crusing speed/RPM

[IOPort51]

for those of us who consider "all terrain " to include asphalt I have a question. What would be considered the best cruising RPM? I have considered the Peak torque to be the sweet spot but I have been thinking about having a little left over at the top for passing and where the most fuel efficient situation can be found.


So, the question is, at what rpm would you choose to run at 55 MPH?? and why? The answer should hold for any engine. gears and tires are not relevant.

 

[gert]

https://www.ringpinion.com/calculators/Calc_RPM.aspx



The chart on there is the one I go buy, I'm trying to set my CJ up to cruise around 2000rpm I don't want to rev the out of the motor down these Texas highways and prematurely wear out the motor or listen to it either. I would think you would want to cruise using your torque band and then pass people using your horse power, so definitely cruise below peak horsepower


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[mtnwhlr]

https://www.ringpinion.com/calculators/Calc_RPM.aspx



The chart on there is the one I go buy, I'm trying to set my CJ up to cruise around 2000rpm I don't want to rev the out of the motor down these Texas highways and prematurely wear out the motor or listen to it either. I would think you would want to cruise using your torque band and then pass people using your horse power, so definitely cruise below peak horsepower


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for a CJ but my truck with a 454 and 4.11 gears runs at about 3200 rpm's and gets about 10 to 12 mpg witch is about the same as my hot rods ran but I didn't care about mpg it was all about light to light performance.

 

[Torxhead]

That is a tough issue the way I see it there IO. 70 m.p.h. is really flying in an old CJ5 . Then to take that vehicle over a nasty rock garden on the trail at a speed where the speedometer needle barely even moves off the stop pin. Probably best to put it on a trailer or flat tow it. That way you can take the doors and windshield off to have fun and squeeze thru places where only a swb vehicle can fit.

 

[IOPort51]

That is a tough issue the way I see it there IO. 70 m.p.h. is really flying in an old CJ5 . Then to take that vehicle over a nasty rock garden on the trail at a speed where the speedometer needle barely even moves off the stop pin. Probably best to put it on a trailer or flat tow it. That way you can take the doors and windshield off to have fun and squeeze thru places where only a swb vehicle can fit.

I agree that 70 may beat bit much I will edit that to 55.

 

[Hedgehog]

Gert - Honestly OMG, 2000 rpm's @ 70mph. My Ford Fusion doesn't run that low an RPM. My CJ has a 360, 31" (Really 30") tires and 3.31 gears. Many have indicated that my 3.31's are totally inappropriate for a CJ that runs off road. I can't tell you right of what the RPM's are at 70 (simple math really) but at 65mph it's at just over 2400 rpm's and the engine seems VERY happy on the interstate.

By-the-way - My CJ will hop right along at 70+. I just get nervous doing it. 65 is just fine though.

Edit: no don't edit it now, it will throw off all the answers so far. In your next post switch your target to 65, most highway travel is just fine at 65.

 

[Torxhead]

Hedge, that 360 w/3:31 gears does sound pretty good as it has the power to push those high gears to a good highway speed. As I recall you did add in a T-18 , so with some low Transfer Case gears that would give you a pretty good crawl ratio also. For me 65 is a bit much.

 

[Bill D]

I recently changed my ring gears and pinions from the stock 3.07 to 3.73. Before, I could run 70 on flat highway at 2000 rpm but half of Colorado isn't very flat and the hilly part was causing me problems. Downhill, I could really fly but going uphill, I spent a lot of time downshifting. Since the gear change, I do much better and my 65 mph tach reads around 2600. I also regeared my Dana 300 Transfer Case with 4:1 gears to get my crawl speed down. I run 12.5X33X15 tires and am now very happy with performance.

Having said all that, there are many variables in the speed you want to drive at, the terrain you have to traverse and how you want to traverse it. I can now get out on the interstate and keep up with all but the crazy drivers and in the outback, get a pretty low crawl speed for all but the very serious rock climbing.

As I said, I am happy with my performance but not everyone has my criteria.

 

[Hedgehog]

Torxhead - The T-18 is waiting for it's day in the CJ. I'm satisfied with the gearing, but the super low crawl guys that push their CJ's harder than I do didn't really understand changing from 3.73 to 3.31's. I understand what they are talking about. The T-18 and low Transfer Case gearing should compensate for the tall gearing.

Bill D - I'm impressed, 2000 rpms @ 70mph's your I-6 must seem like it's loafing along.

 

[gert]

Guys I live in Texas and we don't have hills so I might have problems in Colorado keeping speed we will see, I have the 6.34 gear NV 4500 and 4 to 1 Dana 300 with 3.91 gears running 34.5 Goodyears. When I calculated it I have a 99 to 1 crawl ratio and with my Overdrive I should cruise at 2000 rpms. Right now my combo is untested. I did however think that the torque that's available from my 360 would be enough to cruise at 70. We will have to see. Hedgehog- your fusion has a 4cly? 4cylinders don't have enough torque to move vehicles so the factory compensates with lower gears to keep rpm up. Most 4cly I have had are same way mtnwhlr- your 454 truck is made to haul hell of a lot of weight so those 4.11 gears are there to get you off the line hauling that weight and keeping your motor happy pulling the weight. This is a complicated subject as the above have pointed out because vehicle weight , aerodynamics , horsepower, torque, intended use, amount of cylinders all play a part in how to gear for highway speeds


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[Peanut Butter]

When I lived in Colorado, I would run up and down I-25 at 80 mph all day long in my F350 Crew Cab 4x4. It had the 7.3L Power Stroke in it with a 4 speed automatic with Overdrive . It would purr at 2200 rpm at those speeds on flat interstate. Going up a hill it would down shift to 1:1 and up the rpm to 2500 with the turbocharger kicking in. Best dang engine I've ever had in any vehicle. Got really good mileage too.

Recently I've been driving my son in laws YJ. It has a 5 speed AX15 and 4.0L in it. 4th is 1:1 while 5th is an Overdrive . It will run 50 to 55 in 5th near 2000 rpm, but any grade at all and the speed drops off. In 4th or 1:1 it will drive 55 between 2200 and 2700 rpm consistently. 65 to 70 mph, forget it. It will go that fast, but you have to really work it to do that. My son in law does everything at 70 mph +. So he complains about driving the Jeep. But he is comparing it to his 2016 F250 power stroke.

The Jeep is caught right between both worlds. The gearing is good for mild to medium off road and ok for highway if your not in a hurry. My Custom CJ Truck will have a 3 speed auto and 3:54 gears, and for now a AMC 258 i6 / 4.2l I6. For my driving it will do fine for both worlds, but not GREAT for either. At some point I may put a V8 or a 4BT in it for towing, but I'm happy to drive 50 to 60 mph on the highway, and my off-road will be Jeep trails and sand beaches. I believe that my setup will handle all of this just right for me. 30 years ago, I would have needed a blown 500 hp V8 to be happy.

 

[Belizean]

I think PB is right on the money for a CJ. Cruzing at 50-60 or maybe 65 is the sweet spot. Anything more and if something goes wrong you could be involved in a disaster or something catastrophic. That being said running rpm somewhere a good range of 2300 - 2800 @ 70 being 2800 rpm.

 

[007]

Here's my 2cts.

http://www.grimmjeeper.com. Click on gear ratio calculator. It's astonishingly accurate. I know.

You know your engine. Set the left side up with what you have. Click the copy button to copy it all to the right. Then make changes as necessary to the right side to get to the numbers you want.




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[Torxhead]

Using the grimmjeeper chart it shows my engine rpm at 60 m.p.h. to be 2505. Driving at this speed reading a tach it is closer to 3000, which is probably more accurate as I only run around 28 p.s.i. in the 33" tires to get a good tire wear. In the local mountians there is an on road hill that tops out at around 8400' and I have to downshift into 3rd to make it to the top. I do have a very good cruise thru most rocks except on the famous Hammer trails located in the So. Cal high desert. For most trips it is putting the camping gear and spare gas in the air conditioned truck and flat towing.

 

[007]

Using the grimmjeeper chart it shows my engine rpm at 60 m.p.h. to be 2505. Driving at this speed reading a tach it is closer to 3000, which is probably more accurate as I only run around 28 p.s.i. in the 33" tires to get a good tire wear. In the local mountians there is a hill that tops out at around 8400' and I have to downshift into 3rd to make it to the top. I do have a very good cruise thru most rocks except on the famous Hammer trails located in the So. Cal high desert. For most trips it is putting the camping gear and spare gas in the air conditioned truck and flat towing.

Yep you do need to use your actual tire diameter and not what's on the size of the tire. Forgot to add that.


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[Lumpy Grits]

It all depends on the engine(4cyl, 6cyl etc)and the power band of each.
My Jeep-I can run 65-70mph all day with no issues. I'm turn'n about 2700 rpm with my AMC 258ci/6cyl carb'd engine.
T'head-If your tack is OEM, AMC. Don't belive it.
LG

 

[Hedgehog]

Mostly tire diameter errors. Measure from the ground to the middle of the hub x2 to get your actual tire diameter. It is surprising to see how much they vary from the written tire size. Especially between manufactures. Even speed makes a difference you can see it in drag racing, tires grow with speed, yes even our relatively slow CJ tires.

 

[Neuner]

Not real sure about 55 but I know 60 well. I run at 2,140 rpm at that speed. I only know it because I have my 4.0 connected with bluetooth to my Android phone running TorquePro.

I have run this with BFG MT & AT 33s. With my old AMC 258 i6 / 4.2l and now my 4.0. Both with a T177 and 3.31 gearing.

I was trying to determine if my rpm to gearing ratio was in line for highway driving so I monitored all my vehicles and looked up tranny/R&P ratios of past ones. All of them were running between 1,800-2k at the same speed with exception to my old Mazda 4-banger. It would travel along at around 2,800 rpm at that speed and 3,100 at 70. It was my 2nd 4 cyl to do that and they both started blowing smoke at around 130k. I've read of quite a few others doing the same. They run these small engines at higher revs to get the power they lack but they die an early death. All of this convinced me that running my 6-cyl at around the 2k mark was the sweet spot for commuting. I'm sure it's a poor crawler though.

With the AMC 258 i6 / 4.2l , it stressed out to get to 65-70 but purred at 60. The 4.0 will get to 70 easily. I don't even downshift from 4th to pass. I stomp and it just goes. Like HH, I get nervous at 65-70. I don't trust the CJ to swerve or brake in time for if there was an incident so I just hang out in the slow lane doing 60 minding my own business.

Although it's Texas, there are some longer steeper climbs North of here, nothing like the mountains of CO but still decent, and I have no problems climbing them in 4th while maintaining 60-65. If I want to get to 70 to not hold up traffic, I have to downshift. If I still had the AMC 258 i6 / 4.2l , it would have turned around, looked at me funny and laughed as I dropped to 55 and moved over to the shoulder to let everyone else go around. I could have forced it by stomping on the gas to get the revs up while in 3rd but the further over 2100rpm it got, the worse the blow-by and oil sputtering.

I avg'd 15 mpg with the AMC 258 i6 / 4.2l and now I get 19 mpg with the 4.0.

 

[Lumpy Grits]

2140 rpm @ 60mph is almost lugging.
LG

 

[gert]

2140 rpm @ 60mph is almost lugging.

LG

Man 19 mpg is pretty sweet for a CJ


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