How to calculate whether travelling to get better petrol discount? App or formula available?

Forums › Transport (cars, bikes and boats) › How to calculate whether travelling to get better petrol discount? App or formula available?

Tatou

235 posts

Master Geek

#306633 9-Aug-2023 10:24

Before I attempt to construct something in Excel I wonder if there is an app or online formula that would allow me to do the follow

I wish to compare total cost of refilling at Station A (the cost of petrol to get to a refill station and the savings at that station) compared to Station B (the cost of petrol to get to a different refill station (closer, same distance or further)

Saving x cents more per litre must have a diminishing return depending on the distance, amount of fuel purchased and liters/kilometer of the vehicle

I know the liters/100km my car uses.

I know the approximate distance to nearby petrol stations from my home

I know the prices that are offered per liter

I am assuming a static amount of petrol purchased regardless of where I travel (typically half a tank) to make the calculation easier but if possible would like to have the amount of petrol as a variable.

I know the capacity of my petrol tank

Thanks

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#3113644 9-Aug-2023 10:27

I would enter what you've put above into ChatGPT and it'll give you an answer

nitro

656 posts

Ultimate Geek

#3113649 9-Aug-2023 10:42

do you make trips specifically for a fillup? that may be where what you're thinking makes the most sense. i'm sure there will be people who'd find it handy as well.

i do my fillup along the way home from work, and know which station has the best price (gaspy helps occasionally). that way, in my mind, i don't burn petrol just to fill up or get home from filling up.

richms

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#3113653 9-Aug-2023 11:13

You also need to allow for maintenance etc in your distance, not just the l/100k of fuel used.

IMO it's seldom worth it more than a couple of km when you factor that and the wasted time into it.

I dont have many options for 95 near me, and one is always more than the other so I fill at the Z on onewa on the way down the hill. Sure, its sometimes 2c a litre more than the one along glenfield road, but on the small tank in the toyota that is 80-90c, and on the ute its $1.20 for a 2c difference.

If someone offered to pay you $1 to drive out of the way somewhere to buy something you would tell them to get lost. Same with the gas saving.

Richard rich.ms

eracode

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#3113656 9-Aug-2023 11:22

I sometimes just try to do the calc in my head. e.g. Say my car costs $1 per km (all up) to run and I have to drive an extra five km (return) to save 15c/litre on 50 litres. That’s $5 of cost to save $7.50. Not really worth worrying about - unless the price difference is much greater or something. Also, car cost per km may be more than $1.

Sometimes I just sit and think. Other times I just sit.

wellygary

8312 posts

Uber Geek

#3113657 9-Aug-2023 11:39

eracode:

I sometimes just try to do the calc in my head. e.g. Say my car costs $1 per km (all up) to run and I have to drive an extra five km (return) to save 15c/litre on 50 litres. That’s $5 of cost to save $7.50. Not really worth worrying about - unless the price difference is much greater or something. Also, car cost per km may be more than $1.

And there is the crux of the matter,

There needs to be a pretty big price difference/litre to warrant travelling more than round the corner...

MikeAqua

7773 posts

Uber Geek

#3113662 9-Aug-2023 11:42

I would just use 79c/km. It's the IRD rate. Close enough.

Mike

WyleECoyoteNZ

1049 posts

Uber Geek

#3113663 9-Aug-2023 11:48

It would be something interesting to know. I put fuel in the car on Sunday, and the gap between the cheapest and most expensive was 34c! (Z Johnsonville 2.939 vs Petone Pak N Save 2.597)

I did have to travel around 10 km's there and back to get to the Pak n Save vs less than 2km's for BP. There was a Waitomo (Johnsonville), which was the closest, but that was still 10c more than the Pak n Save.

Heck, even the 3 Waitomo's within a ~10Km radius all had different prices, Tinakori\Thorndon 2.619, Waitomo Tawa 2.679 and Waitomo Johnsonville 2.709.

All the prices I've quoted were for 91 Octane, and pre any discount.

In a similar vane, is there much of a difference to consumption when only driving around town with Full Tank vs Half Tank ? Around town consumption is usually high 9L/100km - 10L/100km ?

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boosacnoodle

963 posts

Ultimate Geek

#3113684 9-Aug-2023 12:51

What is your time worth?

MikeAqua

7773 posts

Uber Geek

#3113689 9-Aug-2023 13:13

boosacnoodle:

What is your time worth?

According to the NZ govt, a maximum of 127/day.

Mike

nitro

656 posts

Ultimate Geek

#3113693 9-Aug-2023 13:23

WyleECoyoteNZ:

It would be something interesting to know. I put fuel in the car on Sunday, and the gap between the cheapest and most expensive was 34c! (Z Johnsonville 2.939 vs Petone Pak N Save 2.597)

I did have to travel around 10 km's there and back to get to the Pak n Save vs less than 2km's for BP. There was a Waitomo (Johnsonville), which was the closest, but that was still 10c more than the Pak n Save.

Heck, even the 3 Waitomo's within a ~10Km radius all had different prices, Tinakori\Thorndon 2.619, Waitomo Tawa 2.679 and Waitomo Johnsonville 2.709.

All the prices I've quoted were for 91 Octane, and pre any discount.

love how pak n save in that corner of wellington influences petrol prices for the services stations there. there's usually a $0.14 difference in synergy supreme (98) between mobile petone and mobil high street.

Tatou

235 posts

Master Geek

#3113711 9-Aug-2023 13:57

Thanks for all the replies.

My practice is fill up the car when the tank is half empty. I do make a special trip to do so unless I happen to be driving pass the refill site that is offering that best price in the area.

I live in Karori Wellington where the pump prices are high even with the 6 cent off on special days. Currently I go to Caltex on Hutt Rd every Wednesday which is 10c off plus 2c off for Supergold. Saving today was $2.86 on 23.84l.

eracode

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#3113723 9-Aug-2023 14:06

Tatou:

Thanks for all the replies.

My practice is fill up the car when the tank is half empty. I do make a special trip to do so unless I happen to be driving pass the refill site that is offering that best price in the area.

I live in Karori Wellington where the pump prices are high even with the 6 cent off on special days. Currently I go to Caltex on Hutt Rd every Wednesday which is 10c off plus 2c off for Supergold. Saving today was $2.86 on 23.84l.

Interested to know why you fill up when the tank is half-full.

It looks like your special return trip is about 14 km. How much do you estimate it costs to drive your car 14 km? Is the saving of $2.86 really worth the time and cost of that drive?

If your tank was closer to empty, you might save $5+ on the fill.

Sometimes I just sit and think. Other times I just sit.

Tatou

235 posts

Master Geek

#3113724 9-Aug-2023 14:06

ChatGPT says this

To calculate the formula for comparing the total cost of refilling at Station A and Station B, taking into account the savings per liter and considering diminishing returns based on distance and vehicle efficiency, you can use the following approach:

Let's break down the formula step by step:

Calculate the effective savings at Station A and Station B based on the savings per liter and the amount of fuel purchased:

Effective Savings at A = Savings per liter at Station A * Liters of fuel purchased
Effective Savings at B = Savings per liter at Station B * Liters of fuel purchased

Calculate the effective cost at Station A and Station B, which includes the cost of fuel and the effective savings:

Effective Cost at A = Cost per liter of fuel at Station A * Liters of fuel purchased - Effective Savings at A
Effective Cost at B = Cost per liter of fuel at Station B * Liters of fuel purchased - Effective Savings at B

Calculate the distance factor based on the distance to each station and the vehicle's efficiency:

Distance Factor A = Distance to Station A / Vehicle's efficiency (liters/kilometer)
Distance Factor B = Distance to Station B / Vehicle's efficiency (liters/kilometer)

Apply the diminishing returns factor, which can be based on a formula that considers the distance factor and the amount of fuel purchased:

Diminishing Returns Factor = 1 / (1 + (Distance Factor A + Distance Factor B) * Liters of fuel purchased)

Calculate the Total Cost considering the diminishing returns factor:

Total Cost at A = Effective Cost at A * Diminishing Returns Factor
Total Cost at B = Effective Cost at B * Diminishing Returns Factor

Now, you can compare the Total Cost at Station A with the Total Cost at Station B to determine which station offers a more cost-effective refilling option.

Please note that this formula assumes a simplified model and might need adjustments based on specific factors, such as the exact nature of diminishing returns, the accuracy of vehicle efficiency, and other contextual considerations.

It's recommended to test and adjust the formula according to real-world data and user feedback.

Tatou

235 posts

Master Geek

#3113745 9-Aug-2023 15:07

eracode:

Tatou:

Thanks for all the replies.

My practice is fill up the car when the tank is half empty. I do make a special trip to do so unless I happen to be driving pass the refill site that is offering that best price in the area.

I live in Karori Wellington where the pump prices are high even with the 6 cent off on special days. Currently I go to Caltex on Hutt Rd every Wednesday which is 10c off plus 2c off for Supergold. Saving today was $2.86 on 23.84l.

Interested to know why you fill up when the tank is half-full.

It looks like your special return trip is about 14 km. How much do you estimate it costs to drive your car 14 km? Is the saving of $2.86 really worth the time and cost of that drive?

If your tank was closer to empty, you might save $5+ on the fill.

Started the half fill when petrol price were volatile ( bit like spreading your currency buy :-) )

Price was $2.537 for 23.84l total of $60.1

Cost to get there and back 14km Car is a Hybrid getting 4.9l/100 hence .049l per kilometer which is .68l for 14 kilometers which is $1.74 hence Total cost is $61.74

Cost of nearest station in Karori today BP is $2.92 a total of $69.61. 1 kilometer to BP hence .049l which is 14c. Total cost is $69.81

Saved $8.09 going to Caltex against nearest station

eracode

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#3113747 9-Aug-2023 15:12

@Tatou Ah - so your saving in terms of your OP was $8.09 - not the $2.86 you mentioned earlier.

Sometimes I just sit and think. Other times I just sit.

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