You are asking some tough questions today … lol … never changed the differential gears in any of my vehicles. But I have a Ford truck with a Ford 9 incher, and replaced the limited slip clutches in the third member some years ago. The job wasn’t actually that difficult, at least not technically difficult. The main practical problem was lifting the third member into place, that was a bit of a physical challenge for one-person, laying under the truck, sans lift. If you are diy’er inclined, didn’t mind getting coated with grease & gear oil, I expect this is actually something you could do yourself. Good idea to ask a buddy to help you with the job of installing the third member once the gears are changed. My truck has 4.11 ratio, seems to work pretty good, slow speed accelerations are indeed pretty brisk, especially for a emissions-era 302.
Not sure how it is configured on your Mustang, but I think my truck’s speedo adjustment (were I to change the differential ratio) is done by changing a the speedo-cable drive gear, part of the transmission (or xfer case) ass’y. Hot Rod magazine’s website I expect has some informative articles on this subject.
Mustangs don’t use that type of axle anymore. They are very much like Chevys. You can’t just drop the gear carrier out, it is integral to the entire axle. It is easiest to change the gears by removing the entire axle and swapping gears while it is out on the bench.
I have done one in the car and one on the bench. The bench is easier. You also need a hydraulic press, dial indicator, 1 inch micrometer, an inch-lb bending beam torque wrench with adaptors and marking grease at a minimum.
Generally, any modification has a downside.
Go to 4:10s and the rear tires break loose easier. Increase the tire size/compound for more bite and the next weakest link in the chain gives up.
I used to have a 69 Superbee with a 4 speed/3:23 Sure Grip. Even with that ratio I left the driveshaft lying on the pavement a few times.
If the car already has enough torque or power to spin the drive wheels with the current gears, then going for a higher gear ratio will just make the wheels spin out more easily, and reduce the time that you can spend in 1st gear. The car may feel faster since you don’t have to press down on the throttle as much to make it go. But you could go just as fast if not faster by pushing down on the gas pedal a bit more. Since stock 1st is taller than it will be after your modification you’ll stay in 1st longer before the shift to 2nd and be able accelerate more. Now if the car doesn’t have enough power to spin the drive wheels then putting in a higher gear ratio should help get more torque to the drive wheels, which is what you need.
i’m actually getting bigger wheels, i got them from a junkyard from another s197, so that my wheels shouldn’t spin out easily after i install them so the gears might be a nice addition
First gear launch should be managed to limit wheel spin, I have done this with manual transmission cars. The advantage of the gear ratio change will be apparent in subsequential gears.
I have some old cars that I have changed the standard 2.76 gear to 3.23 ratio, I can nearly complete the quarter mile in second gear, a 4.10 gear would be an improvement.
What rim size? What tire size do you plan? Spinning the tires is the last thing I’d worry about, anyway. That’s a result of extra performance, just what you’re working to get.
rims are 19"x8.5"
i don’t know much about tyres, i just replied to you on the other post about tyres.
Im not sure what kind of tyres to get. I just want any tyre that fits i guess that is half decent, with a bit of grip.
Was thinking maybe getting some good quality used tyres but who knows.
If you wind up with tires with a larger circumference than the ones you had you will be negating some of the difference in gear ratio you spent money to get.
With stock OE size tires (215/65R16) 27" tall at 70 MPH cruising speed here is the average engine RPM with a 4.10:1 rear gear ratio…
RPM CALCULATOR
Your RPM is about with:4.10:1 and
Automatic without Overdrive: 3774 rpm
Automatic with Overdrive: 2500 rpm
Manual: 3572 rpm
5 Speed: 2857 rpm
With a 3.31:1 (OE I think) ratio and
Automatic without Overdrive: 3046 rpm
Automatic with Overdrive: 2018 rpm
Manual: 2883 rpm
5 Speed: 2307 rpm
I shift my V8 at 6,000 rpm all day long, but it doesn’t like cruising over 3,000 rpm and I am running a 3.91:1 on a spool… I am turning about 3,200 rpm at 65 mph and it is loud af… lol
With my old 2.55:1, with a 3 speed auto I could shift 1-2 at 60 mph, 2-3 at 120 mph… Now with the 3.91:1 1-2 is 40 mph and 2-3 is 80 mph… That is a huge difference…
