You should compare the rpm (round per mile) of the tires as well.
Very likely 40-height tires on 20" rim will be smaller, than stock. With some tolerance it might be acceptable, but you might have to calibrate the speedometer.
The only 100% guarantee is put the wheels on and drive.
When installing wider wheels what needs to be checked is fender clearance at maximum turn and maximum load.
Again -start with comparing rpm to stock.
Generally 6% difference is acceptable. I wouldn't see a big problem even with bigger difference, unless you cross mechanical abilities of the car.
Most of the time by "calibration" drivers understand knowing the difference between speedometer reading and actual speed.
So if you drive in 65MPH zone and speedo shows 65 -you'll know if you are below or over the limit.
The real calibration would involve changing the speedo gears, or software what is factory sealed.
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