RPM's @ 55mph
 

Mine is about 1800 and change. Which feels higher than my other Rovers. Maybe I'm wrong. I have stock gears. 245/75/16 tires. I'm hoping I don't have a transmission issue. For some reason it feels to me I'm not engaging 4th gear. All other gears shift so smoothly it's hard to count and drive. No whining. No slipping. No loud noises. The whole drive train is really quiet. I need a copilot to confirm gear changes. I know at 50 I get an abrupt drop of about 200 RPM presumably from the lock up converter. Will the converter lock 3rd gear?

I've done two drain and fills of the fluid. I have to get time to change the filter. The first fluid was pretty dirty. The second about 50 miles later was pretty clean. I used Valvoline fluid both times.

Also while cruising along at hwy speeds of 50+ mph, if you let off the gas the truck chugs and lurches. Then it's fine. If I feather off the throttle it doesn't do it. It feels alot like having a manual trans and letting off the gas while it's in 4th going down hill. Wondering if this is related.

1800 Rpm's is normal. Do you have any fault code numbers?

No faults. Ultragauge does have one "pending" code for Sensor 1 Bank 1.

With your trucks age, would be a good idea to replace both of those front 02 sensors to regain your 14 to 1 air fuel ration and better power.

Good advise. It's on my list.

Sounds about right to me Shift. Mines circa 2K rpm at 60mph (aTD5)

Ok well how about the off throttle lurching? Can't possibly be normal.

There's nothing wrong with your ZF other than needing the fluid change. Your running larger than stock tires and your on target for rpm's vs. 235's which put's 55 around 2.1k. You will not hear or feel the shift changes unless you uncork the exhaust, then you hear third to lockup.

Lurching - I think your IACV is acting up, mine at times does the same thing. Your idle runs a little high sometimes? Sometimes not?

Yep. Sure does. Occasionally high then not.

That's the surge, I guarantee it.

I watched my tach at 55mph and it's about 1800 and change too so the two tire sizes aren't that much different.

As you know.... if you change out the IACV with new you'll have to have the ECU reset otherwise it will (but not always) go high all the time. Another buddy with a 97 with AEL was able to swap out the IACV without a problem so it's not a rule of thumb.