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got a 9'' from a 61 f-100 with a 3:70 and i'm going to hook it to a 351w with a aod trans running 15'' rubber on my merc m-68 just wanted to know if anybody had a idea what my crusin rpm would be at 65 mph
1800-2000 is about where you would want to be. You don't want to get too low then you will be luging the engine. You do have to remember that the stock 351W has about 110-120 hp at 2000 rpm and it has to push a 4000lb truck down the road.
That's a good site. The numbers match my spreadsheet which I developed when I was building my 54. I guess I'll have to keep my speed under 130 to keep from overrevving my 5.0 HO. Rats!
One thing to keep in mind is that the tire diameter as specified by the manufacturer is not accurate. The only way to know for sure is to lay a level on top of the tire under load and measure the distance to the ground then multiply that by 3.14159. (pi)
Of course this will change with tire wear and pressure.
Tires of the same "size" will vary significantly when measured.