RPMs higher than they should be at highway speeds

I took my Cherokee on a short (140 mile) trip the other day and noticed that at 70 mph, the tach is reading about 2250 RPM. I have a 3.55 axle ratio and 31s, so I should be seeing closer to 2000 RPM at that speed. The transmission is an AW4 automatic and I'm accounting for MPH variation due to the tires.

The torque converter locks up normally, if a bit early at lower speeds (a common complaint, from what I've read). Since this is the case, there wouldn't be any fluid slippage, so the 2000 RPM calculation should hold true. Any possibility as to why I'm seeing higher engine speeds? I know the factory gauges can't be totally trusted, but 250 RPM is a lot to be off by.

Other operation seems fine... no noticeable slipping around town, it goes fine when given gas, and I replaced the filter and fluid in the pan about 4000 miles ago. The neutral safety switch checks out against the FSM diagram, too.

tire size. you need lower gears.

Larger tires would make the engine speed decrease for a given vehicle speed. This is the calculation I performed:

RPM = (MPH * Axle Ratio * OD ratio * 336) / Tire Size

It comes out to about 2020 RPM at 70 MPH with 31s.

did you put the proper speedo gear in?

BrettM said:

did you put the proper speedo gear in?

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Not yet, but I've accounted for that. The conversion factor is approximately 1.0887 (28.29" / 30.8"), so when my speedometer is reading 64 mph, I'm traveling at about 70 mph. It is at this speed I looked to see what the tach was reading.

What are you using for the overdrive gear ratio to determine actual driveshaft rotations at speed?

How about checking that speedo against mile markers, or a board cops radar? I'll bet you'll find it's almost right on with those 31's. Jeeps seem to have there speedos read a bit low for some reason. Also 2,250RPM is about right for 65mph.

It sounds like your overthinking it.

scoobyxj said:

Also 2,250RPM is about right for 65mph.

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I think 2000rpm is right for 65mph, which in his case is 70mph due to the tires.
Assuming he has a 4.0.
He didn't mention his engine.

4.0L engine, 3.55 axle ratio (checked by spinning the tires/driveshaft), 0.75 OD gear (late-model AW4), and of course 31" tires.

I was following a friend in a 2004 Cavalier during my highway trip, and I called him to see what his speedometer was reading. Mine was at 66 mph, his was at 71 mph, so there's definitely the need for my conversion factor. I've also driven past those digital radar things and noticed a variation.

Overthinking, probably. But it's nevertheless intriguing.

iroc86 said:

4.0L engine, 3.55 axle ratio (checked by spinning the tires/driveshaft), 0.75 OD gear (late-model AW4), and of course 31" tires.

I was following a friend in a 2004 Cavalier during my highway trip, and I called him to see what his speedometer was reading. Mine was at 66 mph, his was at 71 mph, so there's definitely the need for my conversion factor. I've also driven past those digital radar things and noticed a variation.

Overthinking, probably. But it's nevertheless intriguing.

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You should be showing 2000rpm and so should I. I used to, till about a year ago when my tach started indicating wrong. Like 1000rpm idle and 2200rpm at 65.
I own two more Jeeps with the 4.0 and both indicate 2000 at 65.
Check other rpm/mph combinations, your tach may be fried.

I think you're over working this but if you want to calculate it accurately then you need to measure your tire diameter. The actual rolling diameter may be twice the height from the ground to the center of the axle instead of the free space diameter.

PaulJ said:

I think you're over working this but if you want to calculate it accurately then you need to measure your tire diameter. The actual rolling diameter may be twice the height from the ground to the center of the axle instead of the free space diameter.

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My tires, Dunlop Mud Rovers, are 30.8 inches tall. This was checked against Dunlop's spec sheet for a given rim size, so I haven't looked at the real-world number -- but it's probably closer than just blindly using 31 inches.

I did a few more tests and at 50 mph, the tach is reading 1450 RPM -- right where it should be according to the math. At 65 mph, it's around 2000 RPM -- but it calculates to be about 1900 RPM. My guess, like falcon suggested, is that the tach isn't accurate. It seems that as the engine speed increases, there's more of an offset from the calculated number (~25, ~100, ~250, etc.). Maybe I'll hook up an aftermarket gauge just for the sake of interest.

If you have a GPS, use that to check your speed. Speedometers are not known to be especially accurate or linear.

Ive got the same setup and I also see about 2200 at 70 mph, but I didnt think it was enough rpm