4.3 L merc I/O perf on Seafari

[rhody98]

McGillicuddy recently requested the 1969 Boating article on the Seafari. With the boating season over in RI ("pulled" is the term recently coined in one of the posts), Gilly's request prompted me to re-read.

The performance #'s for the seafari with a 140 hp, 4 cyl sterndrive can be summarized from the article (pg 80) as follows:
1500 rpm, 6.1 mph, 3.9 mpg
2500 rpm, 19.2 mph, 5.3 mpg
3000 rpm, 26.5 mph, 5 mpg
3500 rpm, 31.9 mph, 4.4 mpg
4400 rpm, 39.6 mph, 3.8 mpg

I have a 4.3 L V6. In very good condition. Based on GPS my speed vs rpm is very similar to the numbers reported in the article, except I top out at low 40's at 4400 max rpm. I don't have fuel use gauge but based on my mileage vs fill-ups I believe my gas consumption does not differ substantially from the #'s in the article either....perhaps I am under 5 mpg. This is with a mirage 19p, 2 persons, full tank, top down.

Motor sounds great, very dependable over 50 hrs this summer and I am not a speed demon. But very curious why the performance on my 6 cyl is same as that reported for 4cyl, while adding 200 lbs in weight?????