Performance Question

[Blue88Comanche]

I currently have a renix I-6, AX-15, NP-242, 3.55 gears, 31x10.5r15 tires.

 

I drove it on the interstate for the first time in a while, I was having issues getting to 70, and I could not get past 70 mph. My RPM was about 2300-2500. The jeep was in full time 4wd.

 

On my way home from work I switched it to 2wd. I was able to get to 75 mph at 2500 rpm. There was also a constant vibe at 60mph and up.

 

According to http://www.grimmjeeper.com/gears.html

My engine rpm should be about

 

Also while driving at 45 mph my rpm is 1500 in both full time and 2wd.

 

My speedometer is correct for 3.55 gears with 31 inch tires.

 

I don't know much about what the running rpm should be but having conflicting data with that chart and not being able to pass 70 mph has me slightly concerned

[cruiser54]

What gear were you in, 5th? Did you try 4th?

[Blue88Comanche]

I was in 5th @ 45, 70, 75 however When on the interstate I shifted out of 4th @ 2500 - 2800 rpm

[cruiser54]

Rev it up a bit, man.

[Eagle]

I currently have a renix I-6, AX-15, NP-242, 3.55 gears, 31x10.5r15 tires.

 

I drove it on the interstate for the first time in a while, I was having issues getting to 70, and I could not get past 70 mph. My RPM was about 2300-2500. The jeep was in full time 4wd.

 

On my way home from work I switched it to 2wd. I was able to get to 75 mph at 2500 rpm. There was also a constant vibe at 60mph and up.

 

My speedometer is correct for 3.55 gears with 31 inch tires.

 

I don't know much about what the running rpm should be but having conflicting data with that chart and not being able to pass 70 mph has me slightly concerned

 

Your speedometer is not even close to being correct.

 

With 3.55 gears and 31x10.50 tires, 70 MPH should be 2100 RPM, not 2500 RPM. 45 MPH should be 1800 RPM in 4th gear, 1350 RPM in 5th gear. 2500 RPM in 5th gear would be driving you at 83 MPH.

[mvusse]

Plus with 3.55 gears and 31" tires an inline 6 should easily be good for 90-95 mph. I think you might have other issues. Check you speed with a gps.

 

The vibe at 60 could be an out of balance wheel, or it could be a driveshaft problem (bent shaft, worn slip joint on front shaft if you don't have a disconnect front axle, or a bad u joint somewhere).

 

Eagle, one small detail you're forgetting: most 31x10.50 tires run small. Most are only about 30", some are as small as 29".

 

Going by a BF Goodrich Rugged Terrain T/A, which lists 682 revs per mile I come up with 78mph on 5th at 2500 rpm.

 

Looking at other brands for with revs per mile are available in the specs I see 684, 679, 682, 683, 683. A true 31" solid tire that does not deflect under a load should only run 647.

[Blue88Comanche]

I drove the Comanche on the interstate again today, this time with the gps

 

in 5th gear at about 2225 RPM the GPS had me going 70 MPH, the speedometer was slow by 3 mph

in 5th gear at about 2500 RPM the GPS had me going 75 MPH, the speedometer was slow by 5 mph

 

Tirerack.com states the tire height is 30.8"

 

I am currently running a 32 tooth speedometer gear, I used this chart when buying the gear

To fix the speedometer, would changing the tire pressure be enough to fix it? or should i get a different gear?

 

and the vibe id not come back while in full time 4wd?...

 

when i got home the other day i swapped the intake setup (tube and filter) to how i had it (Cone filter attached to stock intake tube), it seemed to like it better and i was able to get to 80 this time without much problem..

[mvusse]

If your speedometer reads 70 when the GPS says 75 and you're running a 32 tooth gear, you need a 30 tooth one.

 

Vibe did not come back in full time 4wd, but.... was it still there in 2wd?