I wanted to compare the final drive ratios between my current T/A, a 1984 L69 5SP, and previous, a 1983 LU5 T/A w/ 700R4.


Based on my research, the '83 had an overdrive ratio of 3.23 and the rear was .69.


The L69 came with a 3.73 5th gear and a .73 rear end.


That works out to the following:


LU5: 3.23 * .69 = 2.71


L69: 3.73 * .73 = 2.72


Does that sound right? I figured the L69 would have a lower final drive ratio.

You have it worded a little wrong, and have a couple of the gear ratios wrong

The rear gear ratio would be the bigger number

LU5 has a std. 2.93 rear gear with G92 option of 3.23

L69 had a 3.73 rear gear.

The L69 had the T5 man trans
The LU5 had the 700r4 auto trans

The T5 used with the L69 had ratios...
1st: 2.95
2nd: 1.94
3rd: 1.34
4th 1.00
5th .74

The 700r4 gear ratios are...
1st: 3.06
2nd: 1.63
3rd: 1.00
4th .70

So...
L69 final drive would be
3.73 x .74 = 2.76

LU5 with the 2.93 rear would be
2.93 x .70 = 2.05

LU5 with the 3.23 rear would be
3.23 x .70 = 2.26

Thanks. Even my math was wrong. Yikes.

That's a pretty significant difference, though. I've been thinking of putting some taller tires on the rear. I had larger rear tires on my '83.... probably too large for that drive train.

I wouldn't think so. I mean you are only in OD at highway type speeds. Unless your engine package is far from stock and making power in the 3000-7000 rpm band, I don't see a major prob.

Stock-ish engines have great power off idle to 3000 rpm.

Keep in mind that final gear ratio is for only in OD trans gear.
To see the ratio for other gears just do the math. Rear gear x what ever trans gear

It only protested at highway speed when going up a grade, maybe because it was in OD and/or the torque converted was locked.

They were pretty huge tires but I thought it looked cool in a Road Warrior kind of way.

I like that look myself. I run a 275/60-15 rear tire on my camaro, but I have a th-400 3 speed auto and started out with a 3.42 rear gear, now a 3.89 rear gear.

Here is mine..
https://www.thirdgen.org/forums/atta...icture-575.jpg

https://www.thirdgen.org/forums/atta...d-dscf3555.jpg

There, I fixed it for ya.

Cool. Looks like you went with the same rims front and back. I know this is a tire question, but..... if you were to venture a guess, what size tires / rims do you think I had on the back? I want to get something similar, but maybe not quite as wide. I don't want them to stick out past the wheel well.




I'm totally agonizing over this tire/rim decision.

Your old rear tires has short sidewalls so my guess would be a 50 or 55 profile. They are pretty wide.

You got some room between tire and 1/4 panel lip, so I would say around 26" or 27" tall so I'm wanting to say 275/50 or 295/50

Rear wheel looks to be a 8" wide reverse wheel (not very much back spacing). Kinda looks like an old Cragar s/s or an American racing torq-thrust.

Yeah my wheels are factory 1978 trans am WS6 snowflake wheels. 15x8 front and back.
235/60-15 in front and 275/60-15 in rear

Looks to me like that 'bird had 255/60R15s, but too far out.

I still think the sidewall is too tall for a 60 profile.

I ran a 255/60-15 on mine for a couple years. Here is 2 pics of mine with the 255/60

https://www.thirdgen.org/forums/atta...icture-294.jpg

https://www.thirdgen.org/forums/atta...icture-535.jpg

The car had Goodyear Eagle ST's on Cragar SS rims. The front rims were 15 but the rears were much taller... I can't remember how tall but I had to use coil spring extenders to keep the tires from rubbing, when I hit a bump. I like the way they look from the side, but they stuck out too far when viewed from the back. This time, I want to try for more backspacing, but still want to use Cragars.