74 RR 8 3/4 Gear problem

[Root2812]

I need some help. I got some life breathed into my car for the first time since I have owned it and we took it for a spin. The car screams (high RPM)going down the road. I don't dare drive on the highway or I'll blow it up. The tires on it are a little small (around 26 inch diameter) but thats what I had to use and stock says it should be just under 27 inches anyway. I know the tranny (727) was rebuilt by the last owner and goes into all 3 gears. He also swapped out the bad suregrip center section with an open 741 unit he had laying around. Assuming that no one jacked with the final drive of 3rd gear in the tranny I am assuming I have a crappy set of rear gears. Speedo is way off so we are going to get a GPS and try and check the speed later today.

What would you guys say is a good cruising RPM at 60MPH for my car (360 4 barrel)?

Larger tires will come at some point but what gear ratio should I try and get for the car? Its borderline undriveable right now. I played around with some of those calculators online and it seems like a 3.55 ratio will run 3000 RPM at 60 mph and that seems a little bit high of an RPM to me based on my modern stuff. My target would be to be able to go 75-80 on the highway without blowing up my engine.

Thanks

 

[idrivemopar]

That sounds about right with 3.55's.

I am running 3.55 gears with 255/60/15 and turn about 3300rpm at 75, so if you go up to say a 275 or 295 which gives you a few inches in diameter, you will only drop about 300rpm. If that doesn't make you happy, you might consider 3.23 gears with a tall tire, but if you go too tall it may look funny. As long as you keep it under 5,000 rpm you shouldn't blow anything up. I have pushed my 340 to 6,500rpm in first gear, and floated the valves, and it survived, but that was way before I rebuilt her.

Don't forget to adjust the speedo gear on the tranny when you change tires, or make sure its set correctly for what you have now.

 

[Bruzilla]

I can't imagine driving my Roadrunner at 80 MPH on the highway. I could do it, but I think part of driving these cars is getting to be the belle of the ball by just being there. I get on the highway and just cruise at 60-65 in the right lane and watch everyone come racing up, then slowing down to check out the car, and then slowly going by.

If you got style... flaunt it.

 

[Root2812]

Bruzilla, you make a good point. I never drive 80 in my modern cars mainly because I don't like to speed. I was only thinking of that as a target so if I am on the interstate where the speed limit is 70 but everyone does 80 I can drive without getting run over. You're also right about the fun of people checking the car out. Every time I stopped for gas with it on the trailer people came to ask me about it. It was surprising how many people loved the 74 despite a lot of mopar guys saying it's not worth anything. I got more attention with my 74 on a trailer than I did with my old 68 charger. Infact I drove the same distance with more stops and not one person came and asked about the 68, maybe they are just too well known now and the 74 is unique. Before I started the LS swap I used to go haul all kinds of stuff with my 1950 Chevy truck and that got tons of attention. It looks like a 65 year old truck and people were always surprised to see me loading it with stuff at Homedepot. I always tell my wife that it's the people who make old cars fun.

Idrivemopar, you're right. So I checked the speedo with gps and it was within 2mph so pretty much dead on. I guess I have 3.55s. It must just be the exhaust I have that makes it sound like its screaming. I did some reading and it sounds like 3.55 is a preferred choice by a lot of guys but I used some of those calculators and a 28 inch tall tire with 3.23 may be more to my liking.

If I am going to go and replace gears and add a sure grip would you recommend ditching the 741 case in favor of a 742 or is it not much of a difference for the average guy and a 360? I'm going to live with what I have for now due to the lack of funds but I can watch for deals.

 

[Cranky]

You could always stick in an OD trans then go with a set of 4.30 gears! I have a 3.90 gear in my Durango and running 80 is not a problem and it still will get 13 mpg doing that. It gets 17-18 at 60. Yeah, it's only a 318 but it weighs 4600 lbs and is more or less a brick going through the air.

 

[Root2812]

Hey guys. I knew something wasn't right and it's confirmed. The car has a noise coming from the rear and it feels like it's dragging something. I pulled the axles to check the rear bearings and they were good. Then we just turned the axle with it back in and there is something wrong in the center section.

So I have a few questions about the 8 3/4. My car originally had a 489 case with 3.55 sure grip gears. The last owner said he had a bad opinion bearing so he swapped in a open diff 741 case with 3.55 gears. While under the car I see the drive shaft is new too. If I get a stronger 742 or 489 will they need a different length drive shaft than the 741? Or did they change the drive shaft for some other reason?.....

Is it worth building the 741 and adding sure grip?

Thanks