How Big is Your Rear?

[BigT]

I’m on the verge of changing my 2.73 gears to 3.55. Maybe that’s still too big. Maybe I should think about 3.73 gears too. So I was wondering what other folks use and how do they like them?

I live in a mostly rural area with couple of stop lights as I head into work. I would say I go around 60-65mph unless I take the interstate, then it’s around 70-75mph. Once my confidence is back in the car, I would like to take weekend trips. Driving mostly on the interstates around 80-85mph.

As always, I appreciated everyone’s inputs and opinions.

[jaywish]

Depends if you care about fuel. You should notice a 30% increase in engine rpms at a given cruising speed just going to the 3.55's. E.g. what was 2,700 would now be 3,500.

[Tiggie]

For 3.55 at a given speed (let's say 60), multiply by 1.3. Thats 78 in this case. Go 78 with your currentl setup and this will be the same rpm you would be running at 60 with the 3.55.

If you're going to be doing 75-85mph with any regularity, I personally wouldn't go higher than 3.55. Just too many rpm for my liking.

[gadget73]

3.55 are fine, especially with a non-HO. They don't make any power at high rpm, so there is no point in really low gearing if you do a fair bit of highway work.

Mine came with a 3.55 when it left Wixom in 1986, and its still got those gears.

[BigT]

Sorry, I should have mentioned that it's been converted to Speed Density HO. And I should tell you that I'm looking at a Mass Air setup with 24lb injectors in the future. Does that change any thinking of gears to run?

[IPreferDIY]

In my 2000 MGM with 3.55s, my highway range is about 60-75mph. The highway speed limit around here is actually about 62mph, and I try to stay around that for fuel economy. I really wouldn't want to do 75mph on a prolonged basis. For my purposes, the 3.55s are just fine, but I could see how they might be a bit much for higher speeds.

[sly]

3.55 will need a 21 tooth speedo gear. 3.73 will need a 23 tooth speedo gear. Unless you change the output shaft to a mustang AOD 7-tooth from the 8-tooth shaft our boats came with, I wouldn't go higher than 3.55.

[gadget73]

Sorry, I should have mentioned that it's been converted to Speed Density HO. And I should tell you that I'm looking at a Mass Air setup with 24lb injectors in the future. Does that change any thinking of gears to run?

3.55 at a minimum then. 3.73 would also be fine, though once you get past that the speedo gears get stupid. Too many teeth, and they self-destruct rapidly. My car runs quite happily in the 70-75 range.

The HO has shit for torque down low, so they're pretty dismal with the granny highway gears most of these cars came with.

[dan]

I have 4.10s in my 8.8 rear. I currently love it. In 3rd gear I can be cruising down the highway at 3500 for 60 mph and then shift to overdrive for 2k rpms unless I want to go faster up to 2500 for about 70-75

Sent from my Z956 using Tapatalk

[LSE_Dave]

3.55 would be the lowest I would go for a cruiser.

[Mr Bean]

I got 4.10s. With 5.4 and manual trans it's too much. Was perfect when I had 4.6 with a blower but now thinking of switching to 3.55s.

[BigT]

Thanks for all the recommendations. I feel better about the 3.55 gears now. I didn’t want to go through the gear change only to wish I had gone a step lower or two afterwards.

[sxcpotatoes]

Edit: Why, you looking to buy me pants?!?

I have 4.30s! Came with the silver car. My speedometer basically reads in KPH on the MPH marks (NOS 140mph police speedometer) 100 is 62mph. The rear end gets noisy AF over 63-65mph, but it will pin you in your seats from a stop and chirp the summer or winter tires on the 1-2 shift if you are on it hard. Since it's a '91 I could get a $150-ish stepdown electronic doohickey to wire in and fix the speedo, but meh. Hilarious when you're doing 55 and the passenger is like "are we really going 85?!?!?"

The '82 Mark VI has a non-posi 2.73 or 2.83 rear from a '97 Grand Marquis with disc brakes I had put in.

[87gtVIC]

3.73's here with an HO conversion and some 255/5017's. 75mph and shes rpming around 2400 in overdrive.