What kind of MPG numbers are you expecting for a brick going 86 miles per hour
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| Gearing and MPG's http://www.lostjeeps.com/forum/phpBB3/viewtopic.php?f=5&t=51615 |
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| Author: | mrb91 [ Mon Apr 05, 2010 5:39 pm ] |
| Post subject: | Gearing and MPG's |
I have an 05 CRD Liberty running 32" tall tires and I still have my 3.73 gears. Ive been averaging 18mpg city only 21 on the highway. "But thats with the cruise set at 86mph which is as high as it will go." I'm thinking of gearing to 4.10 to get some millage back is that a good gear or should I go even steeper? Also is there a way to shutout 5th gear. According to my calculations 32" tires 4th gear with 3.73 rear diff is close to stock final drive. I would like to do this on my next roadtrip to see if my millage improves without spending money to regear. |
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| Author: | Joe Romas [ Mon Apr 05, 2010 6:00 pm ] |
| Post subject: | Re: Gearing and MPG's |
What kind of MPG numbers are you expecting for a brick going 86 miles per hour
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| Author: | stoutdog [ Mon Apr 05, 2010 6:11 pm ] |
| Post subject: | Re: Gearing and MPG's |
Joe Romas wrote: What kind of MPG numbers are you expecting for a brick going 86 miles per hour I second that... regearing at that speed might net you 1mpg, but I doubt it. After 60mph, you loose 1mpg for every 2-3mph faster you drive. If you are concerned about mpg, slow it down. 
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| Author: | kapalczynski [ Mon Apr 05, 2010 8:09 pm ] |
| Post subject: | Re: Gearing and MPG's |
mrb91 wrote: I have an 05 CRD Liberty running 32" tall tires and I still have my 3.73 gears. Ive been averaging 18mpg city only 21 on the highway. "But thats with the cruise set at 86mph which is as high as it will go." I'm thinking of gearing to 4.10 to get some millage back is that a good gear or should I go even steeper? Also is there a way to shutout 5th gear. According to my calculations 32" tires 4th gear with 3.73 rear diff is close to stock final drive. I would like to do this on my next roadtrip to see if my millage improves without spending money to regear. Interesting thought, I'd give it a try and see if it helps. I achieve my best mpg around 50-55 in 4th so why not. 
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| Author: | LibertyCRD [ Mon Apr 05, 2010 8:43 pm ] |
| Post subject: | Re: Gearing and MPG's |
So with 32s and 3.73s you are effectively the same RPM-wise as a stock tire with 3.55s. On gasoline trucks, I'm a HUGE fan of gearing lower (numerically higher) as I have always seen an increase in fuel economy when I do so. The greatest improvement was on a Wrangler YJ...in which I went from 13 MPG to 18 MPG when I put 4.88s in it. HOWEVER...on a diesel I think it's a different ballgame because the torque peak is so much lower in the RPM range. Therefore...most diesels actually can achieve better mileage with taller (numerically lower) gears. In my honest opinion, 3.55s would be a better fit for the CRD. I'd bet some 3.23s would even be very driveable and get awesome mileage on these diesels. |
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| Author: | tjkj2002 [ Mon Apr 05, 2010 8:53 pm ] |
| Post subject: | Re: Gearing and MPG's |
I gained 4mpg's city and about 7mpg's highway from gearing from 3.73's to 4.10's with 32" tires.Same principle applies to gas and diesel when your talking about gearing and tires sizes. |
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| Author: | stoutdog [ Mon Apr 05, 2010 11:10 pm ] |
| Post subject: | Re: Gearing and MPG's |
tjkj2002 wrote: I gained 4mpg's city and about 7mpg's highway from gearing from 3.73's to 4.10's with 32" tires.Same principle applies to gas and diesel when your talking about gearing and tires sizes. How much does something like this cost? |
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| Author: | CATCRD [ Mon Apr 05, 2010 11:15 pm ] |
| Post subject: | Re: Gearing and MPG's |
There's an old saying in the trucking industry to get better fuel mileage: "Gear fast, run slow". Meaning low numerical ratio, slow speed. But no amount of gearing is going to help much at 85mph. I have cruised that fast and gotten in the low 20s. Step it down 20mph and gain almost 10mpg. |
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| Author: | tjkj2002 [ Mon Apr 05, 2010 11:44 pm ] |
| Post subject: | Re: Gearing and MPG's |
stoutdog wrote: tjkj2002 wrote: I gained 4mpg's city and about 7mpg's highway from gearing from 3.73's to 4.10's with 32" tires.Same principle applies to gas and diesel when your talking about gearing and tires sizes. How much does something like this cost? Just a regear? About $1000 just for the gear swap,lockers add more but really depends on your local labor rates.The biggest cost is the front diff,finding the correct gears is hard and pricey and the install requires the whole front diff to be removed. I recouped my gear swap,even with the added locker cost,in less then 1 year. |
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| Author: | LibertyCRD [ Tue Apr 06, 2010 9:44 am ] |
| Post subject: | Re: Gearing and MPG's |
We've always done ours in the driveway with basic tools. Costs me about $400 for a gear swap. Last time I did one (on my ZJ) I threw in an Aussie locker too...and that added $200 or so to the cost. It's without question the biggest bang for the buck you can get. |
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| Author: | ATXKJ [ Tue Apr 06, 2010 5:44 pm ] |
| Post subject: | Re: Gearing and MPG's |
Yeah but the principle that applies to both - is to gear for the Torque peak high or BSFC low - on the Diesel that's 1800 rpm - V6 is ~4000 rpm. |
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| Author: | stoutdog [ Tue Apr 06, 2010 6:51 pm ] |
| Post subject: | Re: Gearing and MPG's |
Alright, so this is all news to me... Sorry for hijacking the OP's thread, but I've got a couple of questions of my own regarding regearing. - Are there any benefits, other than mpg? What are the drawbacks (impact on daily driving, etc.)? - Are there any long-term concerns? - Googling around shows mixed responses to the effects of regearing on mpg. Some people claim little gain, some (including a TJ forum) claim gains of 6-7mpg. It's unclear, but it appears that the people who are running oversize tires see the most gain. Is this true? I'm running stock size tires, no lift, and have no plans to raise my KJ or add larger tires in the future. Would I still see a significant mpg benefit running on a stock system? Even a boost of a couple mpg would get me into the 30s in day-to-day driving... - If yes, what gear size would you recommend? The numbers are greek to me... Thanks ahead of time. |
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| Author: | kapalczynski [ Tue Apr 06, 2010 7:17 pm ] |
| Post subject: | Re: Gearing and MPG's |
stoutdog wrote: Alright, so this is all news to me... Sorry for hijacking the OP's thread, but I've got a couple of questions of my own regarding regearing. - Are there any benefits, other than mpg? What are the drawbacks (impact on daily driving, etc.)? - Are there any long-term concerns? - Googling around shows mixed responses to the effects of regearing on mpg. Some people claim little gain, some (including a TJ forum) claim gains of 6-7mpg. It's unclear, but it appears that the people who are running oversize tires see the most gain. Is this true? I'm running stock size tires, no lift, and have no plans to raise my KJ or add larger tires in the future. Would I still see a significant mpg benefit running on a stock system? Even a boost of a couple mpg would get me into the 30s in day-to-day driving... - If yes, what gear size would you recommend? The numbers are greek to me... Thanks ahead of time. On a stock setup/stock tires, you would most likely lose economy if you geared higher numerically. When you start to go big enough diameter tires where you start losing economy, then you need to re-gear to regain lost economy. Also, bigger fatter/wider tires will cause economy loss due to the extra rotational mass, nit just the diameter change. |
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| Author: | geordi [ Tue Apr 06, 2010 7:43 pm ] |
| Post subject: | Re: Gearing and MPG's |
To continue what Kap-I-cant-spell-it just posted... Larger outer-diameter tires (yet narrow!) will have a similar effect to re-gearing with numerically-lower gears. Not a huge change unless you lift to make them fit, and larger tires DO weigh more than stock BadYear Slushballs. What you are doing at every gear is asking more of the engine's torque to be applied to the tires, at a lower RPM. This has the same effect of reducing the number of revolutions at a given speed that the engine will turn in a mile... At the expense of faster acceleration. Horsepower is the application of torque over time. Which has more horsepower-equivalent work... A wheelbarrow moving dirt at one load per 10 minutes... Or a teaspoon that moves the same amount of dirt in 10 minutes... But has to make 5000 trips? HP is not work. Torque is work. We have engines that can EASILY do the work needed. Why apply them with a "less per turn, but faster" mode? Because it makes better marketing, not mpg. Gearing to slow the engine down to the torque peak will increase your economy, as long as you don't go TOO low, and get below the torque peak. Then you will be into the "lugging" the engine space. In addition, at heavier loads (towing) you are asking a heavier burden of the engine. If you are geared lower, you are transferring some of the "tow rated weight" into the "better mpg" department, and you cannot ask it to tow as much weight anymore. I personally would not gear mine much lower than 3.5x b/c I do tow at full-weight of 7000 lbs. I also already have larger tires, so I have already changed the gearing slightly. If I could find lighter weight tires that are in the taller size that I want... I would run them in a heartbeat. It is the size, not the rating I am after, mostly. |
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| Author: | stoutdog [ Wed Apr 07, 2010 3:29 am ] |
| Post subject: | Re: Gearing and MPG's |
Thanks guys! Geordi, let me see if I understand you correctly (sorry, this is going off into an area of vehicle mechanics that I am entirely unfamiliar with). If I were to gear to a higher number, I would be asking my vehicle to "do less per turn, but faster," which would give me no mpg gain - but, and I'm guessing here, faster acceleration? On the opposite side, gearing to a lower number would increase mpg and slightly decrease acceleration and towing capacity. Correct? And, theoretically of course, you could gear the CRD all the way down to torque peak? I have yet to tow with my Jeep and, when I do, I won't come close to 5k lbs. Just a small boat and trailer, at most, so that factor doesn't concern me too much.... |
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| Author: | tjkj2002 [ Wed Apr 07, 2010 7:44 am ] |
| Post subject: | Re: Gearing and MPG's |
stoutdog wrote: Thanks guys! Geordi, let me see if I understand you correctly (sorry, this is going off into an area of vehicle mechanics that I am entirely unfamiliar with). If I were to gear to a higher number, I would be asking my vehicle to "do less per turn, but faster," which would give me no mpg gain - but, and I'm guessing here, faster acceleration? On the opposite side, gearing to a lower number would increase mpg and slightly decrease acceleration and towing capacity. Correct? And, theoretically of course, you could gear the CRD all the way down to torque peak? I have yet to tow with my Jeep and, when I do, I won't come close to 5k lbs. Just a small boat and trailer, at most, so that factor doesn't concern me too much.... Gearing higher(# lower) only works to a certain point. The gear ratio in the diff effects the final drive ratio,you have to factor in the gear ratio's in the transmission also.It's all about torque multiplication. 545RFE trans has a 3:1 1st gear so with 3.73 gears you have 11.19:1 in 1st gear(t-case in 2wd or 4 hi),so if your putting out 300lbs-ft of torque you effectively are putting 3357lbs-ft of torque to the rear wheels(not factoring in driveline parasitic loss).Drop the diff gears to 3.55's and the final drive ratio in 1st gear drops to 10.65:1 and the torque reduction drops to 3195lbs-ft or torque to the rear wheels effectively making the engine work harder to start moving the KJ,harder on the transmission also.Dropping to 3.21 gears(the highest you can go with the D30A) drops the final drive ratio to 9.63:1 and the torque multiplication drops to 2889lbs-ft of torque in 1st gear. |
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| Author: | MrMopar64 [ Wed Apr 07, 2010 7:59 am ] |
| Post subject: | Re: Gearing and MPG's |
Don't forget, though, that at the launch (zero vehicle speed=maximum RPM difference) the T/C is giving you about a 1.7:1 torque multiplication. So your 300ft-lb engine feels a lot beefier. The numerically lower gears will feel somewhat different, but not make it-or-break it different. Keep in mind that although the KK CRD uses the A580 trans (3.59:1 first gear), it also uses 3.21 axle ratios. |
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| Author: | tjkj2002 [ Wed Apr 07, 2010 8:11 am ] |
| Post subject: | Re: Gearing and MPG's |
MrMopar64 wrote: Keep in mind that although the KK CRD uses the A580 trans (3.59:1 first gear), it also uses 3.21 axle ratios. KJ CRD with the 545RFE whne stock gives you 11.91:1 torque multiplication in 1st gear. KK CRD with the different trans and gear ratio gives 11.52:1 in 1st gear. The difference in gear ratio #'s in the trans(lower ratio's) makes up for/needs the higher gears in the diffs to keep the same performance #'s. |
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| Author: | kapalczynski [ Wed Apr 07, 2010 7:50 pm ] |
| Post subject: | Re: Gearing and MPG's |
stoutdog wrote: Thanks guys! Geordi, let me see if I understand you correctly (sorry, this is going off into an area of vehicle mechanics that I am entirely unfamiliar with). If I were to gear to a higher number, I would be asking my vehicle to "do less per turn, but faster," which would give me no mpg gain - but, and I'm guessing here, faster acceleration? On the opposite side, gearing to a lower number would increase mpg and slightly decrease acceleration and towing capacity. Correct? And, theoretically of course, you could gear the CRD all the way down to torque peak? I have yet to tow with my Jeep and, when I do, I won't come close to 5k lbs. Just a small boat and trailer, at most, so that factor doesn't concern me too much.... You're right on man.
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| Author: | geordi [ Wed Apr 07, 2010 8:42 pm ] |
| Post subject: | Re: Gearing and MPG's |
kapalczynski wrote: stoutdog wrote: Thanks guys! Geordi, let me see if I understand you correctly (sorry, this is going off into an area of vehicle mechanics that I am entirely unfamiliar with). If I were to gear to a higher number, I would be asking my vehicle to "do less per turn, but faster," which would give me no mpg gain - but, and I'm guessing here, faster acceleration? On the opposite side, gearing to a lower number would increase mpg and slightly decrease acceleration and towing capacity. Correct? And, theoretically of course, you could gear the CRD all the way down to torque peak? I have yet to tow with my Jeep and, when I do, I won't come close to 5k lbs. Just a small boat and trailer, at most, so that factor doesn't concern me too much.... You're right on man. Yep, the trick to HP is that at higher RPMs (like when you are on the highway wanting passing power) you downshift which increases the RPM and reduces the torque-per-rev... But applies that torque more rapidly. Reducing your gearing lowers the RPM at every point across the power band, and asks each pound of torque to be more fully utilized. The danger is that we have a wide demand range. You don't want to reduce the rpm too much, so that at low speeds you are below the torque peak and REALLY lugging the engine. |
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