3.55 gears for a 87 mustang automatic transmission

520Fox

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Mar 29, 2020
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I was wanting to change the rear gears on my 87 mustang, automatic transmission. Currently I have 2.73 gears, and I want something that is good for the street and the highway, because I would still like to do long distance highway driving. Back in the 90s when my uncle still owned the car he had 3.73 gears and said the RPM's were just to high for highway driving, so he had 2.73's put back in the car. I was wondering if 3.55's would be good for highway driving. Currently when I'm driving about 75mph in overdrive I'll usually be around 2100 rpm's just cruising.
 
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CarMichael Angelo

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You have to decide what the main goal is
, and where it’ll see the majority of the time of its use When you choose a gear. 2100 rpm at 75 mph seems like a good place to be if you’re wanting to get some chance at decent gas mileage.
I have 3.73’s in my car, but I have a really loose converter, and a fairly tall tire. When I engage lock up 75 mph was around 23-2500 rpm iirc. If your car has a short tire, typical of what would be on it stock, then you might see an additional 200-300 rpm dependent of how much difference in height our two tires are. A 3.55 would probably put you right where I am.
 

Mustang5L5

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Back when my car was an AOD, 3.73s were great around town, but I hated them on the highway. Felt like I was revving too high to maintain 80mph flow of traffic. OD cannot be engaged at WOT so passing a car on the highway seemed overly dramatic.

I never liked it, still don’t. I should have gone 3.55 but I value cruising over racing.
 
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KRUISR

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Apr 16, 2015
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Assuming stock tire size (25.8" diameter - 225/60/15, 225/55/16, 245/45/17) RPMs for 70, 75, 80 mph for various gears (its just math).

3.73 - 2275, 2436, 2597
3.55 - 2165, 2319, 2472
3.27 - 1994, 2136, 2277
3.08 - 1879, 2012, 2145
2.73 - 1665, 1783, 1901

I personally have 3.73s, love them but don't usually cruise the highway much above 70, usually closer to 65. With the stock motor I would hit around 24 mpg (10.5 L/100km) cruising with the 3.73s. Fuel economy mainly a function of speed (optimal is somewhere around 55-60 mph for most vehicles).
 

KRUISR

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Sorry just looking back at this one. 520Fox, you said you cruise at 75 mph at 2100 RPM. That would mean your uncle probably put 3.27 gears in it, not 2.73's as they should give you 1800 RPM.

That is fairly close to the 3.55s you are considering. You may want to just leave them.
 

revhead347

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There is a notable difference at highway speeds with the 3.55 vs the 3.73. The difference is just enough where the 3.73 puts the car in the drone zone.

Kurt
 

Ryuk

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Old info here, but my 89 with 3.27's go close to 28 mpg highway back in the day. I drove it 100 miles each way to school a couple of times a week without issue.
 

520Fox

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Mar 29, 2020
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Sorry just looking back at this one. 520Fox, you said you cruise at 75 mph at 2100 RPM. That would mean your uncle probably put 3.27 gears in it, not 2.73's as they should give you 1800 RPM.

That is fairly close to the 3.55s you are considering. You may want to just leave them.

I actually had the 3.55s already installed back in April. I don't know the exact verbiage, but I counted the valleys on the ring and divided that number by the ridges on the pinion, and it came out to exactly 2.73.
 

Willybill32

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I have an '86 with a 5-speed. Stock gears were 2.73, and it droned at 1,500 RPM, right around where I generally drove it. I just put a 3.55 in it, and with only a couple hundred miles on it, I'm happy so far.
 
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