indeed it depends on the rear diff,
ok so with a code GU2 this is a 2.73 ratio diff so at 70 mph you will be around 1600-1700 rpm ish
if you have a GU5 this means you have the optional extra of a 3.23 gear ratio, i.e. better acceleration etc which should see you at about an 18% increase in rpms upto about 2000 rpm at 70 mph.I have installed the GU5 in mine so will check on my next drive this weekend. If you are at about 2400 rpm the prev owner might have installed an even higher gear ratio, so accel is better but revs more in OD. If indeed you have a GU2 or even a GU5 then 2400-2500 at 70 mph does seem a little high to me. It would seem like the Torque convertor might not be locking up. I dont meean to scare you by the way, its just a suggestion. When you drive the car you should have the 3 gear changes, 1-2 then 2-3 and then 3-4. But when in 4th at about 40-45 mph the revs should drop further, like another small gear change, as the torque convertor clutch locks up for better economy.
second answer, re service of auto box, it's recommended that it is done maybe about every 70K miles. However some dont do it at all and some say everything is long life. You have to drop the pan (and get covered in ATF) then pull the filter and renew it. You must carefully remove the metal/plasic filter seal and put in a new one without scratching the bore the seeal sits into. Then refil with ATF. I used some triple QX recently from euro car parts as long as it meets the dexron 3 requirements it should be fine. Mine has been fine and it gets a LOT of abuse sometimes (at strip etc). Some people have reported that servicing a 100K + gearbox if its not being done before is BAD. They say as the deposits are washed out that were previously stuck to the clutch plates in the box that it will start to slip. Some people have said they have had no problems? Just the way it goes.
let me know if you need anymore help, or send me a PM
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'99 Z28 A4, 253 LS2 heads, 228R TSP cam, Patriol Dual Springs, Double roller timing chain, LS6 ported oil pump, OBX LTs, 3" True Duals, LS6 intake, 42lb/hr injectors, SLP lid + bellow, EFIlive COS 3 Speed Denisty Tune (self tuned), 3200 Circle D stall, CAI mod, KYB AGXs, Eibach Pro Front Springs, UMI panhard, LCA + Brackets, Strut Brace, Custom Subframe connectors, WS6 axle with 3.42s, 18" American Racing Rebels, US mirrors, 93 tail lights, 3.5 inch custom cowl hood. Mickey T's Street DRs on Vette-ZR1 wheels for the strip !
Best ET: 12.0 @ 116 mph (on a 0-60ft of 1.8 s)
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