mwm3006

I have a 79 vette automatic. Purchased a supposedly 383 stroker with approx. 380 hp and 415 torque.
Car runs much beefier than the original but still can't spin the tires.
checked my rear end to find it is a 305 sucks!!
I would like to smoke the tires a bit and stick me in the seat.
SO the question... 355 or 373.
Does anyone know what rpm the 355 would be at 70mph vs the 373.
Don''t really want to be wound up like heck at 70 but I do want some beef.
Any help woiuld me appreciated.
Thanks

BKarol

I have 3.70 in my 78 and at 70mph it reads at 3200rpm.

Street Rat

Punch some numbers into this...
https://spicerparts.com/calculators/...rpm-calculator

Craig

MelWff

there was no 3.05 ratio
you either have 3.36, 3.55, or 3.70.
how did you go about determining the ratio?
a 3.55 at 70 mph is about 3,100 rpm with 27" tires
a 3.70 at 70 mph is about 3,500 rpm with 27" tires

mwm3006

I turned the rear wheel one rotation and the dive shaft turned right at exactly 3 turns .

mwm3006

right now at 70 I"m at 2900 rpm

C3 Stroker

You probably have the 3.08 ratio. That being the case, the 383/380 horse stroker should still easily spin the tires........I'd look into engine tune, not the rear gear. 415 torque should rip any street tire loose.

Gale Banks 80'

Rather than change the rear end gear You might consider changing the trans to a 4 speed automatic. Lower 1st gear and overdrive.

mwm3006

Don't really want to spend that much more on it. It is tuned as good as it's gonna get. Has 20 wheels which I'm sure it makes it worse.

redvetracr

but there was a 3:08...

Kacyc3

lets pretend you have 305 like you say. going to a 355 will add 500 rpm more to the same speed you are doing now. so 70 mph with 305 is 2900, with 355 its now 3400, with 373 its now 3600..Are you sure the speed o is correct on that? When you change tire height it affects the speedo reading.

mwm3006

Yes the speedo is correct. I can't help but think the 355 or 373 would stick you in the seat much better. Just wasn't sure which would be the best. Never even really go 70 mph. Just bring it out for short cruises every now and then.

Kacyc3

How tall are the tires on the rear or do you know the size?

mwm3006

would have to check but seem to recall it kept the same overall size as the 15s.

mwm3006

ok. just checked . 18 inch wheels. 27" overall height. th 350 trans.
70 mph 2900 rpm. So again 355 or 373?
thoughts.

resdoggie

The same logic I applied when I decided to upgrade to a 4.11 from a 3.55. You will enjoy a 3.73-4.11 ratio change.

OMF

The 355 will be a better all around gear ratio......the 373 will stick you in your seat better when you punch it.

I'd go 355

c3_dk

27"
TH350 1:1 in 3th
70mph

= 3092 rpm with 3.55 rear


27"
TH350 1:1 in 3th
70mph

= 2927 rpm with 3.36 rear

OldCarBum

What is the first gear ration of your transmission?
Multiply the first gear ratio of your trans times your rear end ratio and see what you get.
8.5 to 9.0 is pretty good for a street car and should light your tires.
9.0 to 10.0 is getting low and should give you excellent acceleration off the line and you should light the tires effortlessly.
Anything over 10.0 is useless for the street.
The difference in tire size unless you are running 34" mudders, really won't change the overall results.

BKbroiler

The difference in rpm level at a given speed between 3.55 gears and 3.73 is pure arithmetic. 3.73 / 3.55 = 1.05. Therefore if you're doing 3,000 rpm with a 3.55 gear, you would be doing 3,150 with 3.73 gears, at the same speed. (3,000 x 1.05)