[bignate]

Putting together a budget build roach of a ‘66 GTO to drive while I finish my Judge and dually. I have a .060” over 455 with 4 valve relief cast flat tops. Using 1970 #64 heads, 2802 cam, and ‘66 Tripower. I feel this combo should make decent low end torque. The car has its original M20 trans and I’ll be running 235/60r14 tires on 14x7 Ralley IIs.

If I did my calculations correctly, running a 2.73 rear gear with these tires (25” diameter) should have basically the same effective gear ratio as a 1970 GTO 455 4 speed close ratio with 3.36 gears and G78-14 (26.7”) tires, which I believe was factory; would have.

Car is being put together to cruise, I’d like to drive it long distances much of which will be freeway. It definitely is not going to be raced. Am I asking for trouble with in town driving? Is this going to be a clutch destroyer?

I’ve done every gear ratio mph calculation I can, and it all seems to work out theoretically; but I know calculations and real world results sometimes don’t always agree.

Thanks, Nate

[The Champ]

FWIW - the original tires on your '66 were 7.50x14's that were 27" tall.

I don't see any problems with the torque of an M20 equipped 455 and a 2.73 rear gear when you run short 25" tires, based on my long term, personal real world experience

I have been running 3.08's with my M20 equipped .030 over 455 with 26.7" tall rear tires since 2017.

I've bought this car as a project in 1992 and started driving it in 1994. I have run a non posi rear gear with both 26.1" rear tires and 27" tall rear tires with the original 389.. I then swapped in a 3.36 posi rear with the 27". When that rear end went south, I went with a 12 bolt with 3.42 gears with the 27" rear tires. In 2013 I built the .030 over 455 while still running the 3.42's.

After the engine swap, I realized that I had much more rear gear than necessary and swapped in the 3.08 gears with the 27" tall tires. In 2017 I went to 17" rims and now run the 26.7" tall tires.

My favorite car show is over 200 miles one way, with most of it being interstate highway. I still have the same clutch I put in back in 2013 when I went with the big motor, so no "clutch killer" by any means based on effective first gear ratio with taller tires than you are running.

A 25" tall tire makes 803 revolutions per mile, a 26.7" tire makes 257 and a 27" makes 745 or roughly a 10% difference between my current 26.7" and your 25". My 3.08 and your 2.73 are about 10% different, so whether started out in first gear or cruising on the highway at 70 MPH - our results should be very close.

[mysticmissle]

I had an m20 backed open 8.2 323 455 in my firebird with short tires i they were on 14” rally’s maybe 235-245 60s. Zero traction but it was a lot of fun decent rpm on the Highway. Took it out for a 5 speed taller tires and 410s not sure how I feel yet as i haven’t had a lot of seat time in her but I’m
Guessing 373s will
Find their way in the 8.5”

[Matt Meaney]

my '70 Gp has a warmed over 455, M20, 2.56 rear gear, with 245/60/14 tires. you pretty much just let the clutch pedal out and your moving. no fuss, no muss. on a steep hill a little more slip is required, but nothing excessive. have never smelt it.
as a fwiw, '70 455 Gp's left the factory with a close ratio M21 and 3.07 gears as standard. a 3.31 gear was optional. you could not get a wide ratio M20 behind a 455. go figure.

[Baron Von Zeppelin]

If I did my calculations correctly, running a 2.73 rear gear with these tires (25” diameter) should have basically the same effective gear ratio as a 1970 GTO 455 4 speed close ratio with 3.36 gears and G78-14 (26.7”) tires, which I believe was factory; would have.

He's doing a comparative with standard factory equipment of a 70 GTO 455 4spd car , and its basic issue tire. To get a feel of where his drive ratios will land alongside an approved package.
Is why he used that tire size.

Small note is the 70 would have had a 12bolt 3.31 , not a 3.36

Just hoping your 2.73 is a posi
If not you might better include a rev limiter

[mgarblik]

With all that torque on tap, I would think you would shred those little 14" tires off the rims before you hurt anything else in that set-up. Should be fine. Have some fun!

[bignate]

Thanks!… it’s nice hearing from people with experience. Now I just need to find a 12-bolt series 2 posi carrier.

Nate

[Matt Meaney]

yup, those little tires don't stand a chance.
you'll enjoy the combo.
https://www.youtube.com/watch?v=jtX-TQVatqE