just because you have a lower number gear does not mean anything about gas mileage.......lower number gears take more power to get moving......they take more throttle to keep steady speeds on the highway(unless your jeep is not like a brickwall in the aerodynamics department).....my 84 cj with a 4.2 and a mopar efi setup and 4:11 gears in a ford 9" with 33" tires got between 17-20 mpg on a about 200 mile road trip...there is no real way to tell because the speedo is so far off......my boss's 05 wrangler with 33's and a 6 speed with the stock low 3 something gears gets between 10-12 mpg max.........
proper gearing with tire size makes all the diffrence......im putting 5:13 in my new axles to match my 38's im putting on it......but thats actually a little off for my 39.5 boggers i have
First of all, I would like to say thank you. Rarely does a poster spend time putting enough information together to make a sound decision.
2K pushing that bread box down the highway at 70 is simply, in my opinion,too slow. The numbers look good, but your right on the edge of your power band. Because of that, your foot is going to be buried in the skinny pedal to maintain power. I would shoot for somewhere in the 2300 to 2500 RPM range(with 55 being in the 1800 range). This allows the engine to get up into the RPM band a bit and produce some more power. Given your pulling 14" at idle, either your timing is retarded or you are running a "healthy" cam. Engines with mid range cams need to, well, run in the mid range to be effective. Lower gears are also going to provide a more usable first gear and close the shift points allowing for better acceleration.
I was thinking something like this.....these numbers (from a buddies older than dirt engine analyzer software) following are only a illustration of what happens to the low rpms TQ output with a performance cam
stock 8.0CR 350 with a typical 196/204 oem cam with a 600cfm carb WOT:
500=85 ft/lbs TQ at the flywheel
What I think mine is doing!
14Hg at 850 rpms idle says my cam is around a 220/230 grind,,,with no other motor changes the TQ output is:
500=0 ft/lbs TQ at the flywheel
the difference doesn't look like much until you multiple it by the tranny ratio and by the rear gears ratio to find the actual total ft/lbs applied to the tires....
picking the 1500 rpms #'s for the example:
stock cam 1500rpms=279ft/lbs from the motor x 1.0 tranny 3rd gear x 3.37 rear gears=940ft-lbs of force on the tires...
stock cam in OD =279 x .70 x 3.37=only 658ft-lbs available at the tires
my cam and 3.37 gears!
220/230 cam 1500rpms =184ft-lbs x 1.0 (3rd gear) x 3.37=620ft-lbs available
220/230 cam in OD=184 x .70 x 3.37=only 434 ft-lbs available to keep the 4,000 lbs car weight moving!!!
YUP a head wind will for sure knock down the Hg!!!!!
Now with 411 gears!
220/230 cam 1500rpms =184ft-lbs x 1.0 (3rd gear) x 4.11=765ft-lbs available
220/230 cam in OD=184 x .70 x 4.11=529 ft-lbs
LOL, that is about like asking one Shetland pony to pull the Budweiser beer wagon at 60 mph!!!
he'd be huffing and puffing same as your motor is doing,,,lowHg=rich carb= the motor is "trying" as hard as it can to make enough power to maintain the speed...
I don't think my vac adv is going to do much because I am so far out of balance for TQ at the tires versus weight...
the gears are no where deep enough in OD to get my cammed motor to the rpm-TQ output needed so the motor isn't huffing and puffing at cruise....
the illustration numbers show my motor needs to be at 3000rpms=277ft/lbs to produce the same TQ that the stock cam does do at only 1500rpms=279ft/lbs of force which the 4.11 gears swap will do....
rather than swap 2 sets of gears maybe I should change the cam.
a "RV" grind is designed to help make the most/best TQ possible "at" normal driving rpms range with just normal pedal...
they are excellent for quickest 0-60mph WOT which is what I do want on the streets for fun....
The jeep doesn't like 55mph because the carb is teetering on the Hg/TQ/rpms point where the springs on the rods do move them up to the richer position....
(at cruise they are normally down because the Hg is higher than the spring rating)
richer rods and/or a springs change would fix that...
BUT only 8Hg at 55 is telling you don't use OD,,,,I may be "lugging" the motor
park and in drive values seem to be normal/typical for a roughly 220/230 cam and stock 1800rpms? stall convertor....
Anyhow I guess this is where the I-6 is actually better than the V-8. However, I like the V-8, the way it sounds and the power it does have when you get the RPMs up higher. Maybe not a mud bogger or a rock crawler but I love it on the street and is why I got it! Just trying ti figure it out and make it better.
Chevy 350, Mild Cam. (325hp)
Summit (600 CFM) Carb.
1987 TH700R4 "K" Case
32x11.5x15 Cooper ATs
3" Body Lift
Black Magic 175 Cooling Fan