Best highway RPM's

[RCP Phx]

What rpm do you think would most benifit the motor.Im running 2200rpm@70mph now(wheres the sweet spot )!Im doing a rear 44 and now is the time to decide.

Btw:thats OD w/ TC locked up!And completely stock

 

[Goatman]

What rpm do you think would most benifit the motor.Im running 2200rpm@70mph now!

Btw:thats OD w/ TC locked up!

It doesn't really care.......just ask it.

The issue is what rpm are you comfortable driving it.

 

[CRASH]

I run 2,800 at 70.

My old Renix starts to push a LOT of oil out the PCV if I take up to the 3,000 RPM range. The HO's like it better up there.

CRASH......awaiting strokage.

 

[RCP Phx]

Im changing to a D44 rear so nows the time to change(if Im going to).Im only concerned with driveability/mileage,Ive got the Tera-Low for off-road!

 

[Faux4X4]

My cam reallyopens up at around 4000, but that is with a 4.2 stroker. I have 33 inch tires and stock gears in an 8.25. I don't even use OD on the freeway anymore. I just cruse 75 at around 2500, and I can take 3rd out to about 85-90. I just love my new tach.

 

[Eagle]

The only problem with 2200 at 70 MPH is that below 70 MPH you're operating below the engine's torque peak. You get optimum economy cruising at the engine speed where the torque peak is. Of course, after Chrysler took over the torque peak in the books went from 2500 to 4500 RPM, and then back down into the 3's somewhere so don't put a lot of faith in what the literature says.

As I've posted previously, when AMC first introduced the generation of engines that became the 4.0L, it was before Detroit used overdrive trannies. Top gear was 1:1 direct drive, and the AMCs with manual trannies were geared to run 2500 RPM at 60 MPH (24 MPH/1000 RPM). 72 MPH was 3000 RPM and we used to cruise them at that all day, and still got excellent gas mileage.

If your normal cruise speed is 70, I would aim for gearing that will run that speed at 3000 RPM. If you cruise at 75, I'd gear to run 3000 RPM at 75 MPH. That gets you into the best part of the power curve at your cruise speed, but you're not geared so "tall" that at anything below cruise you are lugging the engine or dropping out of overdrive.

 

[Goatman]

This really looks like a gear ratio question, if you're thinking about changing rear ends. I normally run 2600-3000 rpm on the highway, and downshift running 3200-3500 on the steeper hills. Gas mileage is a tough question, because it's a combination of load on the engine and overall rpm.

I still got decent gas mileage before I added a bunch more weight in body armor/bumpers, etc, and was running about 2300 rpm at 65 mph. Now, with the extra weight, more miles on the rig, and running around 2600 rpm at 65 mph, my mileage sucks.

 

[RCP Phx]

Yeah Im putting in a D44 and needed to decide about the gears.Ive pretty well convinced myself to go ahead with the 4.56's.It just means more work(gotta do the front again).Right now its comin in about 1500$ for the rear,was kinda hoping I could ignore the front for awhile(need a HP30).

 

[garth]

98 XJ with 4.0 30x9.50 and 3.55 gearing. Running in the midwest I have found that up to 70 I get better (2-4mpg) driving in 3rd with the rpms running in the 2800 range. When I shift into OD it drops to around 2300 and the mileage and power drop accordingly. Have gotten as much as 26 mpg on 65 mph roads. Had to check my numbers three times as I didn't believe it either.

 

[ChuckD]

Good point garth, RCP , do a MPG test. drive a tank around town with the OD and one tank without the OD. See what you get. You have a LP D30 up front, so really 4.88 would be pushing R&P strength.

 

[XJEEPER]

I get better mileage with 4:56's and 32's, AW4 turning 2800 rpms at 75mph than I did with 4:10's and the 5speed turning 2200 rpms at 75.(18 mpg freeway) 89 4.0L with 183k miles, 6" lift, bumpers, armor, skids, FS spare, peeling clearcoat to lighten the body........
FWIW