This past weekend I had the chance to ride in another Dak. 07 3.7 QC and I noticed a big difference. It seemed to "lag" and not be so peppy. We were running down the highway at 60 and he was turning between 15-1700 RPM's. The details on his truck. K&N CAI, Borla muffler, daystar 2" kit with 265/75/16 Definity Dakota A/T's. So I got curious and did a test run. Here is what I am turning at the same speed.
DSCN0962.jpg
DSCN0964.jpg
I now know this is abnormally high when I thought it was normal. Would this explain my horrible highway mpg?. (My city MPG seems better) I am just cruising and sorry no video only had cell phone with me.

 

3.55's at 65 mph taching 1700 - 1750

 

Yea, I figured. I was watching his tach between 45-60/65 mph. Is it possible that I have 4:10's. What do they turn?

 

I thought the 3.7 came with 3.92 and no other options. If I remember correctly from an old thread, the rpm diff between 3.55 and 3.92 @ 65 mph is about 250 rpm, which should put the tach at 2000 @ 65. Wait, don't you have a stick? That would mean no overdrive, add another 200rpm.

 

No stick, he has PRNDL lights

I think you have 4.56's.

You're turning 2,050 RPM +/- at 57.5 indicated MPH.

Your 265/75 R16 tires turn 610 RPM at indicated speed.

610 * 4.56 = 2,501 driveshaft RPM

At a 0.69:1 overdrive (4 speed auto), your crankshaft speed is only 1,919 RPM.

Do it again with 60.5 MPH, and it's pretty close. Either the speedo or tach are a bit off.

But if you had a stick, you would be humming along around 2500. If you had 4.10's, you would be down around 1,800. 3.92's, around 1,700. You are turning way too many revs for your speed.

Did the previous owner tell you he put gears in it? Cuz you got 'em. One quick thing - you were in overdrive and locked, yes?

 

This is a cool gear ratio calculator, punch in some numbers and maybe you can figure out exactly what you have.

http://www.rearendparts.com/pages.php?pageid=16

 

Here put your vin in and it will give you a build sheet.

http://www.compnine.com/vid.php

 

You can use a calculator...or look up your build sheet...but the math doesn't lie. I'd bet $100 you have aftermarket 4.56:1 ratio.

 

if you had 4.56 your low end pull would be stupid.

 

My bet is on 3.92. I've got 3.55s and do 2k at 70mph.