I need some help deciding on a rear gear. I originally had a 3:55 in the back of my roadrunner with a 383. I rebuild the pumpkin and installed a 3:91. I found the rpm was around 2850 at 60mph with a 28inch tire. I am now running a 500 cube stroker motor that makes 500hp. I am in the market for a new drive shaft and I want to go from a 7290 pinion to a 1350. The car is street driven with 75% city driven and 25% highway. The car weight is 4100 lbs with driver. I need to replace the pinion yoke and I guess the proper way is to take apart the diff and replace the crush sleeve to a crush sleeve eliminator. Since I have to spend the money to do that, it probably won't be much more in labour to change the gear back to 3:55. Any recommendations. How hard is it to take the diff apart? I have never taken one apart before. I only installed it in the car.

what rear axle 8.75 or dana ?

rereading your post it sounds like the 8.75...

3:91 sure grips are desireable & I'd pull the pig & sell it then buy a 3.23 or 3.55 clutch type sure grip then install the 1350 yoke if desired. if it's a 742 case (my recommendation) no need to mess with the innards when changing the yoke to the 1350. I think you'd be time & money ahead to do it that way

I forget to mention that it is an 8 3/4 489 case with a new auburn posi and I have a lot of money in this pumpkin, just like everything else on this car. Lol

I'd say put a gear vendors on the ride but it might be cheaper to go another route....Still....drivablity with the GVO off, and the better MPG with it engaged...you get two rear ends for the price of one...I have a 440 spinning 2200 rpm @60mph with 3.55 and 28" tires (final drive is like a 2.76)

Can you fit a 30-31" tire on it?
That would get you back to the (effective) 3.55 ratio range and be a whole lot simpler.

With that big of an engine on the street you may be looking at 2.94s as a viable option.
But I'll hedge and say go to 3.23s.
R.

Perhaps a Passon overdrive 4 speed. I am running 4:10 with passon and 275:60:15's (28 inch)at 75 mph mph doing 3k rpm,,,if tach and speedo to be believed. Running 512 stroker 6 PAC. Performance is excellent. Now wish I had a 390 or better yet a 370 gear for a bit better highway fuel mileage. Car has so much low rpm grunt, could likely even run a 355 gear, but probably would not.

A bit more expensive buying a gear set from Passon, but likely a lot more flexibility and joy. 5 speed even better, but now talking real big bucks,,,not convinced worth it

Back to the original driver of the change... 7290 to 1350. What is the benefit? If you are not drag racing with sticky tires the 7290 should hold up fine. With a high end U joint it should even take hot launches... at least as well as or better than the 8.75 rear itself.

I run the 7290/489 combo behind a healthy stroker and manual transmission without problem. It gets plenty of spirited driving but no drag and no slicks... drag plus slicks would need a D60 I believe. With the D60 the bigger U joint makes sense.

Agree with the moderately large diameter tires you are running and stroker, 3.55 or 3.23 would work better.

Thanks everyone for the feedback. The gv is a nice option but not cheap. I was in the market for a new driveshaft since my original has a few dings in it. I have had it balanced but I am chasing a vibration problem and I am not sure if the drive shaft is causing it. So I figured if I have to buy one I will go with the 1350. Not sure if I can get a 30 or 31 inch tire on the car. That's is good option. Anyone running that size on a 69 RR? The car is a 4 speed