Well I put a Clarke 4.2 gallon tank on my 91 XR250R and thought I would type up a review for any other XR250R riders.
FIT/MOUNTUP: The tank fits great and mounts up easy using the stock tank mounts,petcock,and bracket that seat slides into(same bracket for hooking rubber strap to). I got a Red tank and the finish on the tank looks great.
FEEL OF TANK WHEN SITTING AND RIDING BIKE: With the stock tank if you look at the bike from behind it looks really skinny,the Clarke if you look at it from that angle looks quite a bit bigger so I intinally thought that it was going to be far to wide for the singletrack and take away the nimbleness of the XR

,However this is simply not the case. After I got the seat on I sit on the bike fearing the worse and to my surprise I could not tell the clarke was any wider than the stocker when I was sitting on it. It seems that the tank is skinnier where your knees rest against the tank and then gets a bit wider above that point. I filled the tank up and it ways 30 pounds full

but I was still popping wheelies like I used to even with the added weight up front.
AFTER A FEW RIDES WITH THE TANK ON: Well I have put right around 21 1/2 hard miles on the bike since I put the tank on and I don't miss the stock tank a bit. The weight didn't bother me and I still couldn't tell the tank was anywider than the stocker. The Clarke tank sits down closer to the engine than the stocker thus still keeping the center of gravity low and the bike didn't feel tippy either. One thing I really noticed is that it seemed that I could get the front end to stick better in turns with the clarke, I still slid up to the tank but it seemed to let the front end stick a little better with the added weight up front(Maybe it was my mind playing tricks on me

: )Anyhow I really like the tank and my plan was not to feel it all the way up unless I was going on a really long ride to keep the weight down but I think i'll go ahead and fill it up everytime anyway.
WOULD I RECOMMEND IT TO OTHER XR250R RIDERS: Yes I would my impressions on this tank after the 21 1/2 miles I put on it are alot better than what I was thinking before I rode it. The only thing that is a little aggravating is that it is kinda hard to adjust the idle knob with the clarke tank, It can be done pretty easy utill you get in mud and your gloves and the knob on the screw is wet and slick,You then can't get enough grip on it to turn it but its not to big of a deal,you can always take a glove off if you have to so you can get a better grip.
P.S. You can still operate the choke just fine
Well those are my thoughts on the Clarke tank mounted up to an XR250R. Maybe someone will get something out of this review later on down the road.
Later,Matt