Per gz250 manual downloaded from this site, page 7
FUEL
Your vehicle requires reqular unleaded gasoline with a minimum pump octane rating of 87. Unleaded gasoline containg MTBE (Mehty Tertiary Butyl Ether)(also known as RFG-reformualed gas) may be used in your motorcycle if the MTBE content is not greater than 15%. Blends of unlead gasoline and ethanol (grain alcohol), known as gasohol, may be used in your vehilce if the alcohol content is not greater than 10%. Do NOT use fuels containing more than 5% methanol under any circumstances.

There is no mention of an upper rating limit, however, I do have the 1999 manual which states use gasoline of at least 91 octane.

Attn Sarris and EasyRider here is the octane recomendation from the service manual and in my owners manual from Canada

page 9 in the service manual on this site.

I use 91 mostly, especially in the Summer to help engine temps. Anyway, it's a good idea to use higher octane if you don't ride as much since I read that fuel sitting in your tank loses about one octane rating per month.

I've put nothing but 87 in so far. After reading this, I'm going to move up to 89 and see if I can feel any difference.

I ran 89 in my last tank.

I noticed no difference and experienced a loss of 1.4MPG. Again, 1.4MPG falls within the realm of varying fuel economy. But, that's what I got, none the less.

I ran two tanks of 89, noticed no difference in performance, but lost over 4 MPG. Went back to 87 octane and my mpg is back over 72mpg :-). It dropped to 67-68mpg on the 89 octane. :-(

From what I understand, the higher the octane, the less energy, so worse mileage would make sense. So far, results seem to be mixed as far as performance goes. Several found no difference, one or two found smoother running.

OCTANE
 

I HAVE USED 93 AND 93.5 IN MINE SINCE I BOUGHT IT 3 MONTHS AGO. I HAVE OVER 1200 MILES ON IT , HIGHWAY , CITY AND COUNTRY ROADS. HILLS AND FLAT LAND. I DON'T KNOW IF IT RUNS BETTER,BECAUSE I HAVE NOTHING TO COMPARE TO,BUT I GET 76 - 78 MPG FROM THE FIRST DAY. I KNOW IT BURNS CLEANER,SO LESS DEPOSITS LEFT BEHIND. MY HUSBAND ONLY RUNS 93 OR HIGHER IN HIS 2005 HARLEY AND SAYS THERE IS A DEFFINITE DIFFERENCE. HE PUTS 101 OCTANE IN AT LEAST 2 TIMES A MONTH AND SWEARS THERE IS A POWER BOOST WITH THAT TANK. HIS BUDDY ALSO SWEARS BY THE 101 OCTANE- HE HAS A 2001 SHADOW

The seat-of-your pants testing isn't good for much.

STOP YELLING, please.

yelling
 

sorry, was typing at work. hard habit to break. didn't mean anything by it.