Calculating date I next need to buy diesel

Hi everyone,

I currently have a formula that will calculate the date I will next need to buy diesel, based on the current mileage left in my tank and the average miles travelled per day. It also takes into account weekends and Bank Holidays. What it can't do is take into account days when I do extra mileage.

Example:
I have 300 miles left in my tank.
I normally travel 30 miles/day.
Result = I need to buy diesel in 9 working days (I deduct 1 day as I don't want to run out of fuel all together )

Quandary:
In 3 days time I will be doing an extra 60 miles for a day trip. This would mean I need to buy fuel in 7 working days.
However, if the extra miles were to be done on the 9th working day, I would need to buy the extra fuel on the 8th working day.

It seems so simple to work this out in my head, but I can't think of a way to do this with a formula. Can anyone help please?

tyvmia

Thank you mehmetcik, but I need a formula that will take into account weekends and holidays, plus the actual date(s) and mileage where I travel extra miles to calculate the specific date I will need to top up my tank.

The formula I am currently using is =WORKDAY(TODAY(), FLOOR(Miles_InTank/27.4, 1), Holidays)-1

Miles_InTank is the named cell I enter the current range of the fuel tank
27.4 is my normal daily mileage
Holidays is a named range of dates for UK Bank Holidays and any planned annual leave

I also have a separate table called ExtraMileage with the following columns: Date, Description, Mileage

For budgeting purposes, for instance, if I have 400 miles in the tank now, and I use 270 miles before Xmas and 3 trips of 60, 40 and 90 miles planned after Xmas, I need the formula to provide the top up date to be after the 40 mile journey.