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Our cars do have a cable-operated speedometer. But there is a VSS inside the instrument cluster that takes that cable movement and converts it into an electrical signal that is sent to the ECU. Your ECU will throw a code with the speedometer cable disconnected (BTDT!) -- it does know about the speed of the car.
And as MetroMPG said, you don't have to know the exact flow. Just plug in a guesstimate number (and numbers that other people use are available out there!) and then tweak it based on your gallons used / miles traveled calculations.
I'm not sure what exactly would be causing your problems, but colder weather puts a hurt on fuel economy in many different ways, from making the air denser to push through, to taking more energy to get the engine up to operating temps. So that could be a factor...
-soD
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