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

 

235 75 R15 tire with AOD and 351W will be about 1900ish going 65 mph in OD

 

thanks there havi so would a set of 3:55s bring it down to about the 1500's or so

 

nah, more like 1800

 

what would be the best crusin rpm speed for a stock 351w

 

As fast as you can without getting pulled over!!!! sorry could,nt help myself.

 

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.

 

mercman65

Here's a link that will give you various rpm's based on your rear gear, aod drive ratio, and the diameter of your rear tires: http://golenengineservice.com/calc/calcrpm.htm

 

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!

 

thanks guys looks like i should have about the right gearing with the 15'' rubber 3:70 9'' rear and the 351w/aod set up

 

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.

 

thanks there winfordr think im in the ball park were i want to run not hard to change gears in these 9'' rears if i have to in a very far feature