Is 245/40R18 bigger than 245/45R17?

245/40R18 is 0.04 inches taller and 0 mm wider. In overall diameter that is +0.14%, which stays inside the ±3% rule of thumb for a substitution.

How much will 245/40R18 throw off my speedometer?

Every reading scales by 1.00138. An indicated 60 mph becomes 60.1 mph, and every 100 miles on the odometer is really 100.1 miles.

How much does 245/40R18 change ground clearance over 245/45R17?

The axle centerline moves by 0.02 inches — half the diameter change, because half the extra height sits below the hub and half above it.

Will 245/40R18 fit where 245/45R17 fits?

Nothing derived from the size code can answer that. Clearance depends on wheel offset, suspension travel and body dimensions that no tire size contains. This page tells you exactly what changes dimensionally, which is the part that can be calculated.

245/45R17 vs 245/40R18

245/40R18 is +0.04″ in overall diameter and 0 mm in section width against 245/45R17 — a diameter change of +0.14%, which stays inside the ±3% substitution rule. An indicated 60 mph would really be 60.1 mph.

Within 3%

Every dimension

Difference between 245/45R17 and 245/40R18
Dimension	245/45R17 → 245/40R18	Change
Overall diameter	25.68″ → 25.72″	+0.04″ (+0.14%)
Section width	245 mm → 245 mm	0 mm (0.00%)
Sidewall height	110.3 mm → 98.0 mm	−12.2 mm
Rolling circumference	80.68″ → 80.79″	+0.11″
Revolutions per mile	785.3 → 784.2	−1.1
Axle height / clearance	12.84″ → 12.86″	+0.02″

245/45R17245 mm

245/40R18245 mm

245/45R17 25.68″245/40R18 25.72″

Which is taller, 245/45R17 or 245/40R18?

245/40R18 is the taller of the two, at 25.72″ against 25.68″. The gap is 0.04 inches, or 0.9 mm.

Both are metric sizes, so neither states its height directly. Both figures below are derived from the section width, aspect ratio and wheel diameter in the code. 245/45R17 is 245 mm wide with a 110.3 mm sidewall on a 17″ wheel; 245/40R18 is 245 mm wide with a 98.0 mm sidewall on a 18″ wheel.

What the speedometer will read

Indicated versus actual speed after the size change
Speedometer reads	Actual speed	Error
25 mph	25.0 mph	+0.0 mph
40 mph	40.1 mph	+0.1 mph
55 mph	55.1 mph	+0.1 mph
60 mph	60.1 mph	+0.1 mph
70 mph	70.1 mph	+0.1 mph
80 mph	80.1 mph	+0.1 mph

Fitting 245/40R18 in place of 245/45R17 multiplies every speed and distance reading by 1.00138. Over 10,000 indicated miles the odometer is out by +14 miles.

Effective gearing moves by the same proportion. A vehicle with 3.73 axles running 245/45R17 would need 3.74 to keep the same engine speed on 245/40R18. Work it out for your own axle ratio.

Full specification of each size

245/45R17

Derived dimensions for 245/45R17
Dimension	Metric	Imperial
Section width	245.0 mm	9.65″
Sidewall height	110.3 mm	4.34″
Wheel diameter	431.8 mm	17.0″
Overall diameter	652.3 mm	25.68″
Rolling circumference	2,049.3 mm	80.68″
Revolutions per mile	488.0 / km	785.3 / mile
Measuring rim (estimate)	203 mm	8″

245/40R18

Derived dimensions for 245/40R18
Dimension	Metric	Imperial
Section width	245.0 mm	9.65″
Sidewall height	98.0 mm	3.86″
Wheel diameter	457.2 mm	18.0″
Overall diameter	653.2 mm	25.72″
Rolling circumference	2,052.1 mm	80.79″
Revolutions per mile	487.3 / km	784.2 / mile
Measuring rim (estimate)	216 mm	8.5″

Rim widths differ too: 245/45R17 is designed for 7.5″–9″ and 245/40R18 for 8″–9.5″. Their measuring rims differ, so the same wheel will not be ideal for both.

Can you swap one for the other?

Inside the ±3% rule

The diameter difference of +0.14% is within the ±3% that fitters use as the boundary for a substitution, so the speedometer, odometer and wheel-speed-derived systems stay close enough to calibration. The section width difference of 0 mm is a separate question and is the one that decides whether it rubs.

Neither figure says anything about physical fitment. Whether 245/40R18 clears the strut, the fender liner and the steering stops depends on wheel offset and suspension geometry that a size code does not encode. For maintenance rather than sizing, Car Basics covers tire care.