73 Roadrunner gear set

[Lt Shaffer]

I have a 73 Roadrunner that I’m installing a healthy 340 in. Originally 318 with an 8 1/4 car, now I have an open 8 3/4 for it. I’m rebuilding the 8 3/4 with a sure grip, my question is the ratio. I’ve had 3:55 in a 340 swinger and 3:55 in another 73 Roadrunner with a 400 and enjoyed both. I’d like to try 3:91 for a change. I live in the mountains and I really wouldn’t be doing much if any highway miles. Does anyone have any real world experience with the 3:91 gear set in this combo? Thanks

 

[#41]

I have ran them in several cars until I moved to a more rural place and have to drive the highway more. They are a lot of fun unless you have to drive the highway... then not as much fun as you watch all the Hondas and Kias pass you by as you ride the slow lane with the trucks.

 

[RR Fan Dan]

I have a 74 RR with 3:91 and 440, 4 speed manual. A blast to drive, but wish I had 3:55 for highway. Is yours manual or automatic?

 

[70chall440]

Like the others I have ran a wide range of ratios; I really like 3.55s over all however if you really want to beat on then 3.91's are awesome. As stated however I think you really run out of gear too quickly with 3.91's and end up pushing the engine harder than need be.

All that said, in an 8.75 its not hard to change so try out the 3.91's and see for yourself.

 

[Nacho-RT74]

For today standards, 3.55 is enough fun to drive. 3.23 would be a regular all around use ratio. There is also 3.73 available.

I think would install 3.73 or 3.91 just having 4 speeds… or a lock up converter if auto.

(Well, actually I don’t think would install 3.91 ever really)

Also depending on your wheels size.

 

[MWbirdLOVER70]

I love my 3:91’s but highways and right lane here til I get an a hole wondering why I’m going 55 and stab it and leave him in the dust and then I back off and he gives me thumbs up and I wanna flip him off for being a prick first round lol but then give him the beepbeep horn lol. Oh 3:55 nice gear but in town I love the 3:91 launch

 

[Nacho-RT74]

Well, with an auto, a higher stall converter would solve that launch sensation still keeping the 3.55… I think…

 

[496 polara]

Define healthy? How much cam and what rpm range?

3.91 gear is a great performance gearset for the track. You need to be able to turn 6000rpm or you will run out of gear in the 1/4 mile with 28" or shorter size tires.

 

[Lt Shaffer]

I have a 74 RR with 3:91 and 440, 4 speed manual. A blast to drive, but wish I had 3:55 for highway. Is yours manual or automatic?

4 speed

 

[Lt Shaffer]

Define healthy? How much cam and what rpm range?

3.91 gear is a great performance gearset for the track. You need to be able to turn 6000rpm or you will run out of gear in the 1/4 mile with 28" or shorter size tires.

I’m guessing at least 400. I can’t remember cam specs, old purple shaft, 10.5 compression, x heads and an Edelbrock Torker. There aren’t any drag strips up here…just stop lights.

 

[496 polara]

Sounds like you can make use of it. Nice engine braking coming down the mountain ;-)

 

[GTX JOHN]

We run 391 in all our street/strip cars.

 

[Black_Sheep]

My BIL has a ‘69 Swinger 340 (stroked to 418) with a 4 speed and 3.91’s, it’s a surprisingly quick street combo. He drives the thing everywhere, the fuel economy isn’t great but that’s not what he built it for.

Here’s a shot of it when we dropped the engine back in last summer, because everyone likes pictures…