It should help if you install summer tires after winter is over. Snow tires usually have high rolling resistance. Check if your ignition timing is correct. Make sure the engine warms up to normal operating temperature. If the heat feels weak you might need to replace the thermostat. Check the output from the oxygen sensor to see if the fuel mixture is correct. It should have about 0.5 volts after it warms up. If it's not working right it should turn on the "check engine" light. Replacing the oil in the differential with a slightly lower viscosity synthetic oil should help in the winter. There's a lot of other things you could check out. Read about the
65+ Vehicle modifications for better fuel economy - EcoModder.com if you haven't already.