Darrel1

I have a 1980 L48 stock. I bought it last fall and am new to working on old cars.
I just drove it on the highway for the first time and
at 40 mph the RPMs are at 2000.
At 65 mph the RPMs are at 3000.

Both just cruising ( not accelerating). She idles around 700 RPMs. I am not sure what my axle ratio is but will determine this weekend.

Does the 3000 RPM AT 65mph seem high or is that how these setups were designed to run at. I know I need to know my ratio, but does the RPMs seem in the ballpark for any of the stock rear ends? I used to have 1993 vette and at 75MPH the RPMs were at around 1700 (from what I can remember.

Darrel

Crash80

Welcome to the world of the 3 speed automatic. Sounds normal. Your 93 was a 4 speed auto more than likely, thus lower RPMs on the highway.

Darrel1

Thanks

Yes it is an automatic- just wanted to make sure. Would a small vacuum leak cause the engine RPMs to go up at highway speeds or just at idle?

My headlights stopped working and when I pinch off the hose at the carb it runs much better-so I believe I have a small leak, another to do item on the list.

chevylit

According to My restoration book the only rear end ratio offered
in 80 was 3:07. A vacuum leak would not affect RPMs at higher
engine speeds. I have a 79 with automatic and 3:55 rear end
and its turning around 3400 at 65. Thats the reason alot of folks
are switching to later 4 speed automatics or 5or 6 speed standards.

DaveL82

It's possible someone swap in a slighly higher ratio diff at some point.

My 80 L82 had the 3:07 diff with the auto trans. I have since gone to the Doug Nash 5 speed for much better accelleration. The 1 to 3 gaers and much lower and 5th is 1 to 1 so that works well with the 3:07 rear gear. If I had a 3:55 rear gear I would have needed and overdrive trans.

Please also note that your car should have the 350C trans which did have a clutch lockup to reduce RPMs when in cruise (high vacuum) and in 3rd gear. This may not be working in your car or has been disabled.

Crash80

The RPMs listed sound typical of a stock 80...with the torque converter lock up working. If you want to spend the money to get an overdrive tranny, the 2004R and 700R4 are popular amongst those here. The 2004R is a pretty direct swap with the TH350C that you currently have in the 80. The 700R4 requires the drive shaft to be shortened. There are a few other issues with both like mounting brackets, etc. If I ever get any movey ahead, I am going with a 2004R myself. You can do a search and see all the arguments for and against either one. Of course you can get out the really big bucks and do a swap over to manual.

kona

I did the swap to the 200r4 besy money ever spent. I believe I have 3.07 gears in my 80 and at 75mph I am turning about 2000 rpm's.
Not only is it alot quieter ride but really saves on the gas. My swap ran alittle less than $1,600 and a shop did it.

Kona

srs244

i have a 78 with a 3:07 rear and those numbers sound pretty correct for the same setup (L48, automatic TH350). i turn about the same RPMs (perhaps even a shade higher). does wonders for fuel economy, lol!! since i do not daily drive mine (78 pace car) and do not tour with it except to go on a day cruise or a distant show, i prefer the originality of the car and have no plans to do a trans change-out. those mentioned above certainly will improve improve the shift characteristics and fuel economy based on being a bit more sophisticated technologically.

kdf1986

I have a 75 and have about the same RPMs at the speeds that you indicate. I thought it was too high also. But then I was used to driving my 86 with the RPMs around 1500 during cruise mode.

kdf

Darrel1

Thanks for the replies-
I think I will be adding the 200R4 as a future add on. I am like Earl I have my list.

How much for the 200R4 if installed myself??


Darrel

SanDiegoPaul

Yep In addition, it has a lockup convertor too - thereby lowering RPM a bit more.

One more thing to consider though: Verify the tach's accuracy with a tune up machine or a known-accurate tach before trusting it's judgement too much.

BLACKDOG

Darrel - not sure where you live, but 2 1/2 years ago I took my '80 to BowTie Overdrives, about 30 minutes south of Barstow, Ca. and had them swap out the TH350 for their stageII 200/4R (handles up to 400HP) for $2500. With 3.55's & 255/60/15's I now can cruise 75mph @ 2200rpm's rather than 3300, so there is a big difference. I'm considering swapping out the 3.55's for 4.10's now, since I'd still only be turning 2560rpm's @ 75, but it would be a helluva lot more fun getting up to 75 !! No doubt, you'll definitely like the 200/4R !! Good luck !!

palazzopl

my 81 was turning about the same ,,3000 at 70 ,the lockup wasn't working.. if its the same as an 81 there is a plug under the shift console near the ash tray.. try and ground the plug when your doing about 45mpg and watch for the rpm drop,if it don't drop have the lockuo soleniod checked out.

Darrel1

Does anyone know 1980 would have the same setup as an 81?

The_Dude

No, it wouldn't. In '80 you couldn't get a locking torque converter.

TedH

They are correct: only stock ratio in '80 was the 3.07. I bought a new diff and blueprinted/added 3.55's along with my 700R4 conversion back around 2001... Very nice at higher speeds