RPM's at 75-80mph

hybridp · **Joined:** Thu Feb 25, 2016 1:23 pm **Posts:** 3

Hey everyone,

I have been lurking this forum for the past few days, gathering research on the KJ CRD. I have found a few threads talking about RPM's at certain speeds and whatnot, but none specifically with what I'm looking for. I drive 70-80 miles one way to work, mostly highway at 75-80 for 85% of the trip on cruise control. For those who have the GDE trans tune, what are you guys at RPM wise at that speed? I'm wondering if that speed will be too much for the engine. I'm also wondering how big of a tire you'd need to make a dent in lowering the RPM's. I've seen numbers from people, but not with the trans tune. Thanks

papaindigo · **Posted:** Thu Feb 25, 2016 2:08 pm

I cannot give you rpms for those speeds which are faster than I drive. However I can say at those speeds the rpms will be identical regardless or F-37, no F-37, or GDE trans tune as you are in the top gear fully locked up and in overdrive unless you have locked out overdrive via the button on the shift lever. The only exception that I can think of might be the GDE TCM tow tune that holds off final transmission lockup and OD to a higher (well above 58 mph) speed.

flash7210 · **Posted:** Thu Feb 25, 2016 2:27 pm

Regardless of which transmission tune you have, at any speed above 70 mph the transmission will be in 5th gear.

For mine, 75mph is 2100-2200rpm and 80mph is about 2300rpm.
I'm running a 245-75-16 tire which is about as tall as you would ever want to go on a KJ.

I was running a 215-85-16 tire a few years ago back and I really liked them but they didn't do to well on wet pavement.

hybridp · **Joined:** Thu Feb 25, 2016 1:23 pm **Posts:** 3

flash7210 wrote:

Regardless of which transmission tune you have, at any speed above 70 mph the transmission will be in 5th gear.

For mine, 75mph is 2100-2200rpm and 80mph is about 2300rpm.
I'm running a 245-75-16 tire which is about as tall as you would ever want to go on a KJ.

I was running a 215-85-16 tire a few years ago back and I really liked them but they didn't do to well on wet pavement.

Thanks. I don't know why I thought the GDE tune was going to lower the RPM's at that speed. When I think 2300 at 80mph for a gasser that's pretty awesome. But what's the redline on the CRD? Like 5800 or something? Did you have to lift yours to run the 16's? Do you think that running the engine at 2300rpm at cruise is going to be too high of a load? It seems to be the same issue with the WK's as well. Tough to find a diesel car with some cargo space that isn't FWD or crazy expensive.

papaindigo · **Posted:** Thu Feb 25, 2016 5:13 pm

The various GDE transmission tunes tinker with shift points and torque converter lockup so they can change rpm just not at those speeds. For example the basic tune lowers the "final shift" point from the post F-37 ca. 63 mph to ca. 58 mph and lets you hold that down to near 50mph. Hence if you have that GDE tune and have made that final shift then for speeds in the range of near 50mph up to ca. 63mph your rpms will be lower than a CRD with the F-37 tune but once above ca. 63mph there is no difference. Hope this makes sense.

Also if you think it thru that GDE tune only helps mpg for more or less in town driving where you are not above the final shift point.

WWDiesel · **Posted:** Thu Feb 25, 2016 6:29 pm

Only simple way to lower engine RPM's is to run a taller tire! Taller tires lowers final drive ratio of engine speed vs ground speed....
245/75/16 requires some small lift (new springs) to clear body parts when turning... :wink:

mass-hole · **Posted:** Thu Feb 25, 2016 6:59 pm

It will be around 2200 RPM or so which is perfectly fine. These diesels redline at 4000 rpm but its not because they will explode or anything, its just because diesel fuel doesnt burn fast enough to provide much power above that. Plenty of people have run at 80 mph in these, including me. I drive all over Wyoming, Utah and Nevada, where its 5 hrs between patches of civilization and the speed limit is 80 mph and its totally fine. It doesn't sound like its stressed out, it sounds just like any other vehicle would at that speed. In fact its probably better because at that speed you are right in the peak torque range so it cruises along effortlessly.

I also dont think a bigger tire will pay off in increased mpg's or reduced stress on the motor. All it will do will satisfy you efforts to reduce rpms which don't really matter. Regearing the diffs to a lower ratio is the only way you could do it not take a hit from tire weight and drag.

hybridp · **Joined:** Thu Feb 25, 2016 1:23 pm **Posts:** 3

papaindigo wrote:

The various GDE transmission tunes tinker with shift points and torque converter lockup so they can change rpm just not at those speeds. For example the basic tune lowers the "final shift" point from the post F-37 ca. 63 mph to ca. 58 mph and lets you hold that down to near 50mph. Hence if you have that GDE tune and have made that final shift then for speeds in the range of near 50mph up to ca. 63mph your rpms will be lower than a CRD with the F-37 tune but once above ca. 63mph there is no difference. Hope this makes sense.

Also if you think it thru that GDE tune only helps mpg for more or less in town driving where you are not above the final shift point.

After re-reading it again, that definitely makes sense. Thank you. Just to clarify, F-37 refers to the GDE trans eco tune?

WWDiesel wrote:

Only simple way to lower engine RPM's is to run a taller tire! Taller tires lowers final drive ratio of engine speed vs ground speed....
245/75/16 requires some small lift (new springs) to clear body parts when turning... :wink:

mass-hole wrote:

It will be around 2200 RPM or so which is perfectly fine. These diesels redline at 4000 rpm but its not because they will explode or anything, its just because diesel fuel doesnt burn fast enough to provide much power above that. Plenty of people have run at 80 mph in these, including me. I drive all over Wyoming, Utah and Nevada, where its 5 hrs between patches of civilization and the speed limit is 80 mph and its totally fine. It doesn't sound like its stressed out, it sounds just like any other vehicle would at that speed. In fact its probably better because at that speed you are right in the peak torque range so it cruises along effortlessly.

I also dont think a bigger tire will pay off in increased mpg's or reduced stress on the motor. All it will do will satisfy you efforts to reduce rpms which don't really matter. Regearing the diffs to a lower ratio is the only way you could do it not take a hit from tire weight and drag.

Thanks guys. What kind of MPG's you getting on 16's at 80mph with an eco/hot tune?
Or to Mass-hole, what are you returning with regular tires at 80mph?

papaindigo · **Posted:** Thu Feb 25, 2016 8:30 pm

No. When the CRD came out DCJ had installed an extremely weak torque converter in the automatic transmission. Shortly after they installed a slightly less weak TC. The early TC grenaded almost immediately and the replacement TCs were showing signs of problems all related to shift points, horsepower, and engine torque. DCJs "solution" was not to recall and install a decent TC but rather to recall and do the F-37 which a) detuned the ECU to reduce horsepower and torque and b) change the shift points in the TCM by raising them so shifts occurred at a higher rpm. Pre F-37 CRD final shift occurred at about 57-58 mph and would hold that shift down to say 52 mph; F-37 CRDs final shift occurred ca. 63mph and would hold that down to say 57 mph; GDE eco TCM tune among other things "reset" the final shift to about 57-58 mph and allow it to be held down to say 50mph or perhaps a bit lower on the flat.

mass-hole · **Posted:** Thu Feb 25, 2016 9:06 pm

hybridp wrote:

papaindigo wrote:

The various GDE transmission tunes tinker with shift points and torque converter lockup so they can change rpm just not at those speeds. For example the basic tune lowers the "final shift" point from the post F-37 ca. 63 mph to ca. 58 mph and lets you hold that down to near 50mph. Hence if you have that GDE tune and have made that final shift then for speeds in the range of near 50mph up to ca. 63mph your rpms will be lower than a CRD with the F-37 tune but once above ca. 63mph there is no difference. Hope this makes sense.

Also if you think it thru that GDE tune only helps mpg for more or less in town driving where you are not above the final shift point.

After re-reading it again, that definitely makes sense. Thank you. Just to clarify, F-37 refers to the GDE trans eco tune?

WWDiesel wrote:

Only simple way to lower engine RPM's is to run a taller tire! Taller tires lowers final drive ratio of engine speed vs ground speed....
245/75/16 requires some small lift (new springs) to clear body parts when turning... :wink:

mass-hole wrote:

It will be around 2200 RPM or so which is perfectly fine. These diesels redline at 4000 rpm but its not because they will explode or anything, its just because diesel fuel doesnt burn fast enough to provide much power above that. Plenty of people have run at 80 mph in these, including me. I drive all over Wyoming, Utah and Nevada, where its 5 hrs between patches of civilization and the speed limit is 80 mph and its totally fine. It doesn't sound like its stressed out, it sounds just like any other vehicle would at that speed. In fact its probably better because at that speed you are right in the peak torque range so it cruises along effortlessly.

I also dont think a bigger tire will pay off in increased mpg's or reduced stress on the motor. All it will do will satisfy you efforts to reduce rpms which don't really matter. Regearing the diffs to a lower ratio is the only way you could do it not take a hit from tire weight and drag.

Thanks guys. What kind of MPG's you getting on 16's at 80mph with an eco/hot tune?
Or to Mass-hole, what are you returning with regular tires at 80mph?

On my stock tires I used to get 30 mpg or more at 75-80mph. I could get better if I went slower and had seen up to 34 once.

On the 265/70R16 Duratracs I get about 26 mpg going 80 so I lost quite a bit. That was on a trip from Salt Lake to Las Vegas and back so about 11 hrs of total driving.

CGman · **Posted:** Fri Feb 26, 2016 11:39 am

I too used to get around 30 mpg with the stock setup. With lift and bigger tires it's down about 5 mpg, throw on the Moabs with the BFG ATs and it goes down another 3 mpg. I think the increase in height and weight/inertia/rolling friction quickly outweighs the RPM benefit.

flash7210 · **Posted:** Fri Feb 26, 2016 1:20 pm

CGman wrote:

I too used to get around 30 mpg with the stock setup. With lift and bigger tires it's down about 5 mpg, throw on the Moabs with the BFG ATs and it goes down another 3 mpg. I think the increase in height and weight/inertia/rolling friction quickly outweighs the RPM benefit.

Same here.

I was getting 25-26 mpg city and 28-29 mpg highway.
Then I added a mild lift and some aggressive 245-75-16 all terrain tires and now I'm getting 21-22 city and 24-25 highway at speeds below 75mph.
And at speeds above 75mph my mpg is more like 20-21.

But those tires are great off-road :JEEPIN:

Hexus · **Posted:** Fri Feb 26, 2016 5:54 pm

Should be around 2200 RPM's.

Patrick Henry · **Posted:** Fri Feb 26, 2016 11:21 pm

Same here. 2200 rpm at 75 mph. 245-70 series tires.

RPM's at 75-80mph

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