Stick to below the limits. Concentrate on the road and the potential hazards. This is especially important when in areas, like towns, where you are around things outside your control. For example, over 30 in a 30 is idiotic.
FWIW, I find most modern cars read a consistent mph higher than the real speed, not a percentage. Most old cars tend to be a percentage, possibly because of modifications or different wheels or tyres.
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