Estimating RPMs
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Senior User
Joined: Oct 2001
Posts: 116
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From: Florida
Estimating RPMs
My f-350 has a 460 with 3.55 rear differential. Tires are "standard" 16" I think. Anyway, the truck has no tach - what the best guess on RPM's at 65 or 70 MPH?
Getting ready for a long haul - just curious.
Thanks
Getting ready for a long haul - just curious.
Thanks
Senior User
Joined: Jun 2003
Posts: 439
Likes: 0
From: Burnsville, MN
Estimating RPMs
Long answer from faulty memory...
Determine the circumfrence of the tire.
This will give you feet per axle revolution.
divide this number by 5280 (feet in a mile).
Multiply by the differential reduction (i.e. 4.10) to get driveline revs per mile.
if you have an overdrive transmission, multiply by overdrive ratio (i.e. 0.70) or skip if you don't have an overdrive.
You now know how many engine revs it takes to cover one mile.
Now travel through one mile at 60 miles an hour, it will take you exacly one minute.
If you cover the one mile in 60 seconds then your engine rpm will equal the engine revs per mile.
I hope.
Determine the circumfrence of the tire.
This will give you feet per axle revolution.
divide this number by 5280 (feet in a mile).
Multiply by the differential reduction (i.e. 4.10) to get driveline revs per mile.
if you have an overdrive transmission, multiply by overdrive ratio (i.e. 0.70) or skip if you don't have an overdrive.
You now know how many engine revs it takes to cover one mile.
Now travel through one mile at 60 miles an hour, it will take you exacly one minute.
If you cover the one mile in 60 seconds then your engine rpm will equal the engine revs per mile.
I hope.
Posting Guru
Joined: Dec 1999
Posts: 1,903
Likes: 1
From: Copley, OH
Estimating RPMs
You cannot accurately calculate effective rolling diameter of the tire from the size, there are way too many other things happening. For example 235/75-15 tires are 28.88" dia. That would equal 698 revs/mile. According to the manufacturers most brands roll at 720-725 revs/mile which is equal to a rolling diameter of only 27.93"
Look up the revs/mile for the your tires on the manufacturer's web site or at the Tire Rack.
Look up the revs/mile for the your tires on the manufacturer's web site or at the Tire Rack.
Senior User
Joined: Jun 2003
Posts: 439
Likes: 0
From: Burnsville, MN
Estimating RPMs
Website is best for revs/mile, but lacking that information I was suggesting that one measure, not calculate the circumfrence.
Also, the C-6 adds additional ambiquity as I don't believe that it has have a locking converter. So the best calculated value we can hope for in transmission input shaft speed. Engine RPM will be somewhat higher depending on load.
Also, the C-6 adds additional ambiquity as I don't believe that it has have a locking converter. So the best calculated value we can hope for in transmission input shaft speed. Engine RPM will be somewhat higher depending on load.
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Posting Guru
Joined: Dec 1999
Posts: 1,903
Likes: 1
From: Copley, OH
Estimating RPMs
Even a measurement will be off bt a fairly substantial amount. Measurements are static and a rolling tire is dynamic. There are things happening at speed which don't occur at rest.
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Post Fiend
Joined: Nov 2001
Posts: 14,947
Likes: 6
From: Topsfield, MA
Estimating RPMs
The formula MPH X GEAR RATIO X 336 / TIRE SIZE * OD RATIO works pretty well. Mine comes out exact, mine is 1950 rpm at 65 and the needle is just under 2K at 65. It's about 2500 rpm since you don't have overdrive. The non-locking torque converter will only let it stip about 100-300rpm max.
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