AFAIK, the dry weight of the Pratt & Whitney R-2800 was several hundred pounds heavier than the Packard V-1650.
Agree... the ususal for a radial...heavier. I always think of the radial as "Deisel" and the inline premium petrol...ones heavier and less refined, but can handle more punishment.
Not really an apples to apples comparison. The R-2800 produced considerably more horsepower than the V-1650. Depending on who's numbers you believe, the R-2800 had a slightly better power to weight ratio than the V-1650. Also, the actual installed weight must be accounted for including the accessory section and cooling system, not just the basic dry weight.
Spitfires from the MK V onwards had a mounting point for a drop tank under the fuselage. There were 4 sizes of "slipper" tanks - 30, 45, 90 and 170 gallons (the 170 gallon tank was rarely used). Some "torpedo" style tanks were also used, of 45, 90 and 170 gallons. The slipper tanks were by far the most common. Over 300,000 were produced during the war. In 1944 two Spitfire IXs were sent to the US to be tested for conversion to long range escorts. They were fitted with small fuel tanks in the wings, Mustang drop tanks under the wings and had a total fuel load of 345 US gallons. They were tested at Wright Field. Range at 25,000 ft was given as 1,365 miles at 264 mph true, and 1,241 miles at 306 mph. In both cases that includes warm up, taxi, takeoff, climb to 25,000 ft, 5 minutes at war emergency power, 15 minutes at military power and 30 minutes reserve. The project was abandoned because the Mustang had become available in numbers. The Spitfires were flown back to the UK.