tranny/torque conv/gears guestion

[Viper5035]

I have a 03 f-150 screw xlt 4x4 and i am wondering what gears i have or of my torque converter is not locking up like it should. My rpm's at 75mph is 2.1k i have 265 tires on my truck. im not sure what else i can say about it but can anyone help me out. ooh i have the 4.6 with an auto trans

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[adrianspeeder]

Take a looky at your door jam sticker. There is a code under the axle part that we can tell ya what gears are in the diffs.

Adrianspeeder

[Viper5035]

axle code is 26 is there a formula to find out what the rpm's should be at at a certian speed

[mtnbike_jr]

Originally Posted by Viper5035

axle code is 26 is there a formula to find out what the rpm's should be at at a certian speed

Answer:
http://www.f150online.net/forums/sho...ight=axle+code

[fasterhorses]

Originally Posted by Viper5035

axle code is 26 is there a formula to find out what the rpm's should be at at a certian speed

YES...formulas....have fun

DGR = Diff Gear Ratio
TGR = Trans Gear Ratio (1st=2.71, 2nd=1.54, 3rd=1.00, OD=0.71)
D = Tire Diameter in inches

DGR = RPM*D/336.09/MPH/TGR (not exact but can tell 3.55, 3.73, 4.10, etc)
Viper has 265 tires so D=30.5", MPH=75, RPM=2100
DGR = 2100*30.5/336.09/75/.71 = 3.58 so Viper has 3.55 diff gears

or
RPM = 336.09*MPH*DGR*TGR/D
RPM = 336.09*75*3.55*0.71/30.5 = 2083 RPM

BUT 26 axel code says 3.73 so I must have guessed wrong on the tire dia. Viper, are the tires 31.8" or 32" tall? If I got the tire size right then the RPM should have been 2200 at 75MPH. The best way is to use 3rd, get accurate RPM and tire dia.

[LastSplash]

Originally Posted by fasterhorses

YES...formulas....have fun

DGR = Diff Gear Ratio
TGR = Trans Gear Ratio (1st=2.71, 2nd=1.54, 3rd=1.00, OD=0.71)
D = Tire Diameter in inches

DGR = RPM*D/336.09/MPH/TGR (not exact but can tell 3.55, 3.73, 4.10, etc)
Viper has 265 tires so D=30.5", MPH=75, RPM=2100
DGR = 2100*30.5/336.09/75/.71 = 3.58 so Viper has 3.55 diff gears

or
RPM = 336.09*MPH*DGR*TGR/D
RPM = 336.09*75*3.55*0.71/30.5 = 2083 RPM

BUT 26 axel code says 3.73 so I must have guessed wrong on the tire dia. Viper, are the tires 31.8" or 32" tall? If I got the tire size right then the RPM should have been 2200 at 75MPH. The best way is to use 3rd, get accurate RPM and tire dia.

I found your error. You used 30.5" for the tire diameter, a 265/75/16 tire is 31.6", substitute 31.6 and it works out. I would say his TC is locking up correctly.