question on rpm

[westport70]

i have a 94 half ton 4x4 i cha ged the alternator some time back,did not even think about saving then pulley.Know i wonder if my tack is correct.i wonder if any one could give me the correct diameter. Seems the rpms are high at highway speeds. What should i be turning at 70mph.

[moondoggie]

Good Day!

What size tires does your truck have?

Manual or AT?

Axle ratio? If you don't know this we can guess based on:

1/2, 3/4, or 1 ton?

rpm = (mph) x 1/60 (hour/minutes) x 63,360 (inches/mile) / (tire dia (in inches) x pi) x (driveshaft rev/tire rev (axle ratio)) x (engine rev/driveshaft rev (OD ratio))

My 95 pickup,for example:

rpm @ 70 mph = 70 x 1/60 x 63,360 / (30.47 x pi) x 4.10 x .75
rpm @ 70 mph ≈ 2375

Oops

[joem]

Why would your alternator pulley have anything to do with the tach?
Joe

[bcbigfoot]

6.5's get thier tach signal from the alterator, the faster it spins the more RPM's show on the gauge, so a smaller or larger pulley will throw the RPM guage out of wack. If you ever need to replace alternator make sure it is for a 6.5 application and the pulley size is what you took off or better yet swap pulleys.

Westport, need more input.

[joem]

I did not know that. How do I know if I have the right size pulley on my?
Joe

[DennisG01]

At 70mph, I'm at 2100rpm. I have 265's, but my VSSB is compensated.

[joem]

Well at 70 mph im at 2450-2500 w/ 245/75/16 373gear and a 5 speed. idaling at 900 rpm.

[bcbigfoot]

The dia of the pulley on my 93 (not certain 95 is the same) is 2.25" to the top of the groves that the pully runs on. The other way is doing a driveline calculation which Moondoggie will do if he gets enough input #'s. There is also a meter that most garages have, they put a piece of reflective tape on the harmonic balancer and the meter picks up the rpm, kind of like a timing light.

[moondoggie]

Good Day!

"How do I know if I have the right size pulley on my?" Use the formula.

Blessings!

[bcbigfoot]

Is the OD ratio on the 5sp manual a little different than the 4l80e OD ratio, something like .80 compared to the 4l80e's .75 thought I read that somewhere but could be wrong.