can someone confirm my calculations?

DrNick · #1 10-17-2009, 01:05 AM

I want to switch to 4.10's.(i have 3.55 now) I'm running 305/60/18 which I believe is a 32.4" tire. I don't tow. My truck is kind of a dog. Especially down low. I just want some giddy-up off the line. But, I drive on the freeway a lot too. A lot.
I used the calculator that GLC linked to and came up with RPM of 2232 @ 75 MPH. Is that right? Is that too high of RPMs for constant highway driving? My truck is the "family car" for all our trips. I'm wondering how it would be for that. I'm hoping maybe I figured it wrong and the RPMs would be lower than that at 75. Thanks a lot.

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glc · #2 10-17-2009, 03:33 AM

Your calculations are correct. These engines will run at 4000 all day, don't sweat it.

Right now, you should be doing 2761 in 3rd and 1933 in OD at 75.

Just for comparison, my truck (in sig) is turning 2385 at 75 in 5th.

avfrog · #3 10-17-2009, 11:26 AM

I think mine runs right around 2k-2100 at 75mph and I Have 3:73. If it wasn't so damn expensive I would put in 4:10.

ONELOWF · #4 10-17-2009, 11:51 AM

I see 2,500 at 75 with 410's and 30.5" (?) tires. My little engine is humming pretty good at that speed. I do probably less than 1,000 miles a year on the freeway so I'm happy.

DrNick · #5 10-17-2009, 12:47 PM

Thanks for the input. Going to 4.10s from 3.55 will definitely give my some noticeable off-the-line right? I think I can live at those RPMs on the freeway; even though 2200-2400 is right when my muffler starts really letting you know it's there!

Also, there is a transmission shop out here up in Oceanside that will do Yukon 4.10s for $700 installed. That's not "too good to be true" is it? I am the FedEx guy that delivers there and he said he'd give me a break from the usual $850 they charge.