Gear Ratio upgrade for 69 GTX

[451Mopar]

You need to consider your tire height in the equation. A small tire like a 235x60x14" is only about 25" tall, and with 3.23:1 gears, 70 MPH is around 3,000 RPM. A tall (and wide) tire like a 295x60x15" is just about 29" tall, so 70 MPH with 3.23:1 gears would be about 2600 RPM. To have the same RPM (about 3,000) with the taller tires you would need to change from the 3.23:1 gears to 3.73:1 gears. With an automatic, you may also need to consider the converter stall speed?
The 3.23:1 came in many performance Mopars in the 1960's and 1970's, but most of these cars had fairly small tires around 26" tall, and the gear is good on the highway and daily driving, but not so good for acceleration performance. The 3.54/3.55:1 gears are usually a good compromise for a street car that has short tires. 3.73:1 or 3.91:1 is a good ratio if you take tour car to the track, and highway driving is OK. If you swap all street tires to shorter strip tires, it works good.
4.10:1 or 4.30:1 Highway driving is a chore unless running 29"+ tall tires. More than 4.30:1 should be for the track or a rock crawler.

 

[tallhair]

The tire calculator posted above will help make all that easy to figure out too

 

[NJRR]

Definitely good info and raises some questions for me. As I have posted, I'm converting my 70 RR, 383, from an auto to a 4 speed. It has 3.23's and 255-60-15 tires. I'm considering using Passon's 833OD tranny that he is giving a great deal on, as opposed to rebuilding the 833 core that I have. I didn't really consider it before this thread, but would changing my 3.23 to 3.55 give me any appreciable 1st and 2nd gear performance with the 833OD while still preserving lower highway RPMs because of the OD? When I say appreciable, I mean worth the investment. The 3.23s now pin me to the seat when I punch it from a dead stop. Also, if I switch to 3.55's, would it be worth converting to a Suregrip? Just wondering if it's worth considering all that.

hahahahaha .. it happens once in a while. What fooled me was a lot of the guys posting in 09 are still here. Still good info though so hope it helps someone and that question is timeless

 

[451Mopar]

FWIW, I had 4.10:1 gears in my '69 Coronet Convertible with 255x60x15" tires (calculated 27.05" tall), and even running the now defunct Keisler SS700 5-Speed, I decided to install 3.54:1 gears in the Dana 60. I don't recall if my trans is wide ratio or close ratio, but it has the 0.68 overdrive gear, so now 5th is like if I was running a 2.40:1 rear gear without an overdrive. Now I can cruse 70 at only 2100 RPM.

 

[GTX Mando]

I understand this is an older thread. Great info, i need some confirmation. I am rebuilding a 69 GTX, it has a 4-speed and I need to replace the gears in the Dana 60 rear. I'm leaning towards 3.73 gears. I will be running 18" wheels mostly street driving. any words of advice?

 

[451Mopar]

I understand this is an older thread. Great info, i need some confirmation. I am rebuilding a 69 GTX, it has a 4-speed and I need to replace the gears in the Dana 60 rear. I'm leaning towards 3.73 gears. I will be running 18" wheels mostly street driving. any words of advice?

It really depends on the overall tire height and how high RPMs you will be driving at on the Highway.

 

[HT413]

It really depends on the overall tire height and how high RPMs you will be driving at on the Highway.

X2. I run 4.10's, 30" tall tires and am at 3000 rpm around 60-65 mph. It suits me just fine because I prefer cruising 50mph rural back roads among the hills and farms (yes we have those here in nj!) to highways.

Pick a cruise rpm and a tire size and then you can back calculate your way to a gear.

- - - Updated - - -

Here's a nifty calculator that allows you to compare 3 ratios...

http://www.wallaceracing.com/gear-speed.php

 

[GetX'd]

My 440 GTX came stock with the 3:91s. When I was younger I had thoughts of going to 4:56 or 4:86 then I realized I wouldn't even be able to drive the car to the grocery store down the street comfortably. I wanted to be able to drive on the freeway at a reasonably comfortable rpm. I just bought new gears and stuck with the 3:91s. Happy that I did!

 

[ykf7b0]

3.55's are a great all around gear especially if your not on the interstate often. I've tried them all in my 440 roadrunner before settling on the 3.55's.