2300 -2700 RPM.

It should handle up to 3000 RPM without problem, but that's screaming too fast for my tastes.

Matt

 

At 55-58 mph I'm turning around 2600-2700 RPM. and that is a bit high for my liking. I try and avoid the interstate as much as possible for that reason. Thanks for the reply

 

A slightly larger tire with your 4.10 gears would get you in a more comfortable RPM range on the interstate.

Play around with this gear ratio calculator to see what I mean.

Matt

 

Thanks for the calculator, Interesting to watch the RPM numbers fall with increased tire size but that's really not an option at this point. I only have less than a 1000 miles on them. I guess I assumed they would be a "true" 32 " tire. Thanks for your help

 

I have the same combo as you except my 32" measures 31.5" and have frequently gone 60-65 on the highways here. Ya he screams along, but the wind noise is louder! I think I am around 2600 @ 60mph and 2800@65. I have done 70-75 but it really did sound wound up. Supposedly it'll rev higher, but not sure how it'll do sustained.

Prior to my build I had the stock 3.31 and 31" tires. Those gears were ok for road cruising and good for highway cruising, but terrible for anything else.

 

Yea, mine had the awesome 2.73 originally and was quite the clutch smoking turd. I rebuilt the jeep to mainly backroad and drive in the country and it does well. I venture on the interstate from time to time and I just hate to wind her up that tight and getting blown away by everything including a Prius.

 

86 CJ7, 4.2L, 4.10 R&P, T176 with 32x11.50x15. (tires measure 30.5)

What is a safe or reasonable RPM at constant highway speeds for a tired ole' 4.2 ??

That depends on what you consider "Highway speeds". Lots of folks on here say 55 is as fast as they ever need to go, but others will say 75-80 is where they need to be. There can be a big difference in RPM there.

 

86 cj auto 33" tires 4.10 gears 65mph at 2900rpm

85 cj 177 31" tires 2.72 gears (I think) 65mph at 2300rpm

 

OP-What do you call highway speed?
Have you confirmed your tach is 'rite'? Have you confirmed your speed is 'rite'?
I run 4:88 R&P with a T-5 and 33" BFG, can run all day at 65-70mph. proven via GPS and a certified tach, set for 6 cyl.
BTW-You ain't run'n a MSD ign are you? They can muck with tachs.
LG

 

Interstate speeds (65-70) is what I was referring to. No I haven't confirmed anything other than changing my speedo gear to match my set up. My speedo kinda dances around, so it is not accurate so to speak, but my tach says 2700 rpm, speedo shows between 55-60 and I'm getting blown off the road. I was just asking a safe rpm for an ole' worn out 4.2. At sustained speeds.

 

37" tires, t176 and will comfortably cruise 70 mph at 3k rpm. Just wear ear plugs!

 

I don't have my '83 CJ-7 any longer (sold it to my sister and her husband). My typical cruising rpm was 2,600-2,700 for my rig with fuel injected 258, T-19 Ford transmission, Dana 300 transfer case, 4.56 gears and 35" BFG MT tires. I cannot recall the mph at that rpm as my focus was more on engine performance and efficiency rather than maintaining a particular speed.

IMO, it wouldn't matter in a CJ what rpms the engine was turning at anything over 70 mph because I never liked driving a short wheelbase jeep at those speeds.

 

My '85 CJ7 hits 2100/2200 comfortably with 31 x 10.5 x 15's and 3:31 gears. Top speed without panic is 75 mph (2300) on a good day. Average speed is 65 mph.