6.4 RPM's with 37's - 3.73's

[Mike_D]

Hey guys,

Just looking to see what kind of RPMs you see at 65mph on 37's with 3.73's.

I am around 1900 and thinking thats pretty high for the tire size. I did buy the truck with a different set of 37's on it, but 37's none the less. Thinking the PO could have re-geared away from the factory 3.73's.

Thanks for the info

 

[Jonnydime]

At 65mph with my 33's and 3.73's I'm at around 1750rpm.

 

[mikeeg02]

Have you checked mph with gps? Speedo could be off.

37" tires, 65 mph, .71 OD and 1911 rpm = 4.56 gears
37" tires, 65 mph, .71 OD and 1802 rpm = 4.30 gears
37" tires, 65 mph, .71 OD and 1718 rpm = 4.10 gears
37" tires, 65 mph, .71 OD and 1563 rpm = 3.73 gears.

 

[Mike_D]

Yeah my MPH is dead on with GPS.. This thing has to be re-geared. I am more along the lines of 4.56's from what I can tell.

Kind of high if you ask me.. At 75mph the truck is turning some serious RPM.

 

[mikeeg02]

37" tires, 75 mph, .71 OD and 4.56 gears = 2205 rpm
37" tires, 80 mph, .71 OD and 4.56 gears = 2352 rpm

This is a helpful calculator if you want to use it yourself.

http://www.csgnetwork.com/multirpmcalc.html

It basically is regeared like a stock truck with 4.10s

 

[Mike_D]

Awesome. Appreciate the info!

 

[oldskool]

Mine with 37s i am about 1900 rpm at 65mph.

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[oldskool]

Thats with 3:73s

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[mikeeg02]

Mine with 37s i am about 1900 rpm at 65mph.
......
Thats with 3:73s

And you've verified this with GPS?

Unless your converter is unlocked in 6th (not sure how your tuning works) Im not really sure thats possible at 65mph with 3.73s and 37s.

Locked in OD should be 1563 rpm
Locked in 4th should be 2202 rpm

 

[oldskool]

I rechecked mine this morning and at 65 i was around 1700rpm.

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[mikeeg02]

That sounds much better!

 

[Mike_D]

Reviving this old thread.. I snapped a pic of my RPM while cruise was locked at 65mph, with GPS.

Truck shows built with 3.73's but was lifted before I bought it on 37's.

What do you guys think?

 

[mikeeg02]

Sure looks like 4.30s to me. If you want a guarantee, change your differential fluid and on the ring gear it will have the tooth count stamped for the ring and pinion. Should say 43 10. IIRC

 

[Mike_D]

That is where I am headed. I guess I dont know why I am even concerned about it. Those RPMs seem pretty ideal to me. Maybe a little on the high side.

 

[tbsimmons]

Or just jack the reared up and put the the truck in neutral.
Mark the tire to the ground and the driveline so you an see it.
Rotate the tire one revolution so the mark is back at the ground while counting how many time the driveline spins.
If its over 4 a little, 4.10's, almost 4.5, 4.30's, etc.