Yes, the hub is identical
Same 5x108 PCD and same 63.4 mm centre bore: wheels from Smart #5 bolt on and centre on Volvo EC40 without rings. You still need to check the offset and tyres, detailed below.
≈: width estimated from the original tyre size.
Wheel by wheel: offset and tyres
Each original wheel from Smart #5 is compared with the one from Volvo EC40 of the same diameter: how much closer the wheel gets to the shock absorber or how far it sticks out of the wheel arch, and whether the tyre keeps the same overall diameter.
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7.5Jx19 ET43 · 245/50R19 — Check on the car
Compared with the original 7.5Jx19 ET50.5 on Volvo EC40: shock absorber side −7.5 mm, wheel arch side +7.5 mm, tyre diameter +1.4 %.
Check in detail
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8.5Jx20 ET43 · 255/45R20
Volvo EC40 has no 20-inch original wheel in our data: check the approved fitments and caliper clearance.
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8.5Jx21 ET43 · 255/40R21
Volvo EC40 has no 21-inch original wheel in our data: check the approved fitments and caliper clearance.
Before fitting the wheels
An identical hub is not enough: offset, width and diameter change the position of the wheel, and larger calipers can touch a smaller wheel. Original wheels often use bolts or nuts with a specific seat: keep the fixings intended for the wheel, with the thread of Volvo EC40 (M14x1.50), tightened to the torque shown on its vehicle page. Finally, check that the size is among the approved fitments for your vehicle. This comparison is for guidance only: have the fitting approved by a professional.
And the other way round: wheels from Volvo EC40 on Smart #5?