Ok i know i have 2.77 gears according to the plate right on the diff cover. dont ask about rpo codes they have been removed at some point. its a BW diff if that helps.

there seems to be some issue with rpms according to what others seem to turn at relative freeway speeds. i run 2000 at 60mph, which some say is right and some say is wrong for 2.77 gears. i've tried the 70mph test, but i came up with 3300 rpm at 70 in 3rd, oh this is 700r4 trans. anyway that would be 3.27 but the plate clearly says 2.77.

so what exactly is a normal cruising rpm at 60/70 mph in overdrive with 2.77. i just recently changed the diff fluid so i dont feel like pulling the cover off again just to get a tooth count.

any help would be great.

 

TTT

 

This sounds about right man, I have a 2.73 rear, and I run about 1900-2000 rpms from 55-60 mph. at 70, in OD, I think I run about 2300 or so RPMS. 3rd gear at about 3300 rpms I'm running about 85-90 though. So I dunno man. Hope I was a little help.

 

I have the same rear and I run about the same RPM's but i never go up into 3300 RPM's the most i go is about 2800.

 

I have either 2.73s or 2.77s, don't remember which. And a rebuilt 700R4. I'm right at 2,000rpms when cruizing 75mph. Is something screwy? Why am I different?

 

yeah this doesnt seem to be making sense some are running faster and some slower, i dont understand it. 2.77 at 60 in OD for me is 2000 rpm but 70 in 3rd is 3300.

me and 87TA seem to be running about the same in OD but not in 3rd. i may just have to look the next time i pull the cover.

at 75 i'm doing about 2500 rpm so brian is turning way slower them me.

 

Just checked again-

60mph is 1550rpms. I can't figure out why there'd be a difference.

 

i only wish i was only turning 1550 at 60 in OD. then i would pull down better then 20mpg a tank.

is anyone else going to weigh in? seems like every post here is different even though we all have roughly the same gears, either 2.73 or 2.77. if i have to i will pull the diff cover but not for a while.

 

Yeah, the last tank I got 23mpg on my l98. It was all highway, but still. It would been more except I was runnin 75mph instead of 60.

 

I don't think that the stock tachs are overly accurate. Probably just a differance in everybodys tach.

 

its a thought but when compared to a snap on scan tool a yr ago the rpms where fairly close, at least for me. the scan tool of course was allot more accurate and should so variance do to a slight vacuum leak.

 

i have the 2.77 rear in my 88' Roc. it turns roughly 2000 rpms in od @ 70 mph with 245-50-16 tires.

 

dang i cannot touch that kind of rpm at 70. so mystical and brian seem to be running about the same. whats wrong with the rest of us? i havent run 70 lately and paid attention but it has to be over 2250 easy since i normally run 2500+ at 75mph.

tire size is an issue but it wouldnt be 500 rpm without a major size difference. my tires are off 225/60/16, so my speed is 5mph fast, since i know that i can correct for it.

 

My 700r4 at 60 mph with 26 inch tires and a 2.73 gear is just above 1500, same as brian. I will have my new tach installed this weekend and I will post if there is a diff. between the factory and the aftermarket.

 

I could be wrong, but I don't think tire size makes any difference. Sure, if you have taller tires, you will be going a little faster, but your speedo won't know it.

 

contrary to belief tire size does make a SLIGHT difference.


smaller tire - speed marked higher than actual
larger tire - speed marked faster than actual

(the same goes for the tach. except speed now reflects engine RPM)

 

When I had my 2:77 rear in I was at a little over 1500 at 60 in OD.I agree that a tire size can make a slight difference also.

 

I have 3.42 gears and I am doing 2000 rpm at 60 mph in OD (700r4).

At 70 mph in OD I am pulling about 2400 rpm.

When I had 2.73s (up to about 4 weeks ago) I pulled 2000 rpm at 70 mph in OD.

Compare yourselves to that because it is accurate.

 

i seem to be running closer to Sitting Bull, almost exactly 2000 rpm at 60 in OD, i'd have to double check 70 but 75 was bout 2500.

i guess i just am going to have to pull that cover this is killing me. the tag said 2.77 when i changed the fluid and the car is really quite stock so i highly doubt the rear was swapped.

 

i have a thought where can i check on the rear to be sure it wasnt swapped? i did a smart thing or stupid 1 and left the ID plate off when i bolted the cover back up so i have it right here.

part # 10079887
ratio 2.77
serno 2395

2nd thought is what should the tooth count be for 2.77, 3.27 and 3.42? it may help explain why my trans doesnt shift on time.

Sitting Bull did swapping gears affect your engine rpm shift point? mine is crazy high right now at WOT.

 

Strangely enough, it doesn't seem to have--at least with regard to overdrive. It always went into OD at about 40 mph with the 2.73s and it shifts there at the same mph with the 3.42s.

 

just an update i pulled the diff cover off this weekend and check the ring gear. it is definately a 2.77, it was stamped right on the gear.

i dont know why a .04 diff would be so big comparing the same mph to rpm but apparently it is. some of us run about 2000 at 60 and some run 2000 at 70+. weird why so different, even comparing 2.77 to 2.77 the rpms are different if you just look over the thread.

i though all 700r4 have the same gears! maybe not apparently.

 

2000 rpm's at 60mph
I used to have a 2.77 and ran like 1500 rpm at 60, now w/ the 3.70 I am running 2000rpm at 60mph, someting doesnt sound right.

 

I think you just aren't checking things in Overdrive. Your numbers only make sense at the 1 to 1 Drive ratio.

2.73 compared to 2.77 wil only make a miniscule difference.

Or your tach may be wrong.

 

no its in overdrive and the converter is locked. trust me i have been watching the revs for a while. if you notice from the previous posts though i dont appear to be the only 1 either. i know its weird and if its not the gears then the trans is the only other possibility.

oh that 70mph test as i have seen it explained was to put the car in 3rd (D) and do 70mph the rpm should match the rear gear ratio but on mine its way off, about 3300 roughly. plus when i talk to a freind of mine who has a 3.08 rear vette he is turning slower then me too.

 

looking at whats been posted if i went from 2.77 to 3.42 right now i would end up at 2800 in OD when going 70 which is crazy. seriously my car is turning this kind of rpm though, at 75-80 if i remember i was turning at least 2500rpm the flowmaster is putting out some serious sound and will make your ears ring after an hour or 2.

 

i just double checked my 75mph in OD and it was exactly 2500, even if the tach was off you can tell by the sound that its turning some rpm. i also double checked the tc and it is locking and unlocking fine. i can drop into OD at 40mph and the tc will lockup.

i cannot explain it but it must be in the trans since the gears are a definate.

 

Then your tranny is slipping.

In OD, my 3.42 is 2000 rpm at 60 mph and 2400 rpm at 70 mph.

If you are turning 2500 rpm at 75 mph then that may be sorta correct for a 2.77.

 

i guess that is possible but it sure doesnt feel like it is slipping and it accelerates just fine even in OD. if that where true then 3rd is slipping to and that also doesnt seem right, i can down shift to 3rd and take off. i dont have a spike in rpm versus acceleration as would be normal in a slipping tranny.

i'm putting the car on the dyno tomorrow so it should show something. if the revs dont match the wheel speed when we hit it then the tranny is suspect. i am going to diagnose as much as possible.

 

anyways... I got the 88 iroc 350 2.77 rear end

I run about 77mph at 2000 rpm in OD or so

 

i seem to have 2 that run the same rpms as me right at the top and 5 that run way lower. maybe this explains why i get a best of 20mpg.

oh i thought about the tranny slipping idea some more and its totally not possible. it would have been slipping for a yr now, i would have lost at least 4th and probably 3rd a long time ago. i've done 16K miles with these kind of revs.

 

How about timing and or throttle adjustments ?

 

i got a 3.27 9 bolt and im around 1900-2000 rpm at 70 in od with a 700r4

 

This post is from 2002.