replacing stock 2.75 gear with a 3.5

I've read a lot of posts on this site about various gear ratios, rpms at highway speeds, driving styles, etc.

I have a stock 73 Mex Mach 1 4spd.  I would not call it slow, but I want to accelerate faster.  I will not be racing or doing burnouts, but I like being the fastest car away when the light turns green.  I don't plan to drive on the freeway.  currently around town, I never leave 2nd gear. its good until about 50mph at least.  when I get onto roads where I can go 60, I never need to leave 3rd gear.  so my feeling is that by going to a 3.5 gear, I will use more of the gears, and never have the "crap i'm at 4k doing 65mph" issue.  I was also thinking of a 3.25 gear, but given the time and money, I don't want to be disappointed .  anybody have any advice?  thanks

If you have a true Mexican Mustang it will have a Dana type rear end and not the Spicer type like the Ford 8" and 9". That means that you cannot just jack the car up pull the axles and swap out the center section. The new gear has to be installed into the housing from the rear of the housing. Much more difficult to change ratio. That is unless you have a complete rear housing to swap out. 

Sounds like a 3.50 would be your best bet. If you were drag racking a 3.91 or 4.10 would be better.

The Ford 9" is popular because you can swap out the center section in an hour and be back on the road.

One word of caution NEVER NEVER use synthetic rear end grease. Here we have one guy that is considered the best at installing new rear gears Dana or Ford. He is used by most of the new car dealers for warranty work. He has a room pilled up with new gears out of brand new trucks that come from factory with synthetic lube. That is what causes them to fail. I myself at about 100.000 miles pulled the axles out of my F-150 drained the factory lube and put in full synthetic thought I was doing good. This was years ago. The pinion bearings went out shortly after that. I changed out the entire rear end housing cheaper than putting in new bearings. I went back to regular grease no issue and almost 300,000 now.

Do not know how original you want to stay with the Mexican model you can put a 9" in and also traction loc if you can find one.

there is no question that I have a Mex Mach 1, but I have not actually confirmed that the original rear-end is still in the car.

there is no question that I have a Mex Mach 1, but I have not actually confirmed that the original rear-end is still in the car.

Click to expand...

Just look under the car if a plate is bolted to the back of the rear it is Dana if the center section is bolted from the front then Ford 8 or 9".

so it turns out that the car has a 3.10 right now in the Dana 44. so instead of going with a new 3.50 posi, I'm going with a 3.73.

3.73 sounds like a good compromise. My wife's Lincoln Navigator even has a 3.73 rear, but with a 6 speed automatic so the highway isn't a problem. That thing digs in really well leaving a stoplight with the 5.4L 310 hp engine, even with all of that bulk. You could probably even get away with a 3.91 for the driving style you are talking about. But even after a few times on the highway you might get sick of the high rpms, so 3.73 is probably best. Yep I think many of us did the stoplight to stoplight drag thing at one point. That's really what classic US musclecars were designed for IMO. I don't condone it now of course since I'm a middle-aged IT career type with a family, but around 40 years ago I was doing stoplight racing sometimes up to 100 mph on regular city streets. Also top speed testing up to 140 on certain stretches of highway. Wait, was that really me?

In truth a 3.73 is a lot of fun. With a 4 speed you will not want to drive fast or far on the freeway as you will be over 3250 rpm at 70 though.

A 3.31 would give a small improvement, a 3.54 would be where I would draw the line if I was going to drive on the highways or out of town. I mean who drives 70 anyway?

I have a 3.50 and really like it. I wouldn’t want any more cause I would go through too many tires, lol. I do some occasional freeway driving and with my gears I am running around 3200 at 70 ish. I did just have my engine all rebuilt and I can break the tires loose easily at a 35-40 roll. Anyway hope you like whatever you choose. You will definitely have a lot more fun!

Yeah, its going to be a 3.73. I have no need to drive on the freeway, so the fastest I end up going is 65, which is 10mph over the speed limit on my local roads....

I generally don't cruise at 60mph+ so I currently don't use 4th gear at all. I am also replacing the open dif with a post, and will be putting on the 15x10 rear tires. I'm excited to see the difference in acceleration. I'm doing this work before I make any decisions about what do about adding power.

What size of tires? The larger the diameter the less effect your lower gear will have. Don't forget that you also have to change the speedometer gear to match both the rear end and the tire diameter.

I will be installing 10x15 with 295/50 tires. my speedo is in kph so I run a gps speedo on the dash, so tire size doesn't matter...

drove the car today for the first time with the 3.73. I won't call it fast, but it sure is faster. I'm glad I went with the 3.73 and not the 3.50. I actually used 4th gear today at around 65mph. with the old gearing I didn't need 4th.