Engine rpm

[OldRedFord]

What RPMS are you all turning at cruising speed? Say, 55 mph, with 3:00 gears and stock size tires?

[rubiranch]

http://www.ringpinion.com/Calc_RPM.aspx

Measure your tires and check here.

[willowbilly3]

I can't get that calculator to work but I get about 2175 with a 29.5 inch tall tire and no over drive. If you are running an automatic without a lockup then add a couple hundred.

[OldRedFord]

Heh, didnt know there was a calculator for that. Trying to work out if my cam choice was a good move or not.

[willowbilly3]

I think there are lots of sites that do it for you. I use this. 63360 inches in a mile. Tire diameter (height x pi or 3.14) divided into the inches in a mile gives you tire revolutions per mile. Multiply times gear ratio for driveshaft rpms which will be engine rpms in a direct high gear, multiply times final ratio of overdrive if applicable.

[rjewkes]

for good gas mileage i'd think if you could get an automatic or even a 4 or 5 speed to 2200 or less maybe between 1800-2300 @72 mph you'd see good wear on components and get decent milage to do this you may ned to get up around 3.55-4.11 in your rear ratio unless you can get a realy decent final overdrive that keeps you in that same range.maybe changing tire sizes can help too. All i know is you don't want to tach more than 3k at 70 mph. it'll suck the gas right out the pump ten miles down the road before you get there.

i know my first bmp was a 360 with 3.00:1 rear and i got 5-8mpg around town nd even worse at 60-70 taching 3000-3200.

but it sounds like the calculator says for a 28.8" tall tire(235 75R15) the rpm at 55 is 1977 with a 3.00 at 55 mph.

now my 302 was pushing 3200 the day it died with a 3.25 with 235 75R15's at 70 mph.

according to the calc i should have been doing around 2950 at that speed.