The first step is to find out if the temperature gauge is actually reflecting the coolant temperature. The fact that the temperature gauge is dependant on road speed would suggest that the thermostat isn't sealing very well in its housing. Even a tiny gap will keep the engine running cool when it is cold outside.
If you run the engine for a few minutes, the water temperature in the engine should be noticeably above ambient, whilst the hose from the thermostat housing to the radiator should be absolutely stone cold-right up to the T-stat. If it is getting warm prematurely, you need to look at the fitting of the thermostat more closely.
If the engine is genuinely running cool, you'll get poorer fuel economy and more contamination of the oil with fuel.
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