A change this small is generally treated as a straight substitution. Speedometer error stays under three percent and the clearance designed around 235/45R17 is largely preserved. The section width still changes by 0.39 inches, so confirm your rim width suits 245/40R18 before ordering.
Want to try other sizes against these? Open this comparison in the interactive tool.
This is a plus-size: bigger wheel, shorter sidewall
Going from 235/45R17 to 245/40R18 moves the rim from 17 inches to 18 and drops the aspect ratio from 45 to 40. That is the classic plus-size pattern, and the intention behind it is usually appearance and steering response rather than clearance.
The sidewall shrinks from 4.16 inches to 3.86 — 0.31 inches less rubber between the rim and the road. Less sidewall flex sharpens turn-in and makes the steering feel more immediate, which is the point.
The trade is real and worth stating plainly. A shorter sidewall transmits more of the road surface into the cabin, and it leaves the rim far more exposed to pothole damage. Replacement cost also rises, because both the larger wheel and the lower-profile tire are more expensive than what they replace.
235/45R17 and 245/40R18 measured against each other
Both sizes reduced to the same units. The diameter row is the one that drives everything else on this page — ride height, gearing, speedometer accuracy and odometer readings all follow from it.
| Measurement | 235/45R17 | 245/40R18 | Difference |
|---|---|---|---|
| Overall diameter | 25.33" | 25.72" | +0.39" (+1.5%) |
| Section width | 9.25" (235 mm) | 9.65" (245 mm) | +0.39" |
| Sidewall height | 4.16" | 3.86" | −0.31" |
| Rim diameter | 17" | 18" | +1" |
| Circumference | 79.57" | 80.79" | +1.22" |
| Revolutions per mile | 796 | 784 | −12 |
| Ride height at axle | Baseline | +0.19" | +0.19" |
What the dial shows against how fast you are going
Your speedometer was calibrated to 235/45R17, which turns about 796 times per mile. Fit 245/40R18 at about 784 revolutions per mile and the same wheel speed now represents a greater road speed than the instrument assumes. The error is 1.5% at every speed, so it grows in absolute terms the faster you go.
| Speedometer reads | You are actually doing | Difference |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 mph |
| 45 mph | 45.7 mph | +0.7 mph |
| 60 mph | 60.9 mph | +0.9 mph |
| 75 mph | 76.2 mph | +1.2 mph |
Over distance the same proportion applies to the odometer. Every indicated 1,000 miles on 245/40R18 is really about 1015 miles covered — worth knowing on a leased vehicle, where recorded mileage and real mileage drift apart.
What this particular change asks you to check
- Section width grows by 0.39 inches, so the tire sits closer to the strut and the inner liner on both sides.
- Overall height grows by 0.39 inches, of which 0.19 inches goes upward into the arch — check at full lock and full compression.
- The rim diameter changes from 17 inches to 18 inches, so this swap needs different wheels as well as different tires.
- Speedometer error of 1.5% — check whether your vehicle supports recalibrating for a new tire size.
- Confirm the load index on 245/40R18meets or exceeds the figure on the vehicle's tire placard, particularly if the vehicle tows or carries load.
- On all-wheel-drive and four-wheel-drive vehicles, change all four together — mismatched rolling circumferences load the differentials and transfer case continuously.
235/45R17 vs 245/40R18 FAQ
Is 245/40R18 bigger than 235/45R17?+
245/40R18 measures 25.72 inches in overall diameter against 25.33 inches for 235/45R17. That makes it 0.39 inches taller, a change of +1.5%. It is also 0.39 inches wider in section width.
Will 245/40R18 fit in place of 235/45R17?+
The diameter change is +1.5%, which falls inside the 3% envelope generally treated as a routine substitution. That makes it a plausible swap, but the section width also changes by 0.39 inches, so confirm the rim width suits the new size and check clearance at full lock.
How much lift does 245/40R18 give over 235/45R17?+
0.19 inches at the axle — half the diameter difference. The other half of the change goes upward into the wheel arch, which is where fitment problems usually appear.
How far off will my speedometer be?+
The speedometer will read low by about 1.5%. At an indicated 60 mph you would actually be travelling about 60.9 mph, and every indicated 1,000 miles would really be about 1015.
Do I need to regear for 245/40R18?+
A +1.5% change is small enough that most vehicles absorb it without complaint. Regearing is usually considered from around 10% upward, or sooner on a vehicle that tows regularly.
What is the revolutions-per-mile difference?+
235/45R17 turns about 796 times per mile and 245/40R18 about 784 — a difference of −12 revolutions. That figure is what the speedometer and odometer are calibrated against, which is why both readings shift.
These are nominal figures. Every number above is calculated from the marked size. Real tires vary from nominal by a few millimetres between manufacturers, and by more as tread wears or pressure changes. Use this to plan and to check a quote — measure the actual tire before committing to a tight fitment.