Wheel/trye size affecting speed readings - Alfa Romeo Forum
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Wheel/trye size affecting speed readings

Changed my GT 16" wheels to 18"

On the speedo it is now reading around 10 miles less per gallon than with the 16" wheels.

I would have thought it should be more as the wheels are bigger.

Does the computer have to be reset???

Thank you
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Should be the the same diameter from 205/55R16 to 225/40R18.
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The bigger and wider wheel will often be much heavier and its mass is concentrated further away from the center. It will have larger rolling resistance and require more energy to accelerate which in turn will result in worse economy, slower acceleration times and worse handling due to larger unsprung weight for the suspension to handle. Steering response can be improved.

The larger diameter rims are often only an improvement on looks...
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Here you go....Tire Size Comparison
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The rolling diameter if you have not selected the correct tyre size (as posted) will screw speedometer readings and such.
As for the MPG, just do a full tank to full tank test and not the mileage.
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