Compare current and new tire sizes for diameter change, fitment risk, and speedometer error — free, instant, no account.
Current tire
225/45R17
mm
%
in
New tire
225/40R18
mm
%
in
Ideal match
+0.46%
+0.1"
Safe diameter change for most daily driving.
Size visualizer
Scaled to compare section width and overall diameter.
Current New
Front view
Width 0% (0")
Side view
Diameter +0.46% (+0.1")
Specs
Overall diameter
Current
25"
New
25.1"
Section width
Current
8.9"
New
8.9"
Sidewall height
Current
4"
New
3.5"
Circumference
Current
78.5"
New
78.8"
Revolutions
Current
807.6 / mi
New
803.9 / mi
Spec
225/45R17
225/40R18
Overall diameter
25"
25.1"
Section width
8.9"
8.9"
Sidewall height
4"
3.5"
Circumference
78.5"
78.8"
Revolutions
807.6 / mi
803.9 / mi
Speedometer error
Assumes current tire is OEM. Reading = gauge after the swap; actual = true road speed.
Reading
Actual
Diff
50 mph
50.2 mph
+0.2
60 mph
60.3 mph
+0.3
70 mph
70.3 mph
+0.3
How tire size changes affect fitment and speedometer accuracy
Overall diameter drives clearance, gearing feel, and speedometer error — keep swaps near stock.
Aim for about ±3% diameter change for daily driving; ±1.5% is the ideal band.
Taller tires make the speedo read low; shorter tires make it read high — use the table above to quantify it.
A tire size like 225/45R17 encodes three numbers: section width in millimeters (225), sidewall aspect ratio as a percent of that width (45%), and wheel diameter in inches (17). From those values you can calculate sidewall height, overall diameter, and circumference — the measurements that decide whether a new tire will clear the fenders, change ride height, or throw off the speedometer.
When you plus-size (larger wheels with lower-profile tires) or switch to a different all-terrain size, the goal is usually to keep overall diameter close to stock. Most shops treat about ±3% as the practical window. Inside that band, ABS and stability systems usually stay happy, gearing feel stays familiar, and speedometer error stays modest. Outside it, you can rub on full lock or compression, see larger speedo/odometer error, and change effective final drive ratio.
This tire size calculator compares two sizes side by side: diameter delta in percent and units, section width change, sidewall height, revolutions per mile/km, and a speedometer error map. It is built for DIY fitment checks before you order — whether you are comparing equivalent tire sizes for a wheel upgrade, checking an AT tire on a truck, or validating a plus-size package against OEM.
Speedometer error scales with diameter. If the new tire is taller, you travel farther per wheel revolution than the car expects, so the gauge reads low (you’re going faster than it shows). If the new tire is shorter, the gauge reads high. Use the table above for a concrete reading-vs-actual map, then decide whether you also need a speedometer calibration after the swap.
Treat this as a first pass before you buy. Still verify load rating, speed rating, bolt pattern, offset/backspacing, and real-world clearance with your suspension and fenders — especially on lowered cars, lifted trucks, and vehicles with aftermarket wheels.
Tire size comparison: plus-sizing and equivalent sizes
A tire size comparison is just two sidewalls and a diameter delta. Plus-sizing (17→18→19) usually pairs a larger rim with a lower aspect ratio so overall diameter stays near stock — that is the “equivalent size” idea behind most tire size charts. This calculator replaces a static chart: enter OEM and candidate sizes and read diameter %, width, sidewall, and revs per mile instantly for your exact pair.
Stay inside about ±3% for daily driving (±1.5% when you care about speedo accuracy and ABS). Trucks moving to taller AT tires often exceed that band on purpose — then plan on gearing and speedometer calibration afterward, not just clearance.
Speedometer error after a tire size change
Speedometer and odometer error track overall diameter almost one-for-one. A tire that is 3% taller means true road speed is about 3% higher than the needle shows; a shorter tire makes the gauge read high. The reading-vs-actual table above maps that error at common speeds so you can see whether a swap is “live with it” or needs a calibration / tune.
If you also changed axle gears, diameter alone does not tell the whole story — use the speedometer calibration calculator with both tire sizes and gear ratios. For cruise RPM after a tall tire, pair this page with the gear ratio and RPM at speed tools.
How to compare tire sizes
Enter your current (OEM) tire size. Use the size on the sidewall or door sticker — width, aspect ratio, and rim diameter.
Enter the new tire size you are considering. Try common plus-size or all-terrain options and watch diameter % change update instantly.
Check fitment band and speedometer impact. Stay near ±3% for daily driving, then review the speedo table and related gearing tools before you buy.
Tire size calculator FAQ
A big jump in overall diameter changes how far the car travels per wheel turn. That can throw off the speedometer and odometer, stress ABS and traction control, alter effective gearing (harder launches or higher RPM on the highway), and cause rubbing on fenders or suspension. Extreme mismatches can also confuse AWD systems that expect matched rolling diameters.
Enter the current (OEM) size and the new size — width / aspect / rim — and compare overall diameter %, section width, sidewall height, and revolutions per mile. Stay near ±3% diameter for daily driving (±1.5% is ideal). Use this tire size comparison to decide if a plus-size or AT swap is close enough before you order.
Equivalent sizes keep overall diameter close while changing rim diameter or width (classic plus-sizing). There is no single universal chart for every car — clearance and load ratings differ — but comparing diameter % on this page is the practical substitute for a static tire size chart.
225 is the section width in millimeters, 45 is the sidewall height as a percentage of that width, and 17 is the wheel diameter in inches. Together they determine sidewall height, overall diameter, and circumference — the values this calculator compares.
Read the sidewall of your tire (for example 225/45R17) or check the driver’s door sticker / fuel-filler placard for the OEM size. Use the OEM size as “Current tire” when you are shopping for a swap so the comparison matches what the car was designed around.
Most fitment guides treat about ±3% overall diameter as the practical limit for daily driving. Within ±1.5% is usually ideal for speedometer accuracy and clearance. Beyond ±3%, expect more speedo error, possible ABS quirks, and a higher chance of rub on lock or compression.
Speedometer error scales almost 1:1 with overall diameter change. Roughly, a +3% taller tire means you are going about 3% faster than the gauge shows (speedo reads low); a −3% shorter tire does the opposite. Use the reading-vs-actual table on this page for exact mph/km/h at common speeds.
Yes. A taller tire travels farther per revolution, so at a given indicated speed you are actually going faster than the gauge shows. A shorter tire does the opposite — the gauge reads high while true road speed is lower.
It can. A taller tire acts like taller gearing (sluggish acceleration, lower cruise RPM); a shorter tire revs higher. On AWD and 4WD vehicles, mismatched diameters between axles or corners can make the center differential or transfer case work overtime and wear faster. Keep sizes matched across the set whenever possible.
For most cars, yes — run the same size on all four corners unless the vehicle was designed staggered (different front/rear sizes from the factory). Mixing diameters left-to-right or front-to-rear can upset handling and electronic systems. Always follow the OEM layout if it is staggered.
Keeping diameter close is the main rule, but plus-sizing still changes width, sidewall stiffness, and load capacity. Recheck offset, fender clearance, and the tire’s load/speed ratings before you buy — especially on lowered cars or trucks with aftermarket wheels.
If diameter changes more than a little, yes — or expect ongoing speedo and odometer error. Use the reading-vs-actual table on this page to see the error at common speeds, then decide whether a calibration or tuner remap is worth it. The speedometer calibration calculator can also factor axle ratio changes.