Wow, how's cruising at like 65 with those 4.10's ? Lol... When I had my other set of wheels on with 29" tires, I had it up to like 15 or 16 in town.

Back to the OP: Don't get big tires lol. I'd recommend getting a washable drop in filter. Also try and open up the exhaust (muffler of your choice, not much is available to make the 3.9 sound good). Take off your mechanical fan too, and put in an electric fan. I put one in out of a taurus. All of those together should gain you a few mpg's.

 

its got a wood flatbed/dumpbed that only weighs 120 lbs. stock bed wieghs 320 (in that ball park)

but i understand what yall sayin.. i plan on doin a lot of mods, and a few of em are gonna suck the mpg, others are gonna gain, so hopefully when im done, i have a nice even balance and only see a difference of maybe 1-3 mpg

 

As long as you get it to balance, I think you'll be good.

with the shortie tires and 4.10's, it cruises at 65mph around 2200 and 70-75 at 2500.

The 42re's OD is a 0.69:1 so it makes 3.55's be 2.45's and 4.10's into 2.83's.

I've done 4.10's, SCT, roller rockers, clutch fan delete/Mk8 fan mod, and I'm putting an MSD and methanol injection on it, and gonna try to play around with some smaller tires to get in the powerband faster at the track. Need to fix some vacuum leaks, under 1800 it has no power, but it screams from 2K to the 5000 shift.

 

3.55's would be the *** end ratio right? if thats the case what would the OD on the 42re's make it with a 3.92 and is there a regear kit for the 8.25 corp rear end?

 

I have not done any mod's to it its all stock i am going to be doing a tune up on it soon and putting in a short-ram intake on it to help gas mileage but if 15.5/16 is good then i want great lol

 

yea, and .69 over a 3.92 is 2.70. I always convert the rear to a rear/OD so that it's comparable to yesteryear's data, IE, yesteryear's 727 + 4.10's isn't the same as a modern 46RE + 4.10's because the 46RE has the same .69 OD that makes the 4.10's into 2.83's. And that why you can't tow in OD with a modern truck, gears are simply too tall! A 727 and 4.10's has the same cruising RPM in top gear [3rd, 1:1] as a 46RE with 5.13's in top gear [4th, 0.69:1].

Mine is a 8.25, it's the same that cherokee's and some liberty's use more or less (They also use the 42RE trans). I think the max you can fit is 4.56's on the 8.25 ring but the pinion's gotta be really weak...at that point I'd swap in a 9.25. For all intensive purposes, if it fits a cherokee, it probably fits a dakota.

Blue is stock 3.55's and 29" tire
Red is 4.10's with 29" tire
Yellow is 4.10's and 23.5" racing slick

power-39-1.gif

Screenshot2012-12-11at92101PM.png

Assuming the driveline is 83.3% efficient [15% loss is considered the average, I compared chassis dyno and oem ratings to get the 83.3 number] and neglecting rolling resistance due to traction compound and resistance of inertia,

My best sixty foot is a 2.606 with the 29" tires and 4.10's [7.503/330 @ 41.80, 12.123/660 @ 55.56 womping] and I have a backup of 2.610/60, 7.282/330 @ 48.95, 11.132/660 @ 62.91. Let's assume my truck weighs 4200 pounds.

Force = Mass * Acceleration

Mass = 4200 pounds / 32.168 = 130.56 slugs

Acceleration = 60 feet in 2.606 seconds = 8.835 feet per second squared

Force = 4200 pound-mass * (8.835 / 32.168) G = 1,153.53 pound-force

Torque at hub = Force x Distance [radius]

Torque at hub = 1,153.5 pound-force * 1.208 feet = 1,393.82 pound-force-feet

Torque at driveshaft = Torque at Hub / rear gear

Torque at driveshaft = 1,393.82 pound-force-feet / 4.10 = 339.956 pound-force-feet

Torque after driveline losses = Torque at driveshaft / 0.85 [efficiency conversion]

Torque after driveline losses = 339.956 pound-force-feet / 0.85 = 399.948 pound-force-feet

Torque at input shaft = Torque after driveline losses / 2.74 [first gear ratio]

Torque at input shaft = 399.948 pound-force-feet / 2.74 = 145.966 pound-force-feet

OK, so mathematically, not accounting for external losses, my truck produces 146 pound-feet of torque with 29" tires and 4.10 gears.

Now let's reverse all of that and calculate with a 23.5" racing slick and if I remove the 66 pound spare tire and 12 pounds of weight per corner from the smaller tires:

145.966 pound force feet * 2.74 = 399.948 * .85 * 4.10 = 1,393.818 [this shouldn't change since the tire radius doesn't effect it] = Torque at hub

1,393.82 pound-force-feet / 0.98 feet = 1,423.475 pound-force

Force = 1,423.475 pound-force = 4086 pound-mass * (X / 32.168) G

1,423.475 pound-force / 4086 pound-mass = 0.3438 G

0.3438 G = (X / 32.168) f/s^2

0.3438 * 32.168 = 11.206 feet per second squared [remember, this was 8.835 originally]

Acceleration = 11.206 feet per second squared = 60 feet in X seconds

sixty foot time = sqrt(60/11.206) = 2.314 seconds

per this calculator -> http://www.wallaceracing.com/etcalc.php

actual: 2.606, 7.503, 12.123 @ 55.56 (womping)
actual: 2.610, 7.282, 11.132 @ 62.91
calc'd: 2.606, 7.53, 11.83 @ 57.46
calc'd: 2.314, 6.68, 10.50 @ 64.71

If I can beat the calculated by 0.248 at the 330 [3.4% error] and 0.698/5.45 at the 660 [6.27% and 8.66% error respectfully] and those offsets hold true,

6.68 - .248 = 6.432 sec
10.50 - .698 = 9.802 sec
64.71 + 5.45 = 70.16 mph

6.68 * 0.966 = 6.453 sec
10.50 * 0.9373 = 9.84 sec
64.71 * 1.0866 = 70.31 mph

So basically, I SHOULD pick up 1.3 seconds and 7.25 mph in the 1/8 mile from the smaller tire

Let's complicate things a little. Cutting weight at the tires is a roughly 2.0:1 ratio, let's play conservative at 1.75:1. By cutting 12 pounds per corner worth of lighter tire, or 48 pounds total, that could be mathematically recalculated as a 84 pound technical deduction (combined with the 66 pound spare, 150 pounds total, 36 pound effective difference from before)

Force = 1,423.475 pound-force = 4050 pound-mass * (X / 32.168) G

1,423.475 pound-force / 4050 pound-mass = 0.3515 G

0.3515 G = (X / 32.168) f/s^2

0.3515 * 32.168 = 11.306 feet per second squared [remember, this was 8.835 originally]

Acceleration = 11.306 feet per second squared = 60 feet in X seconds

sixty foot time = sqrt(60/11.306) = 2.303 seconds

So that component [36 pound difference] is only worth 1/100 of a second in the sixty.

2.303, 6.65, 10.45 @ 65.02 -> 2.303, 6.424, 9.795 @ 70.65

negligible at best.

 

holy sh*t. thats math... haha i got dizzy lookin at it.. i always hated math..

 

Needless to say I'm in school to be an engineer cars dont go fast on their own. I editted in more math btw :P

So basically I should gain 1.3 seconds and 7.25 mph from going to a smaller tire, plus other gains I didn't account for [gets to peak torque faster, less rolling resistance, less inertial resistance just to start with]. I'm hoping adding methanol fuel injection, MSD ignition, and bumping timing from 26 degrees to 34 will get me knocking on the door of the 8's.

Something to note: that dyno graph is diagramed wrong. While numerically correct, the curves do not cross at 3200, they cross at 5252, just like every other engine in existence.

Back on topic: Cherish the mileage you get now because losing 3 is probably a good estimate unless you've got some terrible restrictions somewhere that you find [which I doubt]

edit- here's the same graph I posted from allpar, but corrected for how we're used to seeing them.

power-39-1.gif

Screenshot2012-12-11at113145PM.png

 

2200 ain't bad. Now you've got me thinking about 4.10's lol. I think its awesome all you're doing, I'm sure most guys would say just throw in a 318 or 360 but I think it's cool to see what you can get out of this 239. I'm going to leave mine alone for awhile, I've got another truck project that needs a lot of work.

 

Use it till I lose it...then I'll stick a 408 in it and "forget" to change the badging. haha, hahahaha