Can I set up the formula to update automatically?

I have a sheet with 12 columns, 1 for each month. I am updating it monthly
and I want it to take the average of the months I've used as I update it.
The problem is that the numbers I'm entering are vehicle mileage. Example:

Average Usage November December January
91667 91778 91822

In order to get the average usage I would subrtact December from November to
see that 111 miles were used that month, then do the same for January and get
the average of the months thus far. Here is the one I'm using:

=IF(COUNTA(I13:V13)0,((J13-I13)+(K13-J13))/(COUNTA(J13:V13)),"")

J13 - December
I13 - November
K13 - January
V13 is the final month.

I'd like to set it up so that each time I put in the mileage for a new month
the "Average Usage" will be updated. I've got a formula right now, but I
have to update the formula each month in order for it to work.

If you have any questions feel free to ask. Thanks in advance.

How about

(Max(range)-MIN(range))/count(range)

"pugsly8422" wrote:

I have a sheet with 12 columns, 1 for each month. I am updating it monthly
and I want it to take the average of the months I've used as I update it.
The problem is that the numbers I'm entering are vehicle mileage. Example:

Average Usage November December January
91667 91778 91822

In order to get the average usage I would subrtact December from November to
see that 111 miles were used that month, then do the same for January and get
the average of the months thus far. Here is the one I'm using:

=IF(COUNTA(I13:V13)0,((J13-I13)+(K13-J13))/(COUNTA(J13:V13)),"")

J13 - December
I13 - November
K13 - January
V13 is the final month.

I'd like to set it up so that each time I put in the mileage for a new month
the "Average Usage" will be updated. I've got a formula right now, but I
have to update the formula each month in order for it to work.

If you have any questions feel free to ask. Thanks in advance.