It's the size of the tyre used that determines the answer to this question
The larger the diameter of the wheel means that the profile (sidewall of the tyre) MUST be reduced.
I have a couple of shots that I can show you
Now on an 18" wheel the correct size tyre is a 225/40 18 ( we'll keep it simple and say non staggered)
Standard size tyre on a E36 318IS is 205/60 15
As the images clearly show, you can easily go from a 15" to an 18" and the speedo when using the correct size tyre is barely affected.
However, use the incorrect tyre and boom!!! Goodbye license
Using a 225/30 20 I was able to fit 20" wheels to my Coupe
When you are using a Chassis Dyno, you will input the tyre size.
The wheel/tyre size will affect the wheel torque.
Smaller wheels will increase wheel torque and decrease ground speed.
This will mean small wheels will make your acceleration better but reduce top speed. It is another gear system to be acounted for.
Power is not affected by gear ratio, so power will be same.
If there is large changes in rotating mass, this can remove wheel power.
To imagine this, think of big solid steel wheels. The dyno will look at slower acceleration, and the car has less wheel power.
When calculation engine power from the drivetrain, the extra momentum will be measured, so a big correction will be made, and engine power will be OK.
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