tomard

I pulled these numbers off the internet for 1980's: And don't understand what comprises the Final drive ratio numbers and to further confuse the confused, the FDR's are same as rear end ratios, but I know my A/T car came with 3.07 rear gears

Optional Manual Transmission
close ratio four-speed fully-synchronized
floor-mounted gear shifter
gear ratios
1st = 2.43:1
2nd = 1.61:1
3rd = 1.23:1
4th = 1.00:1
Rev = 2.35:1
Final drive ratio: 3.36:1

Automatic Transmission
Three-speed Turbo Hydra-Matic
floor-mounted gear shifter
gear ratios
1st = 2.52:1
2nd = 1.52:1
3rd = 1.00:1
Rev = 1.93:1
Final drive ratio: 3.55:1

7T1vette

Final drive ratio is the gear ratio of the differential unit in your car. Chevy called it a 3.08:1 ratio [for standard unit with auto tranny]. Basically, you divide the number of teeth on the output ring gear by the number of teeth on the input pinion gear.

tomard

I thought that is what it meant, but I have 3.07 (tested it by turning wheel and counting shaft revolutions (just over 3) ....

So why does it say 3.55 on the spec sheet, is that bogus or ?

LB66383

My best guess is that the 3.55 was the standard rear gear, and the 3.08 was optional -- that's what the original buyer selected.

tomard

I think I was wrong on this one too ..... I did do the wheel turning & shaft counting exercise, but today I took it one step further:

1) Took car out on the road
2) Drove her exactly 60mph (pretty much dot on, whole mile)
3) Checked speedo by timing myself between mile markers (exactly 60 seconds)
4) Was running around 2700 rpms (on my tach which I don't know how accurate it is)
5) My tires are 26" across

Result:

(60 mph with 3.55 gears and a 26 inch tire would be 2756.47 rpms)

V-Rod

How did you do the math to get that? the info for 78 says 3.08 or 3.55. I am about to put a 2004r in and want to take it to the track some. I am going to need bigger gears right?

7T1vette

If you want to win at the track, you need to set up the car so it will be at peak rpm in high range as you cross the line. Very few street vehicles will want to be set up this way, especially if you want to drive them at highway speeds. If it is strictly for drag racing, you probably need a 4.11 to 4.56 gear with a 200r4 (if you have rear tires that won't just go up in smoke). A good middle-ground gear for a car that can ride comfortably at highway speeds would be 3.55 to 3.73.

tomard

It wasn't mine, but here it is:

first get the circumference of the tire which is diameter of tire (across)
*pie(3.14) In my case it came out to 6.8 feet. Divide that by 5280 and you get how many revolutions it takes to go a mile. Multiply that by the gear ratio and you have rpms at 60
26"x 3.14 =81.64"div.12 =6.8' into 5280 =776.4x3.55=2756.47rpm@60mph

wfo76

The 80 used a 3.07 in all cars and there were no optional ratios. The 79's used a 3.55 but this was the old style GM rear and the 80-82 Dana 44 offered no such ratio. The 26 inch tires are about an inch shorter than stock and with the torque converter slip at highway speed, your rpms sound about right for 3.07's.

tomard

Converter doesnt have enough stall to be slipping at cruise at 2500+ rpm. Its true that you could get some stall (slip) on the road if you could do WOT without downshifting at 2500 or even a bit more.

3.23 gears would be just over 2500 @ 60mph and I just dont think its going to slip that much, unless the converter it a real POS.

http://www.accperformance.com/350C.htm
level 2: 47402 2200-2800 stall

wfo76

Only the California 305 LG4 got the lockup Turbo 350C in 1980, so you have plenty of converter slip at highway speed.

My numbers with 26 inch tires show 2380 rpm at 60 with no slip as in manual transmission or an automatic with a fully locking clutch type converter. With your standard Turbo 350 automatic, you can have 200 rpm slip on the highway added on top of the 2380 rpm, so an indicated 2700 rpm at 60 should be pretty close to accurate.

tomard

Not sure what you are trying to say, as the stock 350 motor (block now, as I have modified just about everything) and TH350C tranny, I have, is definetely lockup, had the tranny apart and have all the lockup components in there (extra valve body, lockup solenoids, electronics etc.

wfo76

Well, interesting. I thought only the LG4's got the 350C. I guess if it's really locking up, the tach must be reading high. Is the lockup working now?

V-Rod

What is the best guess for a 78 with an L-48. 3.08 or 3.55?

wfo76

L48 4-spd - 3.36
L48 auto - 3.08 (federal below 4,000 feet elev)
L48 auto - 3.55 (california and/or above 4,000 feet)

tomard

Yeah, this is surely interesting ..... enough for me to jack her up and check it .... You were right about the 3.08, 1 full tire revolution is a hair over 3 shaft revolutions.

The tranny is definetely the lockup TH350C. Had to get a special lockup torque converter 2200-2800 stall for it!!!!