With my 3.55 rear, 35" tires and auto transmission I am only running 1700 RPM at 70mph in overdrive.

If I lock OD out, I hit 2,000rpm at 55mph.

I noticed this the other day, don't know why it took me so long to.
It kicked out of OD on a not very steep hill, I think that grabbed my attention.

Is this bad?

If so, what can I do to fix it.

Don't have funds to change rear's, but what would 4.10's do for me?
3.73's (if available, probably not)

I tow nothing, just daily driver for now and forseeable future.


thanx

phox

 

Unless the engine is laboring (ie high fuel consumption) I think this is actually a good thing. Wish mine was like that. Low RPM at cruise should equal longer engine life.<br>Be interesting to see if anybody disagrees.

 

Cummins has a bullitin out at the Cummins shop here saying it's best to keep between 1800 and 2200 for best economy. <br>I've got 3:55's and normal tires and I find the milage slips some if I let the RPM go down below 1900 or 2000.<br>I just got back from an over nighter to Sask. and got a pretty steady 650 km/100 liters. Not bad for hauling 6000 lbs.<br>Ruadh

 

Phox, your not off that much, mine runs 75 at 2000 rpm.<br> Friend of mine ran his like that for allmost 3 years then changed to a 410 rear end. But he did it because he tows a 5th that weighs around 12k lbs and goes up Parleys alot.

 

According to the trip computer, I'm getting 15-16 mpg at 70.<br><br>This is a little more than my current average of 14mpg,<br>(calculated by hand, computer says 13)<br>so I guess the engine isn't laboring too much.<br><br>Don't like those mpg numbers though.<br>At 16,800 miles they should have improved some.<br>Thats the same I was getting when it was new.<br>Tires might account for some of it, but not that much.<br><br><br>phox

 

I have a manual tranny (6spd) at 70mph I run at 2150rpm's. Considering your tire size, I think your in the ball park for an auto. But I am killing you in milage! ;D..I cruz at 75-80mph and I am getting 19mpg! 8)

 

took a trip this weekend the first time since i have had the truck. drove 75mph all the way was crankin 2500rpms she was screamin! so i think i would say i have 4:10's. i have 285 tires little bigger than stock right? i think it was more like 80 to 85mph but still she was screamin!

 

I turn bout 1800 crusing with the speedo saying 65 (I'm actually doing 70) and I'll pull 20 or so to the gallon. That's highway. In the city, I'm lucky to pull off 15. 33&quot; tires with 3.54 gears are not good city gears.

 

Phox, have you corrected your mileage and speed readings for the taller tires? You're going farther with each revolution (Odo &amp; Speedo reads lower than true), so your mileage may be better than you think you're calculating.<br><br>As far as cruising rpms, as long as the engine isn't lugging you should be ok. (I doubt if its lugging).

 

[quote author=bulabula link=board=4;threadid=14190;start=0#133515 date=1051703705]<br>Phox, have you corrected your mileage and speed readings for the taller tires? You're going farther with each revolution (Odo &amp; Speedo reads lower than true), so your mileage may be better than you think you're calculating.<br><br>As far as cruising rpms, as long as the engine isn't lugging you should be ok. (I doubt if its lugging).<br>[/quote]<br><br>Yes, corrected with a Truespeed.<br><br>Doesn't seem like it's lugging, just seems low.<br><br><br>phox