RPM at 70 MPH? - Ford Powerstroke Diesel Forum
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post #1 of 12 Old 02-03-2010, 09:02 PM Thread Starter
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RPM at 70 MPH?

I bought the truck used and I am trying to figure out if it still has the stock gearing.

Would like a little bit better mileage is possible, but anyways I was curious about what RPM you Guys and Gal's are running at 70.

I run at just under 1750 RPM at 70. And get an average of just under 13 MPG but it is mostly city, However oddly enough I don't get much better on the freeway.

Anyhow any input would be great.

Thanks!


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post #2 of 12 Old 02-03-2010, 09:07 PM
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i cant remember off hand what 70 is, i know on mine 80 is around 2250.

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post #3 of 12 Old 02-04-2010, 02:06 AM
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What is your RPM at 60, and what is the diameter of your tires?
Math at 60 MPH is easier
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post #4 of 12 Old 02-04-2010, 02:10 AM
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Here is a posting response from a while back with someones elses question about gearing
If I know your tire diamter and RPM at 60, I can probably figure for you.


To calculate at 60MPH:
Multiply tire diameter by 3.14159 (PI) You: 38 inch tiresX3.14159=119.38 inches (tire circumferance)
Divide Tire Circ by 12 (circumferance in feet of tire) Yours is 9.94
5280 (feet per mile) 5280 divided by 9.94 or 5280/9.94= 531.18 tire revs per mile
531.18X 4.30(your axle ratio)= 2282
2282 x trans ratio= engine rpm's at 60MPH
2282X.712(O.D.)= 1624 engine RPM's
2282x 1.0 = 2282 RPM's in direct drive (gear before over drive)
So...1624 RPM's at 60MPH in overdrive
I'm not calculating RPM's at 65MPH because I've had a few beers and I might hurt my brain!
Will work on though!
Site with Ford trans ratios
Ford Motor Company Transmission Gear Ratio's

Hope this helps
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post #5 of 12 Old 02-04-2010, 05:31 AM
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Mine is doing 2000 @ 75 with 3.73s. Sounds like yours a little different because of the tires.

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post #6 of 12 Old 02-04-2010, 05:35 AM
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Mine at 70 is between 2300 and 2400.......but I have 4.10 gears so thats to be expected.

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post #7 of 12 Old 02-04-2010, 06:05 AM
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At 70 with 3:73's on a 37 inch tire your rpm's should be around 1500. With 4:10's, 1824, and 4:30's, 1913. Hope that help's...

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post #8 of 12 Old 02-05-2010, 01:17 PM
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I am all stock, @ 70 mph with 3:73 gears I am dead on 2,000 rpm.

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post #9 of 12 Old 02-05-2010, 01:21 PM
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I am really close to 2,000 rpm @70mph

Chad
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post #10 of 12 Old 02-05-2010, 02:05 PM
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Quote:
Originally Posted by coolfry73 View Post
I am really close to 2,000 rpm @70mph
yep that sounds right.

studs,egr delete(sinister),dieselsite coolant filter, nuc oil bypass, dj's fuel + regulated fuel return, sct by eric, 4in silverline no cat, isspro gauges, cat elc coolant, hids, lots of other odds and ends......
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