*Blinks*
Not to fill the car when it is raining? Then that would hold true for not driving in the rain, because your vehicle is heavier!
In all honesty, if you are that worried, the amount of extra fuel you would use when the car is wet would be far less than if you had to stop at one extra red light.
(Then again, the extra oxygen content in water vapor that would enter the engine when it is raining. High humidity will help in the combustion process, would far outweigh any negative aspect of added weight of the rain droplets on the vehicle body)
It was noted somewhere that a vehicle uses 80% of it's fuel to move from a standstill. Thus, if you really wanted to save fuel, then lift off the throttle way before a red light, slowing down naturally, and try to arrive at the light when it changes to green, whilst still moving. In 2nd or 3rd gear.
You will far less fuel than if you had to stop, idle, then accelerate from a standstill.
One tip that catches drivers out here in Australia is to fuel up when it's cold (Night time) Fuel is more dense when it's cold. (Expands when it's warm)
All is well if you are immediately driving somewhere. Yet what gets many is, they fill up at night, then go straight home, park and leave the vehicle all day. As it warms up during the day, fuel expands, and in most systems will overflow from either the filler cap or the charcoal vapor recovery canister. On older carby vehicles, it will flood the engine with raw fuel.
Just make sure your vehicle is operating correctly, injectors are clean, tyres are pumped up to their max allowed pressure, take out un-needed weight, drive sensibly, anticipate the traffic and lights. No need to race anyone. Keep RPM's down.
