The tank sat on the front of the seat so eventually I modified the seat mount to adjust the seat back further. I used an old "1000 mile solo seat owners trick" and stuffed a tennis ball between the bottom of the seat and the mounting frame. This keeps the seat from bottoming out on the mounts and relieves some of the stress from the "mouse trap" suspension spring.
I've stuffed 7.5 gallons in mine when low on fuel. I've never filled it to the brim so think it is probably an 8 gallon tank.
My old beemer started life as a Toaster Tank model. After 10 years of riding and wanting to do something different the Toaster was sacrificed (sorry "purists") for a reasonable capacity for a side car. The 4.5 gallons wasn't going to cut it.
If the rig never left town the little tank would suffice but that wasn't the case for our use as we planned on a touring type scenario.
Before our first road trip (and before the Heinrich Tank was acquired) I hunted down a 6.2 gallon touring tank and installed it for the needed range. We made our first road trips with this tank.
After the Heinrich Tank came along I auctioned this off on EBay. It brought a surprising $350.00 sale price, even with a disclosed, stained, rattle can paint job.
This is the same tank after paint, a freshly polished aluminum fuel cap, new repro. tool box key and new BMW Roundels. I haven't installed the rubber knee pads yet. This was a $75.00 Mexico paint job with 4 -5 coats of clear. They did a fine paint job but the right knee pad indent is a little deformed at the bottom rear corner.
This is an R60/6 I bought in Yuma AZ. a few years ago for $1100.00. It was a pretty good little runner. She was original and unmolested except for the painted mufflers. The fuel tank style is nearly the same as the Toaster Tank except for the lack of the chrome panels. I think that after 1973 BMW had the under tank master cylinder cut out on all there tanks. This 1975(?) /6 had front drum brakes and did have the under tank cut out.