258 Top Rpm

Hi everyone. I just read H Tate’s “Top Speed” post June 16th and was surprised at the high speeds folks were describing. I wish!

Im driving a 72 with a 258, 3.73 gears between 33s and a 4 speed manual. I’m spinning 3,800 at 55 mph in fourth, which I thought was top of the rpm range for the 258. How are you guys getting such high speeds. What rpm’s are you turning at 70mph?

I’m thinking either my tac is hooked up wrong, I’ve got a low geared trans or my diffs are lower than I thought. I am going to confirm the diff ratio via the wheel to driveline spin test. Any other ideas? Thanks.

 

Not sure of this holds true but it’s been my observation that a 72-76 intermediate cj with the factory NR T18 comes with a 4.27 differential ratio.

 

That just doesn't add up even with 4.27s 3800 Rpm would be scootin.
3800 at 55mph is like 5.38s and 28" tires.
Get a cell phone app that reads mph and try again.

 

What everyone else says.

• 33" tires and 55mph in high gear should be at 2100 rpm.
• Even 5.89 gears puts you at 63mph at 3800rpm.
• But 33" tires, transmission in high, 3.73 gears, and transfer case in low range puts you at 50 mph. Are you sure you're in high range?

 

I wish there was some follow up on this. I'm really curious what's going on.

 

Thank you for your input. I know it’s not in low range and I’ve done the cell phone thing so I know my odometer is spot on. I don’t know about the Tac but my ear says it correct and I wouldn’t know how to re set it. I am going to do some further investigation when I get some shop time and will let you know.

 

Ones I've seen just have a small selector switch on the back.

 

Your specs almost match up for if you were in low range.

 

yeah, even with 4.10 gears and 33" tires I'm going somewhere around 55-60mph at around 2200-2400rpm with no overdrive. My speedometer is a GPS boat speedometer, so tire size and such doesn't affect it.

 

Well I’ve confirmed I have 3.73 gears so I’m going to look into my linkage to see if I’m not shifting out of low. I don’t see where there is any adjustability in the linkage so I don’t know how to remedy it if that’s the issue. 1st gear doesn’t seem too low but I’m comparing it to a CJ3B I used to have with 5.38 gears. I’ll drive it in 4 high to see if anything is different.


I did not locate a selector on the tac so I’ll have to check it against another device.

I did come across a curiosity when I spin tested for gear ratio. I was curious about whether my rear diff is open or L.S. When I spun one rear wheel, either side, the opposite did not move. I held the drive line static and turned one side and the opposite turned in the same direction. The sorcery of differential function has always befuddled me. I need to pull the cover when I have time, as I detected a lot of play.

This speed/RPM thing is still a mystery to me. Keep the ideas coming. I appreciate all your input.

 

The tachs have seen, you have to take the cover or pod off to select the number of cylinders. Does it sound like it is wound up
at 3800 @ 55 mph or does it feel like you have something left?

 

FinoCJ, PeteL and vtxtasy. You guys called it. I took the tac apart and found a tiny switch inside set to 4. I changed it to 6. Thank you. The question now is…
What’s the max RPM this IL6 258 ci should run at?

on another topic. Any idea if this diff is open or ls based on my wheels up spin test? With wheels up neither spins forward or revers when the opposite wheel is rotated forward. With the driveline held in place, both spin forward. Thoughts?

 

The stock engine makes it's max torque at 2000rpm and max horsepower at 3500rpm. 4200rpm for max rpm
sustained and peak at 4500rpm. The rear is probably a TracLoc that still works?

 

It sounds like you have a very worn out LSD and dragging brakes.