RPM question
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Junior User
Joined: Feb 2003
Posts: 73
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From: Louisville, Kentucky
RPM question
I am new to PSD type super duty fords and I have a transmission question. My 2000 f-350 7.3 has the automatic. When I am cruising along the interstate at 55 mph the tach says 2500 rpm's. Is this normal? It gives a good hard shift 1st, 2nd, 3rd, then OD but it feels like it immediately jumps back out of OD and runs at 2500 rpm at 55mph. If it is in OD what should the rpm's be at 55 mph? I think mine has a 4:10 rear. Any thoughts?
Postmaster
Joined: Nov 2001
Posts: 4,742
Likes: 8
From: OK
Can't remember RPMs at 55 MPH, but you should be turning about 2000 RPMs in OD at 65 MPH with 4.10 gears. Sounds like something is wrong with your truck. Might be worth the $$ to have diags run on the truck to see if there are any tranny codes stored in the PCM.
Postmaster
Joined: Dec 2002
Posts: 3,959
Likes: 9
From: USA
What size tires are on the truck? If the tires are not stock size, has the speedo been corrected?
AxleRatio = (RPM x TireDiameter) / (MPH x TransRatio x 336).
Using your numbers with the trans in 3rd gear, the axle ratio would be:
(2500 x 31.5) / (55 x 1.00 x 336) = 4.26.
Your numbers with the trans in 4th gear, the axle ratio would be:
(2500 x 31.5) / (55 x 0.72 x 336) = 5.91.
Looks like the trans was in 3rd gear and the speedo may be a little off. The ratio is probably 4.10.
RPM = (MPH x AxleRatio x TransRatio x 336) / TireDiameter.
With 4.10 gears and the trans in OD at 55 mph with stock tires and true speedo, the rpms should be:
(55 x 4.10 x 0.72 x 336) / 31.5 = 1732 rpms.
AxleRatio = (RPM x TireDiameter) / (MPH x TransRatio x 336).
Using your numbers with the trans in 3rd gear, the axle ratio would be:
(2500 x 31.5) / (55 x 1.00 x 336) = 4.26.
Your numbers with the trans in 4th gear, the axle ratio would be:
(2500 x 31.5) / (55 x 0.72 x 336) = 5.91.
Looks like the trans was in 3rd gear and the speedo may be a little off. The ratio is probably 4.10.
RPM = (MPH x AxleRatio x TransRatio x 336) / TireDiameter.
With 4.10 gears and the trans in OD at 55 mph with stock tires and true speedo, the rpms should be:
(55 x 4.10 x 0.72 x 336) / 31.5 = 1732 rpms.
