Opinion advice wanted: What axle to go with: 73 RR 4speed, 360-street/HWY cruiser

[greymouser7]

I bought a 73 Road Runner with a 340/833 four speed -odometer reads 85k -original?, don't know - car had a cheap/quick resto and looks great (12 footer)
I believe it has an 8 1/4 that has what 'feels' like 3.91-4.11+ gears in it.

My friend convinced me to save the original power engine/trans and swap in a 360 and another 833. I am going to have him fix the trans, rebuild the axle while I am deployed (subs-nothing glorious)

I want to DRIVE the car back and forth from the east coast where I am stationed and home in San Antonio. I want to drive the car every chance possible-so the motor I am building is a forged 360 rotating assembly-.040 over. Maybe a little nitrous, thinking some kinda cam in the EXtreme energy 268, 274, 282 range

I own: a rebuilt 8 3/4 with disc brakes from a 68-70 b-body (width) -2.71 HWY gears -probably non sure grip -in the garage (no idea of 489/741/742),

and/or: an a-body 8 1/4 axle core (slant 6 vehicle origin),

a F-body Road runner 8 1/4 (maybe 3.55 gears??) core -ran great under a 360/auto -has sure grip (car is lost to rust)

or the original (73 b-body) 8 1/4 axle with different gears

I want to rebuild any of the 8 1/4's if I use them or have Rob just weld in the 8 3/4. I have the spring relocation kit and perches.

I might be able to pull off buying a passon performance 18 spline/overdrive kit for my buddy's trans rebuild (sorry Rob, I just can't remember your id here)

I really want to get decent (above 15mpg??) gas mileage yet be able to light the tires up a little and not be embarressed (sp?) if I stomp the gas peddle from a dead stop in a 4000 pound plymouth - I would like to run fat-asset tires, and I believe that Georgia doesn't allow anything over 12.5" inches wide

thanks for reading everything, I love the Mopar guys on these sights and the support and community here are top notch!

 

[Meep-Meep]

The things that don't mix well are big cams, tall rear end gears and a heavy car. If you want a good performing engine that covers you from acceleration to freeway cruising you need stock or darn close to it. And having cubes in your favor will only help the situation. Or build a turbo motor with EFI and have the best of both worlds.

 

[YY1]

If it were me, and if cost and availability wern't major obstacles, I'd consider that o/d trans with a relatively stock "late model" 5.9 and say, a 3.55 or 3.73 rear.

At 245 NET HP, that's as much or more than the stock 400 for 1973 and close to the 440. With o/d and a 3.55, that would be like 2.6 or someting on the highway.

I doubt that car weighs 4000, more like 3600-3700.

My truck weighs 4200, and gets 14 MPG average with plenty of power, although it ain't gonna win too many at the strip due to poor traction and power to weight.

You can even get a ready made harness to keep the MPI injection (pre-97) from a member here.

 

[Montclaire]

I'd go with a 9 1/4 so you can add RWAL.

As for the rest of your build....

 

[RustyRatRod]

I told you like 2 days ago it is an 8 3/4.

 

[Kowal]

3:55 will probably give you want you want. Good on the highway, but you won't be hitting 80 all day long.

 

[Kowal]

3:55 will probably give you want you want. Good on the highway, but you won't be hitting 80 all day long.

 

[bigredbird]

I told you like 2 days ago it is an 8 3/4.

So it would be a moot point for me to mention that my 360 stroker, 400HP, 3:55 gears, and 8 1/4 rear have given me no complaints?

Okay, won't mention it then. :grin:

 

[RustyRatRod]

So it would be a moot point for me to mention that my 360 stroker, 400HP, 3:55 gears, and 8 1/4 rear have given me no complaints?

Okay, won't mention it then. :grin:

I never said an 8 1/4 wouldn't do it. Since his car is sittin in my yard, and I looked under it, I know what's there. lol Far as the 8 1/4 goes, we got pics of Rodney's Demon snatchin the tires leavin the line out here in the shop. lol It has an 8 3/4 under it now, but not because the 8 1/4 broke.

 

[bigredbird]

No argument here, Rusty. I didn't take it as a slap at the 8 1/4, just that if you already explained there's no 8 1/4 involved here, then this thread done served out its useful life.

 

[RustyRatRod]

I wouldn't say that......I'm open to suggestions and advice since it's our shop doing the work.

 

[67 coronet]

if its 8 3/4 then why not just get a third member with a good posi and figure you rear gear from tire diameter.overdrive units are nice for the freeway but you get into a price vs fuel savings debate there.there is also the cheeper option,which is,keep a set of taller tires for freeway driving and a short set for in town/track use.