Regearing Questions? READ THIS!

[WB9YZU]

Ahh but there is a LOT of anectedotal evidence that points towards lower -higher numerically- gearing improving MPGs (at all but extremely high road speeds) possibly because it gets the 4.0 up into a more efficient RPM zone. I don't know why for certain, but I do know it was the case on both my MJ 5speed and my XJ AW4... tires being the same, 3.xx to 4.56 improved both the driveability and fuel efficiency. The XJ automatic saw a further improvement in MPG and 'fun' going from "33s" to "31s" with the 4.56:1.

MPGs, again not a one size fits all discussion. Here you get into aerodynamics, use, driving habits, inflation pressures, along with the usual engine size, engine condition, performance mods, and even weather/altitude. These are all items I see ignored in Gearing/Tire/MPG discussions.

Throwing a spare on a roof rack instead of putting it in the cargo area can make a difference in fuel consumption. Watch out for "anectedotal evedence" it is not proof, and it certainly is not fact.

You are correct that there is not a one-size-fits-all approach and there will; be trade-offs with whatever ratio one picks, but since there are only five gear ratio choices (for the HP30 - 3.07, 3.55, 3.73, 4.10, 4.56, 4.88) and four (for the 8.25 - 3.07, 3.55, 3.73, 4.10, 4.56) it's easier to speak in general terms.

Consider this chart for example http://www.drivetraindirect.com/t_gear_calculator.htm

If you match the tire and gears to the application, you will get the performace you desire, for the purpose you desire it for.

If you want it to act Stock with bigger tires, adjust the gears to make the RPMs work out the same. If you require more towing power, raise the RPMs, but be aware this comes at a price of maximum speed and fuel ecconomy. If you require more MPGs, lower the RPMs abit and realize this comes at the expense of towing power.

It's also easy to fall into the "diameter trap" ASSuming that your 31"s or whatever are actually the dia. as stated. More accurate predictions can be had by using the working radius x 2 (the actual measurement from the hub center down to the driving surface x 2 as a 'diameter')

I'm not sure exactly what "Diameter Trap" you are speaking of, but tire specs can be obtained from each manufacture. The relative few hudreds of an inch are not that relevant as that will change with rim diameter and inflation pressure.

The usefull diameter is not as important with Jeeps and other on-road equipment, though it make a large difference with drag slicks since the radius at the line at launch can be an greatly different than the radius at the top end of the track.

 

[Gojeep]

I have a 2001 with the auto trans with 3:55's, and 4.0L. and I'm sure that my tires are bigger than the stock ones that I took off. The 255/70/16's that I put on it are the same size as the tires that Jeep is putting on the new Wranglers which they claim are 32's. So I think either Jeep or you guys are doing the math just a little off, you guys tell me. I bet they are closer to 31's but the same size as the 30's that I took off, I think not. As far as the Jeep being used on flat land, well have you ever been to Pennsylvania? We have a boat load of mountains around here and I can see them out of every window in my house. I'm not saying that 4:56's aren't the best choice for power, I'm sure they are. What I'm getting at is that most of you run your Cherokee's off road more than on road. There are a couple of us that use our Cherokee's on road more than off, and are looking to have better MPG's then we are power off road. If you can tell me that 4:56's are going to give me the better MPG's and power with 32's then the 4:10's and 32's then that is what I will run. But you guys have to understand that your telling me one thing and the shops are telling me another. I would like to run 33's but don't want to cut on the Cherokee and have to have the tires covered to be legal. So I will be sticking with the 32's for not only that reason but if I go higher it's not a good DD anymore and it gets a lot more expensive after the 4.5 inch lift. So yes I have lots of hills around here, yes I do run it off road ( I'm going tomorrow with 2 Wranglers and want to spank their ass in the Cherokee), and yes I'm looking for the best on road MPG's and off road compromise. Thanks for the help.

You are just confused as no Jeeps are 32's and only the Rubicon's with 245/75/16's, both now and the new model, are only 31's and the rest of the range is lower still. As you dont believe the whole principle of how to caluclate out the tyre height from the sidewall size, look at this OFFICAL BFG site giving true heights which are even lower! http://www.bfgoodrichtires.com/assets/pdf/all_terrain_ta_kd.pdf
If you are going to stay with the small 255/70/16's, then 4.11's will be better than 4.56's. You will have to trim, run 4" of lift and drop your bumpstops to run 32's like I do in the 265/75/16's which just shows that you are not running this size now as you dont have any of that!

 

[Semper Fi]

I'm not confused on what size the new Jeeps are running. The new 2007 Jeeps are running the 255/75/16 which Jeep claims is a 32. I never said I didn't believe the whole calculate the tire thing I just said that someone was a little off on their math. Well I guess Jeep is just trying to pull us in by saying you can get 32's from the factory. And to run 32's after I put the RE 4.5 inch lift on I will should not have to trim but I will lower the bump stops and I didn't say I was running 265/75/16's on my Jeep.

But you guys have still not answered the question, can you tell me that 4:56 gears will give me better, or worse MPG with 32's than the 4:10's with 32's? Again understand we are coming to you guys because you have the experience and the shops are telling me different. If all they are doing is looking in a book but have never tryed this setup, they don't know, but you guys should know better than them. You have run the different setups and that is why we come to you guys. I am not new to the Jeep thing, as this is my 8th Jeep, I am new to the regearing thing. Again thanks for the help and remember this is a DD and used off road about 8 to 10 times a year.

 

[woody]

But you guys have still not answered the question, can you tell me that 4:56 gears will give me better, or worse MPG with 32's than the 4:10's with 32's?

Based on the personal experiences stated above (89XJ, 225,000 miles, 4.0l Automatic trans, 5-6" lift, 3.54:1 axle gears, 33x12.5" tires on 8" wheels) when I swapped in 4.56:1 my MPG went up from 15-16 to 17-18. A bit better than a 10% improvement. When those 33"s got bald/cut/destroyed, I put on 31x9.50 and maybe got another 1 MPG improvement from that... Note that the smaller TSLs on 7" wheels were likely lighter, as well as presenting less rolling/wind resistance

Not purely scientific data collection, but I did measure my odometer reading over several passes, 10 miles each, on relatively flat/straight interstate. And I made extra effort to fill the tank from the same pump over two+ years of daily commuting (@ approx 1 tank per week) over the same (rural, hilly 2 lane) route. In other words, I didn't just 'dream up' the improved efficiency. It was real. How far off from actual the MPG increase is a good question. If it was really 8% or 12% :dunno: but it was an improvement, and that is good enough for me. I didn't make before/after quarter-mile passes at The Rock to verify any acceleration increase, but the butt dyno told me all I needed to know.


Now to the root of your question... Based on what I have experienced first-hand, "IF" I were certain that 32" was all I was gonna run... at least for this set & maybe the next set of tires (30k?) "I" wouldn't even consider 4.10:1...Unless they were free. I'd either suffer with less fuel efficiency and lower performance from the OEM 3.55:1 or spend on the more fuel efficient and better performing 4.56:1.

I am sort of in the same place with the current 88 XJ as I was before with the 89: 3.55:1 and 30" (really 28" at my desired inflation PSI) and getting approx 17 MPG over the same route as outlined above. I have a choice of rear D44s to swap in, and will probably swap the HP30 for an unbeat unit when I redo the front suspension. The 30"s (28"s) are at approx 50% at 12k, so I have approx one more year's use left on them. Option "A" would be to find an unbeat 4.10:1 HP30 and swap it in (regear the 44 to match) or regear my unbeat spare -3.55:1- HP30. Option B is regear both to 4.56:1 and see how that goes until new tire time... when I will be going to 31" or possibly 32". I am leaning towards Option B... smallish tires and 4.56:1 for awhile. IF (big if) I did a LOT of interstate travel, I may lean towards 4.10:1 more... but I don't, and don't see it in the future. <edit/add> Option C is swap in either 44, leave the gearing OEM at 3.55:1, and continue to tolerate less than optimal performance & efficiency.<end edit>

Bottom line... With 32" tires I think you will see MORE OF an improvement in both MPG and drivability (driveability being both on and off road) with the 4.56:1 than with the 4.10:1.

As they say, YMMV

 

[Semper Fi]

ok well I have to thank you for that because that is what I wanted to hear. Not only that you have tried the setup but you have tried it another way and from experience you know that it works. And thank you for answering the question from a DD point of view. I don't have plans on running anything bigger than 32's but this will leave my options open for 33's in the future if gas prices don't get to high. I will be driving the Jeep to AR from PA on the 10th and on the open road it doesn't do to bad with the 3.55's. Thanks again for the answers

 

[WB9YZU]

Woody, I believe your info is flawed. Not from a measured MPG standpoint, but because your conclusion is incorrect.

Most tire manufactures will recomend a wheel size for each tire. If you choose the smaller rim, there is a real chance the tire will wear in the middle at a normal 32psi inflation pressure. To counter that, most folks decrease the tire pressure to get the tire to lay flat. Did you do that? If you did, you also increased the length of the patch and increased rolling resistance. That's a doubly wammy. A tire/Rim which weighs much more (65-75lb) and which also has more rolling resistance.

When you changed to a smaller, lighter tire with a smaller rim, the inertia and rolling resistance of that combination will be different than the previous combination; taking less power. However, the Jeep is still geared for power and is not running at a RPM which is optimal for fuel efficiency. You get an increase in fuel milage due to less rolling resistance, but loose the gains you made because of having the wrong gearing.

If you took a stock Jeep and ran down the HWY at 65mpg without using OD, that would be comparable to using 31's and 4.56 gears. Not at all optimal, but you will have more power.

Back to the premise of my 2nd post.
If you want MPGs, choose a combination for that. If you want good performace on the trail, choose a combination for that. Just remember that no matter what you choose, the gear/tire combination will not be optimal for the other application.

The optimal DD combination would have a Tire/Gear combination set up for the HWY, and a 4:1 LO kit installed for the Off-Road stuff.

 

[CRASH]

Original post updated for a new crop of newb questions.

 

[Gojeep]

Just came across this little bit of information.

"According to the engine design text I have‚ maximum fuel economy occurs with open throttle and at a piston speed of 1200 – 1500 ft/min·  So if you know the stroke of your engine‚ you can calculate the rpm for best fuel economy·  Usually comes out somewhere between 2000 – 3000 rpm·


The 4.0L has a stroke of 3.411" and a 4.5-4.8L stroker has a stroke of 3.895" so the numbers would be as follows:

1200-1500ft/min piston speed = 2110-2640rpm on a 4.0, 1848-2310rpm on a stroker

Maximum theoretical safe piston speed with cast iron crank/rods and cast pistons is 3750ft/min so that equates to 6600rpm on a 4.0 and 5800rpm on a stroker.

Here are some gear rev and tyre sizes to help make sense of this.

RPM's at 65 MPH w/ 3.55:1 axle gears running 28's: 2,076 RPM
RPM's at 65 MPH w/ 3.55:1 axle gears running 30's: 1,938 RPM
RPM's at 65 MPH w/ 3.55:1 axle gears running 31's: 1,876 RPM
RPM's at 65 MPH w/ 3.55:1 axle gears running 32's: 1,817 RPM
RPM's at 65 MPH w/ 3.55:1 axle gears running 33's: 1,762 RPM

RPM's at 65 MPH w/ 4.10:1 axle gears running 28's: 2,404 RPM
RPM's at 65 MPH w/ 4.10:1 axle gears running 30's: 2,244 RPM
RPM's at 65 MPH w/ 4.10:1 axle gears running 31's: 2,172 RPM
RPM's at 65 MPH w/ 4.10:1 axle gears running 32's: 2,104 RPM
RPM's at 65 MPH w/ 4.10:1 axle gears running 33's: 2,040 RPM

RPM's at 65 MPH w/ 4.56:1 axle gears running 28's: 2,668 RPM
RPM's at 65 MPH w/ 4.56:1 axle gears running 30's: 2,490 RPM
RPM's at 65 MPH w/ 4.56:1 axle gears running 31's: 2,410 RPM
RPM's at 65 MPH w/ 4.56:1 axle gears running 32's: 2,334 RPM
RPM's at 65 MPH w/ 4.56:1 axle gears running 33's: 2.264 RPM

I am looking right in the ball park there with my 32's and 4.56 gears that are going in next week hopfully.




Cheers

 

[nhrocker]

Nice little chart there Gojeep. Any way you could add the list for 4.88's? I'm kind of wondering how my 4.88's with 30"s compares mathmatically.:laugh3:

 

[DaJudge]

Just to add my experience for those who are still not convinced.

2001 4.0 AW4 that is my daily driver.

I had 31s with the stock 3.55s. My gas mileage was terrible (I actually got 8.5mpg once) and the power sucked and driveability sucked. Trans was either hunting or engine was lugging.

Based on my research I went ahead and opted for 4.56 since I am eventually going 33s. I have been running this for 2 weeks now and I love it. I can't give you real mpg #s yet because I just corrected the speedo today. But...In the past I was lucky if I got back and forth to work for 5 days on 1 tank. The first full tank after changing to 4.56 gears I wen back and forth for 8 days plus driving around on the weekend. It has made a huge improvement in my gas mileage. When I figure the next tank I will post the numbers up here.

Also, It is SOOOOOO much better driving now. I am glad I did not go 4.10s.

Glen

 

[Gojeep]

Nice little chart there Gojeep. Any way you could add the list for 4.88's? I'm kind of wondering how my 4.88's with 30"s compares mathmatically.:laugh3:

Ask and you shall receive.

RPM's at 65 MPH w/ 4.88:1 axle gears running 28's: 2,855 RPM
RPM's at 65 MPH w/ 4.88:1 axle gears running 30's: 2,665 RPM
RPM's at 65 MPH w/ 4.88:1 axle gears running 31's: 2,579 RPM
RPM's at 65 MPH w/ 4.88:1 axle gears running 32's: 2,498 RPM
RPM's at 65 MPH w/ 4.88:1 axle gears running 33's: 2,423 RPM

 

[holeshot]

How about 35's?

 

[nhrocker]

Ask and you shall receive.

Thanks Marcus! It wasn't nearly as high as I thought it would be. I guess I should just invest in a tachometer one of these days...

 

[Gojeep]

How about 35's?

RPM's at 65 MPH w/ 3.55:1 axle gears running 28's: 2,076 RPMX
RPM's at 65 MPH w/ 3.55:1 axle gears running 30's: 1,938 RPM
RPM's at 65 MPH w/ 3.55:1 axle gears running 31's: 1,876 RPM
RPM's at 65 MPH w/ 3.55:1 axle gears running 32's: 1,817 RPM
RPM's at 65 MPH w/ 3.55:1 axle gears running 33's: 1,762 RPM
RPM's at 65 MPH w/ 3.55:1 axle gears running 35's: 1,655 RPM

RPM's at 65 MPH w/ 4.10:1 axle gears running 28's: 2,404 RPM
RPM's at 65 MPH w/ 4.10:1 axle gears running 30's: 2,244 RPM
RPM's at 65 MPH w/ 4.10:1 axle gears running 31's: 2,172 RPM
RPM's at 65 MPH w/ 4.10:1 axle gears running 32's: 2,104 RPM
RPM's at 65 MPH w/ 4.10:1 axle gears running 33's: 2,040 RPM
RPM's at 65 MPH w/ 4.10:1 axle gears running 35's: 1,916 RPM

RPM's at 65 MPH w/ 4.56:1 axle gears running 28's: 2,668 RPM
RPM's at 65 MPH w/ 4.56:1 axle gears running 30's: 2,490 RPM
RPM's at 65 MPH w/ 4.56:1 axle gears running 31's: 2,410 RPM
RPM's at 65 MPH w/ 4.56:1 axle gears running 32's: 2,334 RPM
RPM's at 65 MPH w/ 4.56:1 axle gears running 33's: 2,264 RPM
RPM's at 65 MPH w/ 4.56:1 axle gears running 35's: 2,127 RPM

RPM's at 65 MPH w/ 4.88:1 axle gears running 28's: 2,855 RPM
RPM's at 65 MPH w/ 4.88:1 axle gears running 30's: 2,665 RPM
RPM's at 65 MPH w/ 4.88:1 axle gears running 31's: 2,579 RPM
RPM's at 65 MPH w/ 4.88:1 axle gears running 32's: 2,498 RPM
RPM's at 65 MPH w/ 4.88:1 axle gears running 33's: 2,423 RPM
RPM's at 65 MPH w/ 4.88:1 axle gears running 35's: 2,276 RPM

Anyone know the overdrive ratio on a AX-5?
I have a AW4 auto in overdrive 4th, ( 0.75 ). Pre '91 will be slightly slower at 0.70, Peugeot had a 0.72 and faster for AX-15 manuals at 0.79

 

[johnlv6]

I just want to make sure i pulled the correct information...

A 2001 XJ Auto with 33's = 4.56? (highway/city driving mostly...not sure how often i'll get off-road). I'm more concerned with fuel economy than anything else at the moment.

I'm looking at buying another XJ (the one i mentioned above)...it has an Auto w/ 3.55 gears. The owner gets 11mpg on average. Will installing 4.56 gears in the auto yield a drastic improvement in fuel economy? That's what i got out of most of the threads....just wanted to make i'm correct here (i've never had to change gears in a vehicle before). Thanks.

John

 

[Gojeep]

BTW, I just added a heap of calculation tables to my Speedo gaer write up that I found. Will let you feed in any tyre, gear, rev etc combinations right down to working out your low low crawl etc.

www.go.jeep-xj.info

 

[Kejtar]

I just want to make sure i pulled the correct information...

A 2001 XJ Auto with 33's = 4.56? (highway/city driving mostly...not sure how often i'll get off-road). I'm more concerned with fuel economy than anything else at the moment.

If you are considering fuel economy, then honestly you should consider the following: stock height, no roof rack, street grade tires and so on. With proper gears on a lifted jeep expect economy of probably 16 to 19. Some get way better (I think that the speedo gear there might be wrong though ) and some get worse (again possibly speedo gear, or something else).

I'm looking at buying another XJ (the one i mentioned above)...it has an Auto w/ 3.55 gears. The owner gets 11mpg on average.

Did the owner install a new speedo gear when he got the bigger tires or did he kind of figured things will magically work out? Cause the 11mpg sounds like what you'd get from traveling further on a single rotation of a tire with the speedo set for a tire of a smaller diameter.

Will installing 4.56 gears in the auto yield a drastic improvement in fuel economy? That's what i got out of most of the threads....just wanted to make i'm correct here (i've never had to change gears in a vehicle before).

Yes it will improve your economy. Drastic? I don't know, that depends on your definition of drastic. It will also make all the rest of the drivetrain parts work more efficiently and last longer.

 

[johnlv6]

Kejtar, thanks for taking the time to reply. I'd be happy with anything over 15mpg..i guess i should have been more specific on my idea of an improvement . I forgot to ask if he swapped out the speedo gear...i'll ask next time we talk. Would this have an impact on the vehicles mileage?

I'll be honest and say that i'm leaning away from purchasing this particular Jeep. I'm not going to pay good money for a vehicle when i know i'm going to have to run right out and spend $1,000+ to get it running properly ($500 for gears plus install). I know my limitations...a R&P install is more than i can handle at the moment. The other thing i'm wondering...if he was too cheap to install the proper gears, what else did he skimp on?

 

[Kejtar]

Kejtar, thanks for taking the time to reply. I'd be happy with anything over 15mpg..i guess i should have been more specific on my idea of an improvement . I forgot to ask if he swapped out the speedo gear...i'll ask next time we talk. Would this have an impact on the vehicles mileage?

It doesn't have an impact on the mileage but rather on the calculation behind it. Speedo gear is what controls your speedometer. Imagine this: your stock speedo gear has X number of teeth and it know that each time your wheel turns you have traveled X feet. With a 29" tire I believe that would be 91 inches or so (2PIr). If you put on a 33" tire it would be 103 inches. So that's about 12% loss due to bad calculations. Now granted that's not where it ends. With 33" tires your engine is working harder to make things run, but it definately is a starting point: you can't fix something until you know precisely what's wrong with it. I think that if it turns out that he gets 11mpg and has a proper speedo gear (you can check it by taking a ride on the highway and going about 10 miles and checking distance traveled as reported by the vehicle vs the mile markers: you do want to go all 10 though for a more accurate check).

I'll be honest and say that i'm leaning away from purchasing this particular Jeep. I'm not going to pay good money for a vehicle when i know i'm going to have to run right out and spend $1,000+ to get it running properly ($500 for gears plus install). I know my limitations...a R&P install is more than i can handle at the moment. The other thing i'm wondering...if he was too cheap to install the proper gears, what else did he skimp on?

Not that I'm trying to say that this jeep is a good purchase or not, but quite a few are too cheap to install proper gears. The 11mpg sounds kinda scary, the $500 for gears sounds a bit much and you gotta make friends in the local jeeping community to get a better priced install.

 

[19CHEROKEE89]

how about some info on 38's? pretty sure im prolly gonna run 5.38:1 or lower since gas is no big deal and this is a toy. but i would like to know rpms and so forth

Thank you!