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.
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.
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.
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).
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.
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.