Do you test your aux capacity?


IBA Member
I am looking to put a little 3 gal aux tank on my R1200GSA and was wondering how you go about making SURE you don't go over the required 11.5 gallons. My tank is 7.9 gal and I was looking at either the 3 or the 3.5 gallon aux but the 3.5 puts me pretty close to that marker. I don't have any idea how much fuel is in the fuel lines and I know that is part of the calculation.

What do you do to test your capacity?



Premier Member
IBR Finisher
I do not run an aux tank on my GSA, and several others on the last IBR didn't either. Tim Masterson did a math model showing the time savings of an aux tank on a GSA (for his riding speed and style) and found little benefit. A fuel cell might be nice, but is not required for long distance riding on a GSA. Just a thought for you!

The IBR rules usually have a long section talking about fuel cells and how they determine capacity during tech inspection.
Last edited:


IBR Finisher
One fairly simple way to test as an upper bound is, assuming the cell is a simple geometric shape, to measure the outside dimensions. Measure length, height, and width and figure the cubic inches. Fuel is 231 cubic inches per gallon. This was the initial method I used as an initial inspector in two IBRs.

Cells that were close to the limit would then be elevated for additional scrutiny and involved a more complex process using scales, temperature, and deft calculator of Austin.


Premier Member
IBR Finisher
If you really need to be sure, you arrange to ship it to Tom Austin, well before the IBR and he will test/measure it and let you know.
You run into the law of diminishing returns. The larger your OEM tank, the less benefit there is in an aux cell, but the costs remain broadly the same, as do the disadvantages.

With close to 8 gallons stock, it is a toss-up what benefit you would get. One of the main advantages is the removal of any "gas anxiety", and the ability to plan fuel stops that benefit you most, rather than having to stop because you are low on gas. For example, if I know there is a long ride between bonuses I like to fill up midway, rather than close to a bonus because it gives me a break.

8 gallons is not enough to entirely dismiss aux gas though. In that situation I might try to see if the IBR would sanction a 1 gallon RotoPax, properly mounted, even though they usually forbid loose gas cans. It can't hurt to ask.