is the biggest real-world issue with plus-sizing: shorter sidewalls mean stiffer ride. The sidewall is what absorbs bumps. A 45-series tire has about 2.3 inches of sidewall. A 35-series has about 1.8 inches. That 0.5-inch reduction doesn't sound like much, but in real driving—especially on rough pavement or at highway speeds—the difference is noticeable.
Most people can handle Plus-1 without complaint. Plus-2 starts to feel firm, especially on less-forgiving suspension. Plus-3 and beyond becomes genuinely stiff and is usually only acceptable on cars with compliant suspension or those specifically set up for low-sidewall tires.
If you're plus-sizing an economy car or family sedan, consider stopping at Plus-1. If it's a sports car already tuned for handling, Plus-2 or Plus-3 might work fine.
Your speedometer reads off wheel rotation. If you change tire diameter, the speedometer becomes inaccurate. A 2% increase in tire diameter makes your speedometer read 2% slower than actual speed. So when the speedo says 60 mph, you're actually going 61.2 mph.
This has real consequences: you might get a speeding ticket (though courts typically allow a 5% margin), your fuel economy calculation becomes useless, and your ABS/traction control system might work slightly differently.
Good plus-sizing keeps diameter change under 2%. Most factory fitments are engineered with a 2–3% margin, so slight variations are acceptable. But if you're plus-sizing, you must verify tire diameter before buying. Calculate it:
Overall diameter (mm) = (wheel diameter in inches × 25.4) + (tire sidewall height in mm × 2)
Use a tire calculator online—most tire shops or fitment tools will show you. Compare the new diameter to factory and aim for within 3mm (0.1 inch).
Bigger wheels are heavier and wider, which affects fitment. The wheel offset matters more with larger diameters because the same offset change translates to more visual difference. A wheel that tucks nicely in 18 inches might look flush or even poke slightly in 20 inches at the same offset.
Also, if you're plus-sizing, you probably want a slightly different offset to maintain the look you're going for. A 19-inch wheel might use +45mm while an 18-inch used +50mm, just because of how the wheel looks on the car at that size.
Check clearance carefully. Larger wheels can contact suspension components on lowered cars, especially at full bump travel. Always verify fitment with a calculator that accounts for your ride height and suspension modifications.
| Factory Spec | Plus-1 Option | Plus-2 Option | Notes |
|---|---|---|---|
| 205/55R16 | 215/50R17 | 225/45R18 | Small cars, economy sedans |
| 225/45R17 | 215/40R18 | 205/35R19 | Compact sedans, small SUVs |
| 225/50R17 | 215/45R18 | 225/40R19 | Mid-size sedans |
| 235/55R18 | 245/50R19 | 255/45R20 | Large sedans, crossovers |
These are guidelines, not rules. Always verify sidewall height and diameter for your specific tire model, since different brands measure slightly differently.
Bigger wheels are almost always heavier. The increase in unsprung weight affects acceleration, braking, and ride quality. Going from 18-inch to 20-inch wheels might add 3–5 pounds per wheel, or 12–20 pounds total. That's noticeable in acceleration and feels heavier in braking.
The shorter, stiffer sidewall also changes how the tire responds to cornering. Some people appreciate the more direct feel; others find it makes the car feel harsher. It's a tradeoff between sportiness and comfort.
Fuel economy typically suffers slightly with plus-sizing due to increased rolling resistance and heavier unsprung weight. Don't expect big losses, but plan for 1–3% worse fuel economy.
Before committing to plus-sizing, use a fitment calculator that knows your specific vehicle and suspension height. These tools account for your fender shape, wheel well dimensions, and suspension geometry to verify that your new wheel and tire combination actually fits.
If you're running aftermarket suspension, make sure to input your ride height. Plus-sizing on a lowered car is much trickier than on a stock car because you have less fender clearance.
When you've selected your new wheel and tire, physically verify fitment with the suspension at full compression and full extension (turn the wheel fully and have someone bounce the car). This catches problems that calculators can miss, like minor contact at full lock or near-maximum compression.
Plus-sizing is worth it if you want the look and visual impact of bigger wheels and can accept a stiffer ride and slightly less comfortable driving experience. It's not worth it if you're trying to gain any performance benefit—the stiffer sidewall and added unsprung weight actually hurt acceleration and braking performance slightly.
If you do plus-size, keep it to Plus-1 or Plus-2 and maintain tire diameter within 2%. Verify fitment with a calculator and confirm clearance in person. And be aware of the ride quality change—it's usually the one thing people regret.
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