Car MPG and MPH

Have you tried comparing your car's speedometer vs GPS to see how accurate it is? Is it accurate?

How about comparing your car's estimated MPG vs reality (track miles vs gasoline use)? Understanding that if the MPH is inaccurate (e.g. reporting higher than reality), your computed MPG would be off (e.g. lower than reality) too.

We have four cars and they're all pretty close to what GPS says for MPH-- like, within 1%.

Yes same here - very close.