hi i have a 85 z28 i recently swapped a 406 into and i was wondering about gear ratios when im doing 70mph my rpms are over 2500 its suppose to have 3:23's in it i thought maybe my tach was off but it seems to be reading correctly at idle anyway, im about to go for a drive ill take a picture i guess.


41 : 15 = 2.73 (RPO GU2)
41 : 14 = 2.93 (RPO GU3)
40 : 13 = 3.08 (RPO GU4)
42 : 13 = 3.23 (RPO GU5) / Approx. RPM’s at 70 mph--2133
41 : 12 = 3.42 (RPO GU6) / Approx. RPM’s at 70 mph--2255
41 : 11 = 3.73 (RPO GT4) / Approx. RPM’s at 70 mph--2463

wondering if this is accurate? found it on here.

 

so my iphone is a ***** and wont let me send the pic but at 70 mph the tach is just a hair below 2500 at 70 mph this is the rpm at 60 mph

 

Sounds like it's time to change out the rear diff oil for new and do some counting.

And then pull speedo gear for some more counting of both gears.

Guaranteed better results than the popular post on the internet method.

 

check the rpm in third gear, with your foot off the gas to eliminate torque converter slippage. this will drop your cruise rpm a few hundred rpm and more accurately represent what is really happening. this will allow you to get a ballpark guesstimate on which gears you have without pulling the cover on the differential.

is your speedo correct? pull up a GPS based speedometer app on your phone and check it... you can also do it the old fashioned way and check the odometer against mile markers on the highway- find a straight stretch of road and check it against 5 or more miles to see how much the odometer gains or loses over distance.

 

The tach in these cars is ALMOST ALWAYS farkled.

Usually (in fact almost always) it will read high. Sometimes WAY high. Sometimes even HIGHER than WAY high. I have one that reads 3 times actual RPMs.

Like jmd said, if ya wanna know what gears ya got, ignore the tach, ignore the RPO "codes", and step away from the keyboard and monitor. Go change your rear end fluid instead. Then you'll have killed 2 birds with 1 stone: you'll (a) have taken care of some badly overdue "deferred maintenance", thereby increasing the likelihood of being able to use your car without sudden at least one particular source of catastrophic $$$expensive$$$ failure; and (b) you'll know with ABSOULTE 100% IRONCLAD GURANTEED CERTAINTY what gears you have.

Then fix your tach. (because it's WRONG just like all the rest)