Best rear end gear for acceleration

[Michaelarchangelo]

All this talking about gears and motors has got me thinking. What would be the prime gearing for a stock motor, or stroker, for best acceleration?

 

[BrettM]

depends on transmission, tire size and a few other things. Are you simply concerned with 0-60 or are you looking for max acceleration beyond 60mph also (1/4 mile?)?

 

[woody]

I am going to be running P235/75-15s on the street and 30x9.50 off road for the next year or so, and will be running 4.56:1 and an Aussie Locker.

Obviously your driving conditions (+ tire size, axle types, transmission type etc) will guide you into a range of choices. If I lived in the pure flatlands, I'd consider 4.10, and if I lived in the mountains, 4.88 would be my only choice... assuming using a HP30f/44r. My stock 4.0 motor is pretty old though, and likes to rev some to come alive... a freshly built & torquey stroker should be a lot more adaptable with less than optimal gear ratios.

Keep in mind that the OEM HP30 tops out at 4.88:1 and a 8.25 tops out at 4.56:1. The OEM D35 axleshafts make nice horseshoe-throwing pins and the housing makes a (nearly) vandal-proof small mailbox... Any money spent on a D35 should be towards those uses IMO.



Cliff notes: The lowest (highest #) gear that will fit in your axle will give the best acceleration. These low-ratio gears also limit the top-speed in any particular gear. Do the Math!

 

[SCW]

I am going to be running P235/75-15s on the street and 30x9.50 off road for the next year or so, and will be running 4.56:1 and an Aussie Locker.

Holy crap, 4.56 and 235/75R15 seems pretty low. That would be sweet!

 

[woody]

I expect it will be fun. This isn't my first rodeo on low gears and smallish tires, so I don't see any bad surprises in the forecast.

 

[Michaelarchangelo]

Well living in Florida means that I never see a mountian (except Mt Dora, which I never understood). I want max acceleration


Oh, and it is an auto with the 4.0

 

[BrettM]

max acceleration up to what speed?

 

[Michaelarchangelo]

Freeway speeds, not more than 100

 

[1 tuff XJ]

4.56 is good for 33 or 35 inch tires if your only running 30 or 31's 3.73 is what you would want. i'm currently running 3.55 (stock) with a slightly modded 4.0 and auto tranny on 33's i can hit 35 to 40mph before i shift to second and around 60, 65mph when i shift into 3rd, man if i just had the torque to spin the wheels when i got to 4th! the XJ's have got plenty of balls in my opinion, i mean you don't have to go to the lowest gear. that give you the best acceleration but it means your tranny has to shift more often, you limit your top speed, your engine is reving higher causeing things to wear faster and use more gas. so use a chart and decide what you need, they give you a line to follow. theres a tan line for optimal highway and fuel consumption and a green line for more power and offroad use.

 

[j99xj]

I'd say go as low as your axles will go if all you want is pure acceleration. That puts you around 4.56s. Your acceleration will increase but your gas mileage will go down accordingly.

Going this low will kick your rpms up a lot. So if 4.56s are your intentions, I would swap in a hot cam with a highly ported cylinder head along with all the bolt on goodies to build as much power in the mid to top rpm range. The hot cam will reduce low speed torque, but since the rear end ratio is lower you will still have far better acceleration than stock.

Strokers can get away with taller gears because they make more torque at the flywheel. But the gearing must be matched to the powerband in order to have good performance. You don't want 4.56 gears in a turbo diesel, and you certainly don't want 2.73 gears in a race car that doesn't develope good torque until 4500 rpms.

 

[tealcherokee]

4.56 is good for 33 or 35 inch tires if your only running 30 or 31's 3.73 is what you would want. i'm currently running 3.55 (stock) with a slightly modded 4.0 and auto tranny on 33's i can hit 35 to 40mph before i shift to second and around 60, 65mph when i shift into 3rd, man if i just had the torque to spin the wheels when i got to 4th! the XJ's have got plenty of balls in my opinion, i mean you don't have to go to the lowest gear. that give you the best acceleration but it means your tranny has to shift more often, you limit your top speed, your engine is reving higher causeing things to wear faster and use more gas. so use a chart and decide what you need, they give you a line to follow. theres a tan line for optimal highway and fuel consumption and a green line for more power and offroad use.

27" tires, 3.55's and a stroker is almost undriveable in 2wd, just spins them

 

[MogifiedXJ]

27" tires, 3.55's and a stroker is almost undriveable in 2wd, just spins them

Drop them down to 4.56's and keep that 242 of yours in full time...run two posi tracks or a limited slip in the front and you shouldn't spin then.

 

[Kittrell]

4.56 is good for 33 or 35 inch tires if your only running 30 or 31's 3.73 is what you would want. i'm currently running 3.55 (stock) with a slightly modded 4.0 and auto tranny on 33's i can hit 35 to 40mph before i shift to second and around 60, 65mph when i shift into 3rd, man if i just had the torque to spin the wheels when i got to 4th! the XJ's have got plenty of balls in my opinion, i mean you don't have to go to the lowest gear. that give you the best acceleration but it means your tranny has to shift more often, you limit your top speed, your engine is reving higher causeing things to wear faster and use more gas. so use a chart and decide what you need, they give you a line to follow. theres a tan line for optimal highway and fuel consumption and a green line for more power and offroad use.

I don't agree with much of this. I live in Denver, and 33's with 4.88s is perfect. 35's and 4.56s would be a joke. If I had 31's, I'd run 4.56s.

 

[Dr. Dyno]

All this talking about gears and motors has got me thinking. What would be the prime gearing for a stock motor, or stroker, for best acceleration?

With stock 27.7" tires, optimum gearing would be 3.55's for a stock 4.0 with either the AX15 or the AW4.
For a stroker, optimum gearing would be 3.07's with either the AX15 or the AW4 on stock 27.7" tires.
If you use bigger tires or add a suspension lift, you'll need numerically higher axle gears.

 

[GATMOG]

i run 3.07's and 31's talk about joke..