Question about gearing...

[StangGrrLy]

Just a quick question for you guys....

What is the stock rear end gearing on a 5-speed 4 cyl. Mustang? The reason I ask is because ever since I did the T5 swap, my speedo/odo is out. Everything still works, it just reads that I'm going faster than I really am and adds phantom kilometers. They must have forgotten to do the speedo cable mod, and I don't really care enough about it to do it. I just want to be able to calculate what speed I am actually going lol.

IIRC, the auto 2.3L's have 3.73's right? I just need the stock gear ratio to do some calculating.

 

[RustBucket]

5spd cars had 3.45 gears.

 

[StangGrrLy]

Excellent, thanks.

 

[bhuff30]

When you changed to a 5 sp, you changed the drive gear from 6 tooth to 7 tooth. You didn't change the rear end, remember?
You'll probably need a 21 tooth tranny gear to make it read right... unless you used a 90+ V8 tranny which has an 8 tooth drive gear.

 

[RustBucket]

There's also a chance (albeit slim) that the trans could have a 6 tooth drive gear if the donor was an early turbo coupe.


Assuming you have a 7 tooth drive gear (yellow)
If you're running a 225/60 -15 tire you'll need a 20 tooth (black) driven gear.
If you're running a 205/65 -15 tire you'll need a 19 tooth (pink) driven gear.

Anything other tire sizes can be calculated with a little help from Hanlon Motorsports

 

[StangGrrLy]

When you changed to a 5 sp, you changed the drive gear from 6 tooth to 7 tooth. You didn't change the rear end, remember?
You'll probably need a 21 tooth tranny gear to make it read right... unless you used a 90+ V8 tranny which has an 8 tooth drive gear.

True enough, as far as I know they didn't swap the rear end. Oh I'm confused now. I was thinking it was doing the same thing as if you swapped to steeper rear end gears in a car and left the same tranny in, it makes the speedo read high.....since my auto has slightly steeper gears in the rear end than a factory 5-speed, my speedo reads high. Am I way off here?

So here's another question about gearing...When I am at highway speed (100ish kms/60ish mph), my tach is at 3000-35000 rpm depending on how fast I am going. This seems high to me, as my auto would rev about that high on the highway. I also have a friend with a factory 5-speed 4 cyl and he says that this seems a little high to him. I am totally stupid when it comes to trying to figure this out, so is that high of an rpm normal and if not, why does it do that? I always thought it had something to do with the rear end. Can someone provide a dummy proof explanation? lol this totally has me stumped now.

 

[JB66]

My 90LX reads 3000 at 70 mph and 2400 at 60 mph. 2.3; 5spd; 3.45 rear; 205/60x15 tires on 10 holes.

 

[RustBucket]

Here's something else to throw into the mix. My '91 was a bone stock, absolutely untouched, owned by a little old lady (my mother) until I got it. Now I haven't done anything to the drivetrain (yet, 5spd goes in tuesday). Anyway. On a recent roadtrip to see a friend in Tennessee, I took along my hand held GPS just to play around with it on a long trip. What I found out was that my speedo was 4-5mph slow. This is an A4LD, 3.73, 205-60/15 equipped car. I plugged some numbers into a speedo gearing calculator, and found that if I were running 225-60/15's that my speedo would be closer. Since that's the factory 5.0 tire size, I wonder if Ford screwed up, or just skimped when it came to that detail and figured it was close enough.

Anyone else checked their speedometers against an accurate measurement device? Cops radar doesn't count

 

[Wannabe2.3T]

i dont think 205/60/15 tires vs. 225/60/15 tires would make your speedo read ANY different, as 205 and 225 are just refering to the width of the tire. the only thing that would change it would be the profile of the tire and the size rim you are running.
225 refers to the width, 60 refers to the profile (distance from rim to tread), and 15 refers to the size rim its made for. just thought i would clear that up for those would didn't know.

 

[StangGrrLy]

i dont think 205/60/15 tires vs. 225/60/15 tires would make your speedo read ANY different, as 205 and 225 are just refering to the width of the tire. the only thing that would change it would be the profile of the tire and the size rim you are running.
225 refers to the width, 60 refers to the profile (distance from rim to tread), and 15 refers to the size rim its made for. just thought i would clear that up for those would didn't know.

Yeah except that the second number (60) is a percentage of the first number, so it does alter the overall profile of the tire. A 205/60 isn't the same profile as a 225/60 because 60% of 225 is more than 60% of 205.

So the conclusion is basically to give up on ever figuring out how fast I'm going?

 

[Dr_EluSivE]

mine is about 5% too fast with my swap. Following my gilfriend on the same route, when she hit 100 Miles, I was hitting about 105miles. I think mine reads roughly 5mph too fast at highway speeds, less at lower speeds. there is no set amount. I like it this way, and i wont be changing it. I can speed and not get tickets (cause im not really speeding)

Dr.

 

[Bearded Forever]

You should change the gear on the your speedo cable StngGrrly.
Now that you have a 3.73 rear gear it doesn't match with the speedo gear for the t-5 so it's off.

 

[StangGrrLy]

You should change the gear on the your speedo cable StngGrrly.
Now that you have a 3.73 rear gear it doesn't match with the speedo gear for the t-5 so it's off.

OK now I get it. I wasn't too far off then..... lol

 

[JB66]

For some reason, Ford used the same driven gear for the 3.45, 3.55, & 3.73 axles. Mine is also 5 mph too fast (verified by 10 miles of mile markers and a stopwatch).

 

[Shawneebear93]

mine is about 5% too fast with my swap. Following my gilfriend on the same route, when she hit 100 Miles, I was hitting about 105miles. I think mine reads roughly 5mph too fast at highway speeds, less at lower speeds. there is no set amount. I like it this way, and i wont be changing it. I can speed and not get tickets (cause im not really speeding)

Dr.

That would never work for me because I would just figure that in and go those 5 MPH faster!

 

[140CILX-Coupe]

the gear could be worn out and read less than or more than it should...the cable could be rounded a lil bit and is slipping inside the gear.

my old 77chevy truck I had the speedo would rear all sorts of ****ed up and it was cause of the gear box the 2 cables came from. it was causing the cabel and teh gear to get ****ed up.

 

[Wannabe2.3T]

Yeah except that the second number (60) is a percentage of the first number, so it does alter the overall profile of the tire. A 205/60 isn't the same profile as a 225/60 because 60% of 225 is more than 60% of 205.

So the conclusion is basically to give up on ever figuring out how fast I'm going?

Is that why my 275/40/17's look about the same profile as my 245/45/17's?
That's weird, good to know, i never knew that. I've got a girl tellin' me about cars will you mary me???