How to get a running total with some blank cells

Hello,

I have a spreadsheet which I developed as a Log Book for my vehicle where I enter all costs incurred including cost of fuel and cost per litre when it was purchased.
I have columns for Date, Odometer, Trip, Total cost, Cost per litre plus fuel usage (calculated) -
I have a column which calculates litre used per 100 kilometers (L/100km) for each separate purchase which works fine.

I would like to add a column which automatically calculates average fuel usage since the log book commenced.
I have used =SUM(a1:a2)/(SUM(b1:b2)/100) which works until it encounters a cell in a column (column a and/or b) which is blank which can happen when I record a cost which was NOT fuel and then it returns "divide by zero" which I feel is because it is attempting to add a value for a blank cell. I think I need to use IF but not sure how to.

Could someone please advise how I can modify my formula to work.

Thanks

Thanks for your quick reply.

It is a new day since I created the thread and I now find that the error I reported doesn't seem to be occurring anymore BUT I still have a problem.

In the past when I created a 'formula' in a cell using the values from other cells when I then 'drag' that cell down the column the formula is copied
to each cell and the cell 'addresses' are changed automatically by incrementing them by 1.
I don't want this to happen for some of the cell addresses.

In each row (a fuel purchase) I calculate the fuel economy for that current trip.
I also want to automatically calculate the AVERAGE fuel economy since the beginning NOT taking an average of each trip fuel economy but using the total
fuel used and total distance travelled since the beginning.

I have attached a shortened version of my spread sheet.
Note column I (formula is =SUM(F4:F5)/(SUM(C4:C5)/100) )
When I drag cell IF down the column the formula is copied to each cell BUT the first 'parameter' of each 'SUM' is changed and I want it to remain as F4 and C4.

I have manually changed the formula in cells I6 and I7 to be correct.

NOTE that in the following cells the first cell in each SUM is increased by 1 eg F5 and C5.

Hope I have explained enough.

It's been many years since I was working in the IT industry and so don't use Excel very much these days.

I have uploaded my spreadsheet using 'manage attachments' but can't work out how to attach it to this post, sorry

I thought I had replied to you Jete - well I had typed in a reply which must have disappeared into the 'ether'.

Yes it works exactly as I wanted.

Thanks for your reply. I suspected that using the $ symbol would cause that reference to NOT increment as you dragged the cell's formula down the column as boopathiraja has now confirmed.

Thanks for both your replies.

Thanks Ali for reminding me.
Most forums don't require this so I forgot. Will now close it.