RPM Variation ???

I've got what might be a dumb question for you all.

Since my build I've noticed that my RPMs have decreased at the same speed, over time. If that makes any sense.

4.88 gears, 8.8, auto, detroit locker, 33's

When new I was running at 2400-2500 RPMs at 65mph.
Now after a good bit of wheeling, almost a year later, I'm at about 2100- 2200 RPMs at the same speed.

My question is what might cause this type of variation? It doesn't make any sense to me. It runs the same as it always has.

Thanks.

Tire wear!

Ah, makes perfect sense. Thanks

But tire wear would make the tire diameter smaller, causing RPM (of engine) to rise.

It's something else, probably your transmission.

Check your fluid level or make sure you have the right AT fluid, (dexron compatible).

j99xj said:

But tire wear would make the tire diameter smaller, causing RPM (of engine) to rise.

It's something else, probably your transmission.

Check your fluid level or make sure you have the right AT fluid, (dexron compatible).

Click to expand...

Also, a slipping AT would cause the RPM to go up not down. You have a very unusal occourance,... I sure hope someone here could explain it.

A bad RPM guage is the only thing I can think of. That too is unusal in an XJ.

Honestly, What did you change and is not telling us????

Someone stole your gears!!!

Before you were paying attention to RPM's before the torque converter locked.

Now you're noticing it after.

cal said:

Before you were paying attention to RPM's before the torque converter locked.

Now you're noticing it after.

Click to expand...

I don't know man, after my TC locks up I'm still turning 2500 rpms at 65 exactly.

4.88s and 33s as well.

MoparManiac said:

I don't know man, after my TC locks up I'm still turning 2500 rpms at 65 exactly.

4.88s and 33s as well.

Click to expand...

Whats wrong with my rig then? I'm on 32 bfg at's with 4.56's and I turn 2500 at 65 mph. I'm pretty sure I can feel it shift and lock the tc.

DeftwillP said:

Whats wrong with my rig then? I'm on 32 bfg at's with 4.56's and I turn 2500 at 65 mph. I'm pretty sure I can feel it shift and lock the tc.

Click to expand...

I couldn't help ya without some information?

This is how I came up with what mine should have been, and it's been accurate.

2500/0.75/4.88 = x

x*2*3.14*16.5 = y

y/12/5280*60 = z mph

I get 65mph for my setup and that's dead on for what I've experienced.

j99xj said:

But tire wear would make the tire diameter smaller, causing RPM (of engine) to rise.

It's something else, probably your transmission.

Check your fluid level or make sure you have the right AT fluid, (dexron compatible).

Click to expand...

Yeah, I was thinking backwards when I read that.

You know what, my numbers are off a little bit above now that I've been thinking about it. It was 2500-2600 RPMs at 65 and 2100-2200 now.

I thought that it might be extra weight I've been carrying around, not that that would make any sense either, but it's not.

Trany fluid is valvoline dex III-V, IIRC. I know it's what I read out of my chiltons.
Maybe the torque converter is gripping more than it should? I have no clue. It's not really that big of a deal, I'm getting better mpg than before. I just don't want it to be a sign of failure to come.

The only thing I've done differently was switch to full synthetic motor oil and gear lube. I changed the trany fluid and filter about 4k miles ago. I've also been driving around in 3rd gear around town because 4th (OD) it wants to bog down and down shift too much.

cal said:

Before you were paying attention to RPM's before the torque converter locked.

Now you're noticing it after.

Click to expand...

That could make sense but I've been paying attention to it now, all the time, and I can't get it to even reach 2500rpm without going a lot faster than 65.

I think I'm going to try and calibrate my exact speed. Maybe the speedo is acting up.