RPMs @70mph?
Posting Guru
Joined: Mar 2004
Posts: 1,321
Likes: 2
From: MI
Originally Posted by Danko
Yup, I think I am onto something here with the stock setups overreading speed and therefore mileage and cheating on the warranty. Hate to break it to ya'll but your mileage may be worse than you think when you find out how far off your speedo/odo is.
try tracking your speed and milage with a gps, I have found most speedos are within two mph.
Senior User
Joined: Apr 2005
Posts: 430
Likes: 0
From: s.e. Michigan
Checked yours for accuracy or lack of it yet? Not paranoia. Good information for fellow drivers. If your speed/odo overreads by x%, then the warranty coverage expense for FoMoCo stops x% sooner and having owners think they get better gas mileage than they really are getting is not an accident. I noticed on my 1994 15 passenger van the same thing. I went from 245/75 x 16's to 265's up front and 285's on back and AFTER the taller tires went on my speedo was accurate. The good news here is that with speedo's reading too high, FoMoCo may have saved some of us some speeding tickets.
Thread Starter
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Senior User
Joined: Nov 2003
Posts: 224
Likes: 0
From: Trophy Club Texas
Hey Tim, try these simple steps.
1. Go to garage
2. Open drivers side door and get in. (adjust seat and mirrors, fasten seat belt)
3. Insert key in ignition
4. Turn on, wait for glow plug light to vanish.
5. Turn until hearing engine crank and start.
6. Back out of garage (unless you are lucky and have a drive-thru).
7. Put in drive and DRIVE.
8. Smile all the way home!
2. Open drivers side door and get in. (adjust seat and mirrors, fasten seat belt)
3. Insert key in ignition
4. Turn on, wait for glow plug light to vanish.
5. Turn until hearing engine crank and start.
6. Back out of garage (unless you are lucky and have a drive-thru).
7. Put in drive and DRIVE.
8. Smile all the way home!
Cargo Master
Joined: Jun 2003
Posts: 2,203
Likes: 0
From: Carlsbad, California
RPM = (Speed x 0.712 x 0.373 x 1076) / (PI x 31.65)
RPM = 1974 if speed is 70 MPH
Where:
0.712 is the final drive ratio of the Torqshift in OD with converter locked
0.373 is the rear end ratio
1076 converters miles, inches, minutes, hours, etc. to the right result
PI is 3.14159...
31.65 is the nominal stock tire diameter.
You can substitute your rear end ratio if different and if you have a measured tire circumference, substitute that for the (PI x 31.65) term.
RPM = 1974 if speed is 70 MPH
Where:
0.712 is the final drive ratio of the Torqshift in OD with converter locked
0.373 is the rear end ratio
1076 converters miles, inches, minutes, hours, etc. to the right result
PI is 3.14159...
31.65 is the nominal stock tire diameter.
You can substitute your rear end ratio if different and if you have a measured tire circumference, substitute that for the (PI x 31.65) term.
Freshman User
Joined: Jun 2005
Posts: 27
Likes: 0
Originally Posted by 93stillstrong
Just picked up my 05 F350 CC 4x4 Lariat 6.0 on Friday, so I am looking at those types of things to get familiar with the truck.
I believe that at 2000 RPM I am close to 72 - 74 mph, noticed it yesterday on the way home. I have the 3.73 std gears, 18" rims.
I believe that at 2000 RPM I am close to 72 - 74 mph, noticed it yesterday on the way home. I have the 3.73 std gears, 18" rims.
More Turbo
Joined: Jan 2005
Posts: 589
Likes: 0
From: Mililani, Hawaii
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Senior User
Joined: Aug 2003
Posts: 248
Likes: 0
I call mine 2000 RPM's @ 70 - (3.73 rears and stock 265 tires) - close enough.
My speedo reads 1.5 - 2 mph fast when checked against GPS.
I honestly never thought about the "reduced warranty" as a result of a fast reading Speedo.....
Now I'm bummed out thinking about it. 
Imagine a new conspiracy theory for the New York Times.
My speedo reads 1.5 - 2 mph fast when checked against GPS.
I honestly never thought about the "reduced warranty" as a result of a fast reading Speedo.....
Now I'm bummed out thinking about it. Imagine a new conspiracy theory for the New York Times.
Elder User
Joined: Sep 2004
Posts: 821
Likes: 1
From: Nashville
Originally Posted by dsweger
I honestly never thought about the "reduced warranty" as a result of a fast reading Speedo.....
Senior User
Joined: Apr 2005
Posts: 430
Likes: 0
From: s.e. Michigan
Photog, Assuming your 18" ers are the 65 series for the 2WD, the overall diameter is EXACTLY the same as your 17"s and the rpm's should be the same. TireRack.com will show you all of that by clicking onto the tire size you want to look at (skip all the input about your type of vehicle) and then click on specs to see diameters, etc. So, as I read the difference in your two, above, it tells me your 18's must be the 4WD 70 series which are taller, right? Otherwise, considering the same axle ratios, then your trans's would have to be different.
