Full tank vs quarter tank-what saves more fuel money?

With fuel price increasing on daily basis, we are forced to constantly innovate and find every possible way to save a few rupees on fuel. I was dealing with this dilemma for a long time. Which approach saves more fuel? Buy full tank petrol so that you don’t regret much when price goes up next day or carry just enough fuel and get better fuel economy as the car won’t have to lug along weight of full tank petrol.

Option	Model/approach	How it works?
1	Buy just enough quantity of fuel, save from reduced fuel consumption (as carrying full tank petrol burns more fuel)	Your car may have 40-45 liter tank capacity- fuel pricing increasing every week but instead of buying full tank fuel, buy just what is needed for a week (say 10 liters). As the car carries less fuel (35 liters= 30 kgs), there will be a marginal increase in fuel efficiency, so you get to save a few rupees per liter in terms of distance you can travel. But you've to pay little more next week when buying another 10 liters, as fuel price would have gone up for sure
2	Buy full tank petrol and save from price hike	Buy tank full of petrol (40-45 liters)- this may last for say 1 month during which fuel price is likely to go up by 4 to 5 rupees. You save a bit from this fuel price increase, though your car will burn more fuel compared to option 1, as it has to tug along a heavier fuel tank.

I had this dilemma for a long time. 

I was not sure which of the two model saves some money. Is the savings in fuel economy due to lesser fuel tank weight is good enough to offset the imminent price hike next week or the savings from not having to pay extra for petrol next week is higher? I did some calculations to decide on this. There were many variables at play.

1.       1. How  much petrol price increases every week? As of now it is fair to assume approx 1 Rupee per week (refer screenshots below for average monthly increase)

2.       2.How much is the increase in fuel economy if petrol tank is lighter? I found some online research which suggested 35% loss in weight could result in 12-20% increase in fuel. 1 liter petrol weighs roughly 750 grams. So not having to carry 35 liters of fuel roughly increases fuel efficiently by 1%

3.       3.Unknown factors- would the price drop by a few paise tomorrow or better to buy today itself? Cost of driving to fuel station frequently-time and effort spent

I did some intense calculations.

Option 1: carry less fuel, save on fuel economy, fill when needed

35% drop in weight increases FE by 12-20% (16% assumed for this calculation) Kerb weight of a typical car/compact SUV: 900-1200 kgs (1200 assumed for this calculation) Fuel tank capacity in most cars: 30-45 liters (45 is assumed for this calculation) 35 liters of fuel = 26 kgs (density of petrol is about 715-780 grams per liter, calculating at 750) 26 kg out of 1200 kg= approx 2% 2% drop in weight = 1 % increase in mileage Mileage with full tank= 15 kmpl (you do the math for your car) 1 % increase = 15.15 kmpl Per km 7 Rs, .15 km extra = 1 Rs savings per liter Per week 10 liters= 10 Rs savings Week 2 filling: 10 liters at 1 Rs extra = break even (10 Rs extra & 10 Rs savings) Net savings: 0 Rs Week 3 filling: 10 liters at another 1 Rs extra = 20 Rs loss extra minus 10 Rs savings Net savings: 10 Rs loss Week 4 filling: 10 liters at another 1 Rs extra = 30 Rs extra at 10 Rs savings Net savings: 20 Rs loss Monthly gain/loss (compared to carrying full tank load of 40 liters)= 10+20 = 30 Rs loss per month If petrol price is increasing only 50 paise per week: 5+0-5 = breakeven

Option 2: Buying full tank save from daily price increase

Fill 40 liter Savings on 30 liters: 10 Rs on week 1+ 20 Rs on week 3 + 30 Rs on week 4 =60 Rs saving Loss because of lost mileage at 1 Rs per liter= 30*1 = 30 Rs Net gain: Save 30 Rs per month if price increases a rupee per week, Breakeven if increase is only 50 paise per week

Final verdict: I feel it makes better sense to carry full tank petrol. What do you think?

	Pros	Cons
Fill just as much needed	Less weight to carry around, slightly higher fuel economy	Multiple visits to petrol station, more time and effort
		Will be forced to fill even if on the day your tank is empty price is up by a Rupee or two Example: if fuel station is not on your way and you've to take 2 km diversion, that is 14 Rs spent in reaching and returning from fuel station + 15-20 mins of time
Carry full tank	Don't have to track daily price fluctuations. As you are left with 25% fuel whenever you feel daily price has dropped a bit or stable you can tank up again	Not much savings if the price remains stable and doesn't skyrocket after you buy
	Some protection from daily price hike at least for a month	Higher loss from evaporation etc if car is not covered well in parking
	Less frequent visit to fuel stations
	Full tank fuel means less rusting during lockdown