Which should I use..87, 89, or 91 octane?

I have been using 91 octane since I bought my 1965 pv544 in August '03, as per the advice of my dad, my mechanic, my intuition. A friend indicated that his 60's mustang ran better on 87, low octane , and used the reasoning that 91 octane wasn't around at the times these cars were built?!? (we're in our early 20's by the way). This week I put in some 89 octane (tank: probably ⏜%] 89 oct and ⎠%] 91 oct) and the only difference I might have noticed is that it took longer to reach the high end of my trans Βspd] gears (i.e. I was able to shift a few seconds later than usual for each gear). It was actually nice. I alsways thought I should get more 'stroke' out of my gears. Does anyone think that 91 octane burns too quickly? Maybe its not designed for the older cars? Of course with gas prices spiking (up to $2.45 for 91 octane in southern CA), it would be great to use the cheaper, anyways. Or should I keep using 91 oct? what are the long term ramifications, either way?

Any input would be great. What do you guys/gals use on your b20s or even b18? Of course my b20 has the original '65 HS6 SU Carbs, so keep that in mind. I think by the time b20's came around, Volvo was pioneering their EFI - eletronic fuel injection? So the b20 - HS6 might be an odd match.

Doug
1965 pv544 Sport b20 w/ HS6 SU's


INTRO: my dad started the family Volvo "disease" in 1962 when he bought his first red 1959 pv544 with barely 20,000 miles ---for $500! When it was T-boned in '65 he bought a white 1962 to replace it. Since then, we have had approximately 4 1800 coupes, 3 1800ES wagons, 1 122S, 7 244 sedans (ranging from 77 to 86), 2 744 sedans, and now my red 65 pv544 to keep the tradition going. My first car was the 84 white 244 sedan that was passed down for 10 years (my dad -to my mom -to my sister -to me), with 196K miles by the time I sold it to a friend. Volvos are probably the only car I wouldn't mind selling to a friend, based on their dependability/longevity.