I've currently got an excel template which I've been using for a number
of years (which can be improved, but it works) and I'm wanting to
update the way I calculate the consumption figure per vehicle.

What I currently do is paste the data from a website and sort it by
date & odometer readings and work out the fuel consumption based on the
total number of quantity of litres purchased, less the most recent
odometer record minus the first record.

or in simple terms : ((Total Fuel Qty * 100) / (End ODO - Start ODO))

What I'm hoping to accomplish is to calculate it over a 3 month period,
without having to manually edit the data.

I think a series of SUM(IF.... statements would work but how would I go
over a 3 month period programatically? Max(Tran-date - 3 months) ?
The Start ODO will be the odo record at the period commencement date
i.e. Trandate - 3 months.

Any ideas?