Gear Ratio to RPM aint working out right

[Cowracer]

Something is fishy here...

For the longest time, I thought I had the 4.56 gears. But my RPO codes list a GT5 for the rear axle, which is 4.10 gears.

I run the followng rpm/speeds:

55mph = 2200rpm
60mph = 2490rpm (touch under 2500)
72mph = 3000rpm

Something aint jive-ing here :mad: I did a search and NOBODY else with 4.10s are running these rpms

I have a 4L80e automatic. The overdrive works (3 distinct shifts) and the TC locks (RPM jumps by a couple hundred if I tap the brakes while holding a steady throttle pressure, and drops back after I let off the brakes). I run 215/85/16 tires on all six spots.


any clues?
Tim

[DmaxMaverick]

Your alternator pulley is likely too small. Search this forum for "alternator pulley". There are several very good posts that cover it well.

[john8662]

Maverick got it!

here is a recent post (similar)

http://forum.thedieselpage.com/ubb/ultimatebb.php?ubb=get_topic;f=1;t=007193

[moondoggie]

Good Day!

Speed vs. rpm? You might want to look at my 9 Mar 04 post in Speed vs RPM with 4.10 rearend (http://forum.thedieselpage.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=1;t=005936)(Click on the colored text.).

Assumptions:

* 4.10 gears
* 4L80-E AT (0.75 OD)
* 215/85R16 tires (30.39" OD ≈ 95.47" circumference)


rpm @ 60 mph = (60 mph) X (1 hour/60 minutes) x (63,360 inches/mile) / (95.47 inches/tire rev) X (4.10 driveshaft rev/tire rev) X (0.75 engine rev/driveshaft rev)

rpm @ 60 mph ≈ 2041

With 4.56 gears:

rpm @ 60 mph ≈ 2270

Blessings!

Brian Johnson, #5044

[More Power]

The a correct stock alternator pulley for a 6.5 diesel application should measure 62mm in diameter when measuring across the peaks of the V's in the belt groove area.

MP

[catmandoo]

my 94 sub 3/4 with 4:10 has the exact same rpms you describe.and i'm running 265/75/16

[moondoggie]

Good Day!

62mm

[catmandoo]

moondogie by your equasion 4:10,4l80e and 215 tires it shows 2041 rpm at 60 i have 265 tires and turn right at 2250-2300 at 60 so i think hey the pulley must be to small,so just go out and check and the pulley has an outside diameter of 2 5/8 inchs or to big so i should be showing low on rpms!!!and yes i DO have 4:10s

[catmandoo]

oops measured wrong,measured by the way it's supposed to or top of the peakes it is right at just under 2 1/2 inchs so pulley should be right is there a way to calibrate the tach??

[moondoggie]

Good Day!

I have no idea. I guess a guy would have to get at least a shop manual & investigate the circuit. I've never done this - both our 95's are pretty accurate.

Blessings!

Brian Johnson, #5044

[More Power]

If the pulley is correct, you might need to pop the tach needle then push it back on at the correct reading.

MP