Posted by jdemaris on November 21, 2008 at 06:34:09 from (72.171.0.149):
In Reply to: o/t dodge pickup posted by johndeereman on November 21, 2008 at 05:37:06:
2000 RPM at 55 MPH in your truck doesn't make sense. Not unless your overdrive isn't working. You've got a 3/4 ton so I assume you've got the standard 245/75-16" tires that are 30.5" diameter. Standard axle ratio is 3.55 and RPMs at 55 MPH should be 1500. When your overdrive is working, your truck should do this - depended on axle ratio.
55 MPH - with 4.88 axle - 2000 RPM. 55 MPH - with 4.10 axle - 1730 RPM 55 MPH - with 3.90 axle - 1650 RPM 55 MPH - with 3.55 axle - 1506 RPM 55 MPH - with 3.07 axle - 1300 RPM
If your overdrive is NOT working -
55 MPH - with 4.88 axle - 3000 RPM. 55 MPH - with 4.10 axle - 2480 RPM 55 MPH - with 3.90 axle - 2400 RPM 55 MPH - with 3.55 axle - 2150 RPM 55 MPH - with 3.07 axle - 1860 RPM
If your overdrive isn't working, and also your converter isn't locking up, you can add another 15% to the RPMs.
Most truck engines are running a peak efficiency at 1800 - 2000 RPM when working hard, so your's doesn't sound real bad - but still, something must be wrong.
My 94 Ford F250 with 7.3 turbo diesel in OD runs 2200 RPM at 65 MPH - and it has 4.10 axles.
My 92 Dodge W200 with 5.9 Cummins diesel in OD runs 1800 RPM at 65 MPH and it has 3.50 axles.
My 87 Chevy K20 with 6.2 diesel and no overdrive runs 2700 RPM at 65 MPH and it has 3.73 axles.
My 82 Chevy K10 with 6.2 diesel and four-speed manual overdrive runs 1600 RPM and it has 3.07 axles.
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