8-3/4 gear changing...

Hi all. I'm looking to change the gears in my 489 cased 8-3/4. Currently, it has a 3.55 in it but it sure could use some more gear. I'm thinking about going to 3.91 but I'm not sure if it would be a big enough change. If I go to a 4.10, will it bolt right in, with proper adjustment, to the existing carrier or do I need to change more than just the ring and pinion gears? If I recall from years ago, in a Ford rear, if you went from a 3 series gear to a 4 series gear you had to change carriers, that's why I ask. Will a 3.91 make a big difference in the way the car launches from a stop over a 3.55? I'm not overly worried about highway cruising as it is only a local hot dog and ice cream getter with a rare trip out to the local dragway. Currently, the car is running about 2000-2300 somewhere around 55-60 mph. The motor is a 440 with an indy top end and it really likes to run above 32-3500 and seems to pull hard all the way up to about 6200.

just looking for some previous experience from all you mopar experts before I have to buy several sets of gears and a mechanic to install them. Thanks in advance!

the 391, 410 gears will work in your 489 case I have 410 in my car at 60mph it at 3000 rpm with a 275 60 15 tire

Good to know, that's my tire size

I but 4:30s in my 489 case with no issues.

747mopar said:

I but 4:30s in my 489 case with no issues.

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What tires and combo do you have? There's a guy on here selling a 489 case with a 4.30 in it and I wasn't sure if it would be too much on the street or not.

285-35-19 (27") tall and a T56 with a .5 6th gear. With the standard I didn't like them at all, felt like I couldn't go 50ft without shifting but a 3 speed auto would help with that. I went back to 3:55s and won't be looking back. The only reason I went with them is it computed up good on paper with 6,000 rpm shifts and a cruising rpm at about 2,000 rpm on the freeway at 70 mph. Some things look good on paper? My personal opinion would be don't do it, 410s is about all I'd recommend if your not getting on the freeway. Steep gearing sounds like a blast to drive but in reality if you wind every gear out in a matter of feet your car really starts feeling slow unless of coarse you've got a motor that revs well above 6,000rpm.

Well, I guess I must have short tires or something? My car is a 4spd without overdrive. I drove it for years with 3.55's and it would run about 3200-3300 rpm on the highway. I put in some 3.91's and now I'm running maybe 3700rpm on the highway and the engine sounds like it's going to blow up any second. So, for JUST going to the hot dog place, sure. If you want to take it to the track, you'd better trailer it....unless you have very tall slicks on it before you leave your house.

You don't have to change a thing about your carrier. The one thing you might consider is buying a complete 8-3/4" center section with gears already in it. You save the money/time of gear setup and you can change gear ratios in about 3-4 hrs. The 8-3/4 center sections pull out just like a Ford 9".

I switched down from 3.91 to 3.55 and I am happy with it. The engine still revs high on the highway, but does not sound as if is an the verge of exploding. I lost a little on the red light to red light action, but with a bigger horsepower engine, it is negligible.

I just switched from 3.91's to 3.55's in one of my cars. Probably gained 10 mph in highway speed that I feel comfortable driving at. The 3.91's were in a stock 383 motor. Pulled that and built a 451 that now has 545 hp and 545 torque. I am happy with the 3.55's with my current motor power, but anything less and I would want the 3.91's back. I think 4.10's are getting too high with a non overdrive tranny on the freeway.

I'm not too worried about highway driving because i prefer to take side roads and if it's further I'll just trailer it. But, it sounds like I'm in the market for a new 489 case. Anybody have one with 3.91 or 4.10 gears they'd be willing to part with? Thanks for all you help and opinions everybody!