I can speak to that... plenty!

I’m not sure how to calculate that really. A lot is the closest I can guess. We did a 750 mile trip up the Tennessee and Cumberland and I bought 300gal. So I have a pretty good idea of what I average.

My 28 offshore with twin 454/485 hp’s use to average around 1 1/4 to the gallon. Although traveled many great lengths at cruising rpm’s I never checked those.

My 14,000lb 42 Harley gets 1.0 mpg running 35mph @ 3500rpm +/- depending on how heavy.

Twin 509s Arneson Drives AFR heads, Stainless Marine exhaust, Holley HP EFI. Approx. 550 HP.

Ordered some parts yesterday, ordered a set of icon forged pistons, .030 over with a 17cc dome, on back order for 30 days but couldnt find them sooner elsewhere or in a different brand, set of summit racing floating pin connecting rods, the promaxx 317 heads, with recommended arp head bolts and felpro head and intake gaskets, also a set of comp retro roller lifters, havent ordered a cam, i havent heard from melling if their cam is steel or cast but guessing cast so may order the crane 731 or 741 that many use here.

I wouldn’t have ordered the head gaskets without knowing your deck clearance figures first.

Also, a 17cc dome with a 119cc chamber with some assumptive calculations is over 10.5:1. Even higher if zero decking the block. A flat top at zero deck would come in around 9.25. Shave the heads and you could get to closer to 9.5. That’s much easier to manage fuel-wise vs always being on the lookout for premium.

Just my take.

9.8:1 with a .040 head gasket, i checked on summits compression ratio calculator before ordering, this is going to be a 509, stock crank was forged and in good shape so decided to use the money elsewhere on the engine.

Missed the 509 thing. I thought you were going 540.

I use the calculator at Diamond Pistons website as it’s more detailed. With a zero deck, I get 10.24, and 10.03 at .010 in the hole.

Icon’s advertised compression ratio is 10.3 with a 119cc head.

I sometimes split hairs on this stuff, but even at 9.8 you’ll be locked into premium fuel. That’s my point. If you can get it all the time and can trust it, great. I know I cannot trust what I’m getting from most pumps around here so not pushing it (and some friends say that even with 9.5 I’m pushing it).