• Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

Power loss Vs Ground Clearance

98XJSport

98XJSport

Destiny is the rising sun
Location
Western Maine
I have a 98 XJ that I use as a daily driver and to occasionally compete in stock class mud runs. I have about 4.5" of lift with 31x10.5s. I can easily fit 32s or trim a bit for 33s but im concerned how this will affect my speed over the 150 foot slick timed runs as well as my ability to cruise easily at 65 for the hour and a half trip to work. I have rustys air tube kit, a tb spacer, and flowmaster catback exhaust, with a 62mm TB soon on the way. My last run I was slowed by needing just .5" to 1" more clearanceBut would bigger tires negate the benefit of the added clearance?. Any suggestions?
 
Go with bigger tires and lower gears.
 
Any suggestions on what gears would match 33s good for this type of application? Searching brings up mostly posts about crawling gearing with 33s, not mud run type speeds. Or would they really even be needed on 32s?
 
Lower gears give you better acceleration... I would say 4.10 or 4.56 would fit the bill for manual or auto tranny respectively.
 
The only problem ive noticed with lower gears is most vehicles built with low gears and much larger(like 37s etc) tires built for deeper runs than the one i compete in accelerate to fast and are not attaining much of a top speed, which has given me an edge over the big trucks in the past. Plus would I still be able to hit 80+ on the highway, or loose much drivability day to day? Those are my main concerns with going with lower gears... What would be a good choice for a prerunner type setup? I think this might be somewhat of a similar need to mine at this point.
 
Last edited:
The ratios I recommended will take you close to stock equivalent gearing for the size tires you mentioned. You haven't noticed that your rig accelerates more slowly than when it had stock tires on it? Over a 150' course, limiting top speed will not be an issue. You won't lose highway cruising ability either. Crunch the numbers, there are links to all sorts of tire size & axle gearing calculators if you use the search feature.
 
Last edited:
I've got 33's and recently regeared to 4.56s (I have a 5-spd). Highway driving is fine, and off the line accel is great. I haven't come across a situation where I wished I had a 6th gear to increase top speed.
 
at least 4.56 if not 4.88, I have 33's and stock gears and It will do 65 in 2nd turning like 3500 rpm's. if you only do 65 on the freeway I would go 4.88. Over 150' I do not see how you would even need to get out of 2nd. the power band is in the low 4's anyway. I have a 4.2 stroker and the power band is around 4600, and after 4000 she is a monster.
 
How about gas mileage? I know it wouldnt improve any but I need it to at least stay around 18 mpg give or take. And would it hurt anything to run 4.56 gears on 31s? I might switch back and forth between the two depending on events.
 
98XJSport said:
How about gas mileage? I know it wouldnt improve any but I need it to at least stay around 18 mpg give or take. And would it hurt anything to run 4.56 gears on 31s? I might switch back and forth between the two depending on events.
Click to expand...
gas mileage would improve with 33's and 4.56 over the 33's and 3.55's

31's with 4.56 will seem torquey, but shouldnt be a problem... free-way driving might turn high RPM, but for the events, that shouldnt be a problem - just put your 32's or 33's on after the event...
 
Basically Im just concerned about being able to cruise at 80 withought cranking the rpms that high, Im on road 75% of the time with it. To me the lower gearing I run then at highway speeds I would need to be turning higher rpms, thus lower gas mileage. Regearing definately seems the way to go, just which gears, 4.10s or 4.56s........
 
XJ_ranger said:
gas mileage would improve with 33's and 4.56 over the 33's and 3.55's
Click to expand...

are you sure thats right ranger? not challenging that I Just thought the bigger tires (33's) with the 3.07's-3.55's would give you better gas mileage as apposed to a Jeep with 4.88's on 33's which your gas mileage would suck but your pulling and power/acceleration would be awesome...

I Just curious cause I seen this on a chart, and I'v never had better gas mileage on my XJ then now with 33x12.50x15 BFG A/T's and 3.55's, I think now Im geting great gas mileage compared to these gas prices at 2.40 a gal. but I could be wrong I dunno?
 
Have you tried to calculate your effective gear ratio and use that to see what gears you need? Effective gear ratio= (old tire diam./ new tire diam.)x current gear ratio.

For example, I originally had 27.4" tires and currently have 235/75/15 tires (which equals 29"), but the true diameter is 28.5."
EGR=(27.4/28.5) x 3.55 Now my EGR is 3.41.

If you already knew all this, I'm sorry, I wasn't trying to insult you, just thought it might help.
 
This should be helpful:

http://www.drivetraindirect.com/t_gear_calculator.htm

Keep in mind that the chart on the bottom of the link page is for a 1:1 tranny gear or no overdrive. With overdrive you will be running 200 or so RPMs less due to the less than 1:1 overdrive gearing. I ran about 2,500 RPMs on my AW4 tranny with 37" MTRs and 5.43 gears in my Ford 9" at 70 MPH and about 2,900 RPMs or so at 80 MPH. I love the power I have now that I'm within a nice powerband of 2.5-3K RPMS on the highway. I'd suggest, like many here...4.88's with anything but the 8.25 where you can only get 4.56 gears for.

Troy
 
Last edited:
That formula was exactly what I needed, it suggests 4.14 gears, so obviously that suggests 4.10s which i was kinda leaning towards anyways. I found a calculator which backed that formula as well as suggested 4.56s for a low gearing. With this being a DD and mud runner not a crawler I think that looks like a good route to take. Any suggestions on a good place to get gears?

Edit: That calculator gives me 3714 @ 80mph on 4.56s, whereas 3340 on 4.10s. Not sure if this is a good indication or not, this is where I get confused with gearing...

Translation: Most here suggest 4.56 Gears (I have 8.25) but most do way more offroad/rock crawling than I plan on doing so would not really benefit from low gears. I just need stock feeling gears maybe a little more of a lower powerband but not really low.
 
Last edited:
For 33's, run 4.56's, for 31's or 32's go with 4.10's. I think most folks are recommending 4.56's so that when you DO eventually go to 33's (as most folks end up doing), you'll already be set. Also, most of us are probably running heavy rigs (ie: bumpers, winches, skids, etc), so the extra power from the lower gears is helpful on the highway.

As far as gearing vs mileage goes, lower rpm's = less power, so there's less power pushing your taller, heavier rig down the road or through the mud. When I was running 33's and stock 3.55's, my mileage was in the crapper (11-12mpg) and I couldn't go over 65mph in 4th/OD because the rpms simply weren't high enough to move the vehicle any faster. If you're concerned about mileage, you shouldn't be cruising at 80mph anyway. Try to stick to around 65-70mph. My old '01 XJ got the best mileage cruising at 65mph on the highway. Faster than that and it would start going down. The 4.0L runs most efficiently around 2700'ish rpms from what I've read. When I finally went to 4.56's, I saw an immediate increase of around 4mpg if I'm remembering right, along with a much welcomed increase in power, and 4th/OD was usable on the highway again.
 
XJ99 said:
are you sure thats right ranger? not challenging that I Just thought the bigger tires (33's) with the 3.07's-3.55's would give you better gas mileage as apposed to a Jeep with 4.88's on 33's which your gas mileage would suck but your pulling and power/acceleration would be awesome...

I Just curious cause I seen this on a chart, and I'v never had better gas mileage on my XJ then now with 33x12.50x15 BFG A/T's and 3.55's, I think now Im geting great gas mileage compared to these gas prices at 2.40 a gal. but I could be wrong I dunno?
Click to expand...

a few things - your speedo is off unless you corrected it with the bigger tires...

Your torque curve in the 4.0L peaks around 2700 or so (IIRC) and so freeway RPM of about that will net the best gas mileage - the peak of the torque curve is when the engine is operating at the most efficent - regardless of what may make sense (higher RPM =NOT= more gas per mile)

also your tranny workes harder to start from a stop with the numericaly lower gears and gets hotter - Heat=lost energy...

conservation of energy states that a finite amount of energy can be attained from burning gas, the more of that that is heat, the less you are using efficiently to move the vehicle...

but then again -

"This is what i love about the internet - I dont have to know a damn thing about what i am talking about and can still get people to think i do"
 
A cherokee with the AW4 is at the stock gear ratio with 33's and 4.10's
 
PFunk242 said:
A cherokee with the AW4 is at the stock gear ratio with 33's and 4.10's
Click to expand...
HOWEVER - you're moving heavier tires and a vehicle with even less aerodynamic efficiency. 4.56 is a better match for those 33s...

2750rpm @ 75(corrected)mph... using 4.56s and 32s.
 
The calculators are fine & all, but in real world use, I have found 33 and 4.56 to be a bit on the weak side(5-speed and AW4)... 31" with 4.56 (AW4) was fun, and my current rig on 30" (AW4) will get 4.56 as soon as I get some $$$ for a locker and alloy shafts.

If anything, gas mileage improved going from 33/3.55 to 4.56, and didn't drop much or any going back to 31".
 
You must log in or register to reply here.

Similar threads

M
2001 Jeep Cherokee XJ Sport (114,854)
Replies
0
Views
975
mastershakexj87
M
RedJeep2.0
  • Sticky
Naches Trail Info
Replies
0
Views
4K
RedJeep2.0
RedJeep2.0
cheapcherokee93
Significant power upgrades: CA Edition
2 3
Replies
44
Views
4K
Hoooper
H
Bronzewyrm
naches trails
Replies
4
Views
4K
jonesinxj
jonesinxj
M
junkyard upgrades thread condensed
2
Replies
36
Views
47K
hollandbr2
H
Back
Top