Which gears to get?

[TheFleshRocket]

I've been trying to decide which gears to install to replace the factory 2.73s in my '89 GT vert, and I made up this chart to help me decide:

- - - - - - - - - - - @70mph @140mph, Redline @5500rpm @5000rpm
2.73 = 0% shorter, 2000rpm, 4000rpm, 210mph 192mph 175mph
3.27 = 20% shorter, 2400rpm, 4800rpm, 175mph 160mph 145mph
3.55 = 30% shorter, 2600rpm, 5200rpm, 161mph 148mph 134mph
3.73 = 37% shorter, 2740rpm, 5480rpm, 153mph 140mph 128mph
4.10 = 50% shorter, 3000rpm, 6000rpm, 140mph 128mph 116mph

Obviously the 2.73s are ridiculously tall. Before I'd made this chart, I figured that the 3.27s would be too tall as well, but depending on how much power my car makes, they might end up being just right. With the mods done to the car now, I'm estimating that it puts between 280-300hp to the wheels. (I'm going to the dyno in a couple of weeks with a buddy of mine, so I'll know for sure then.)

The motor feels like it makes all of its power from 3000+rpm, with the biggest boost coming from about 4800rpm on up. I'm going to wager that the HP peak is around 5500rpm, which would equate out to about 160mph with 3.27 gears. I figure putting around 300hp to the wheels should be enough to get me up to 160mph in fifth with 3.27s. In addition, if I end up bolting an S-trim to this motor, I figure that'll put me at around 400rwhp, which should be enough to easily put me up to a redline-limited 175mph.

Basically what I'm looking to do is spice up the car's acceleration but not go with gears so short that my top speed is redline limited. Now granted, I won't be spending much time at all near the car's top speed, but I still like its potential to be there.

So you guys who have gone with 3.27s, what do you think of them?

And of course, anyone else who would like to comment on my logic here is encouraged to do so.

 

[TrophyHead]

http://www.corral.net/tech/gearcalc.html

I would go with 3.55's if you want the top-end speed. Do you have a auto or stick?

 

[TK_05]

3.73's stick or auto... either way

 

[willys1]

I just put in 3.73's from the lame 2.73's also.Its pretty good on the road, 60mph=2100rpm/ 70mph=2300rpm/ 80mph which I usaully do puts me at 2500 rpm,,never had it past 2700rpm-YET!!

 

[90mustangGT]

Just go with 3.73's, you won't regret it. You'll be in around 110-115mph at the top of 4th gear. Although 4th gear with 3.27's would top around 130-something, with 3.73's 5th gear won't be such a dead gear.

 

[TheFleshRocket]

I've got a 5-speed.

I checked out the gear/rpm/speed calculator at corral.net (very helpful, thanks TrophyHead!) and if it's accurate, then my speedo and/or tach aren't.

I calculated the following speeds:
1st @ 3000rpm = 24mph
2nd @ 3000rpm = 40mph
3rd @ 3000rpm = 60mph
4th @ 3000rpm = 80mph (from memory, I didn't try it out tonight)
5th @ 3000rpm = 105mph (extrapolated from actual reading of 2000rpm @ 70mph)

According to corral.net's calculator, assuming 225/55R16 tires and using the T5 (1985/9, 5liter) trans:
1st @ 3000rpm = 25mph
2nd @ 3000rpm = 43mph
3rd @ 3000rpm = 65mph
4th @ 3000rpm = 84mph
5th @ 3000rpm = 123mph

The differences are pretty minor (only a few %) in the lower four gears, but the difference in 5th is HUGE. The numbers from my own personal experience seem to jive with the gear rations the tranny would actually have. I can't imaging 5th being 50% taller than 4th gear--NO WAY!

I'm going to play around with the corral.net calculator a bit more to see if I can figure out what the deal is.

 

[TheFleshRocket]

I tried the T5 (1985/6 SVO) and its numbers came out much closer to what I've measured:
1st @ 3000rpm = 24.0mph
2nd @ 3000rpm = 39.3mph
3rd @ 3000rpm = 61.8mph
4th @ 3000rpm = 84.1mph
5th @ 3000rpm = 107.9mph

So either the numbers corral.net has for the T5 (1985/9, 5liter) and the T5 (1990/93 5liter) are off or my car has a T5 (1985/6, SVO) tranny in it.

 

[TheFleshRocket]

So I decided to punch in the numbers for the 3.27 and 3.55 gears using the T5 (1985/6, SVO) tranny:

3.27
1st @ 3000rpm = 20.0mph
2nd @ 3000rpm = 32.8mph
3rd @ 3000rpm = 51.6mph
4th @ 3000rpm = 70.2mph
5th @ 3000rpm = 90.0mph

3.55
1st @ 3000rpm = 18.5mph
2nd @ 3000rpm = 30.2mph
3rd @ 3000rpm = 47.5mph
4th @ 3000rpm = 64.7mph
5th @ 3000rpm = 82.9mph

It looks like the 3.55s will essentially make 5th gear act like 4th did with 2.73s and 4th gear act like 3rd did with 2.73s. Since my car will already wind out 4th gear, the 3.55s will be too short. The 3.27s should bring the revs up enough in the upper gears to make acceleration snappier without being so low as to artificially limit my top speed.

I think I'm going to get ahold of a GPS unit and mount an aftermarket tach so that I can be sure that my RPM and MPH readings that I got from the in-dash gauges are accurate before I commit to a set of gears, but at this point, 3.27s look like the way I'm going to go.

 

[mike93lx]

top speed is something i wouldn't even consider. have you ever tried to drive your mustang above 130? besides that it's not worth the risk for the quick thrill (unless of course you're on a track) our cars aren't exactly the most stable and confidence inspiring at that speed.

worry more about 4th gear. think about what speed your car will hit in the quarter and make sure you will at a good rpm for that speed in 4th.
i.e. my car likes the top end and it'll run (hopefully) just over 110 n/a in the quarter. 4.10's will give me about 117 at 6,000 in 4th which will have me close to redline as the motor is right now and give me a little room to grow

personally, i am worrying more about 1/4 mile performance than highway engine speed and top speed. i love to drive fast, but getting that extra tenth or two is more important to me than being able to get my notch to 150mph.

just my $0.02

 

[FreeGT]

"Potential to go there"????

I mean we've all topped out the 5.0 on the freeway, but how many times do you do that versus going to the dragstrip, its like a 1:30 ratio. I mean i'm sure anything bigger will feel like an improvement over 2.73's. Is that brief 160mph run worth it if it means a lack in performance? Just do what ever is more important to you. From the people I talk to that got 3.55 instead of 3.73's they wish they would of gotten the 3.73's. Not much RPM difference, but if you think 3.73's are too steep, I still wouldn't go to anything (numericaly) lower then 3.55s. One other thing to consider is the blower you plan on adding, I've never launched with a centrifugal, but it's something to consider with high gears. I'm putting kennebell on in a week, and I'm already regretting those 3.73s with full boost at 2,500RPM.......
This is all assumed unless you drive 50 minutes on a highway daily, but who goes over 100 miles and hour daily, anyway..........

Don't sweat it, if you don't like it, new gears are cheap.

 

[FreeGT]

Another minor opinion that I forgot to add, Mike93Lx pointed out, racers will pay ANYTHING for a cheap tenth or two. So are you racing this car for grins at a test and tune, or a competing weekend warrior?

 

[TheFleshRocket]

I will probably take the Stang to a 1/4 mile dragstrip on a couple of occasions to find out "what she'll do" but this is by no means going to be a drag car. (Mainly I'm just concerned about big-pen0s bragging rights by being faster than my friends which I'm pretty sure I've got accomplished even with 2.73s.) Most of the track time will be spent autocrossing, with perhaps a roadcourse day or two thrown in the mix.

The main reason I want to go with shorter gears isn't for improving acceleration, it's to make 5th gear a "usable" gear. With 2.73s at interstate speeds, 5th gear is fine for cruising, but any sort of acceleration requires a drop to 4th gear. According to the corral.net calculator, my GT is turning 2224RPM in 5th gear at 80mph.

Here's some more numbers from the corral.net calculator:
2.73
5th gear @ 80mph = 2224RPM
4th gear @ 80mph = 2851RPM

3.27
5th gear @ 80mph = 2664RPM

3.55
5th gear @ 80mph = 2892RPM

4th gear with the 2.73s provides plenty quick acceleration for most purposes on the interstate. 5th gear with 3.27s is less than 200rpm below 4th gear with 2.73s. That should make 5th gear plenty "usable".

The 3.55s would actually put 5th gear a few RPM higher than 4th with the 2.73s. So, to approximate interstate driving with the 3.55s, I could just drive along in 4th with my 2.73s. I don't care to have gearing that short.

 

[TheFleshRocket]

"Potential to go there"????
I mean we've all topped out the 5.0 on the freeway, but how many times do you do that versus going to the dragstrip, its like a 1:30 ratio.

I won't be hitting the drag strip nearly as much as some of you guys do; this car is primarily a daily driver.

As far as speed potential, that does matter to me. I've hit 183mph indicated on my '03 GSX-R1000 once, and the bike has seen 160+mph indicated pretty regularly when I make backroad runs. Obviously the Stang won't get up to those speeds anywhere near as quick as the bike will, but I'd still like to be able to do it. Say, for example, if the guy I know with the souped up 3000GT VR4 and I are cruising along the interstate and decide to put the pedal down--I don't want to run into redline and then just watch him walk away from me.

 

[TheFleshRocket]

top speed is something i wouldn't even consider. have you ever tried to drive your mustang above 130? besides that it's not worth the risk for the quick thrill (unless of course you're on a track) our cars aren't exactly the most stable and confidence inspiring at that speed.

Actually, I've run it up to a bit over 5000rpm in 4th gear. (Yes, this is being measured on the stock tach, so I'm sure there's some margin for error.) According to corral.net, 5000rpm in 4th is good for 140mph, and the car felt stable at that speed. At that point, I shifted into 5th (I dunno why I shortshifted) and that's when acceleration went from "still pretty impressive" to "a whole lotta nothing".

Currently, the suspension is stock, but over the next couple of months, it is getting:
--MM strut-tower brace, K-member brace and full-length subframe connectors
--Tokico 5-ways and MM roadrace springs
--17" Cobra Rs with W or Z-rated rubber
--MM torque arm and panhard bar (most likely won't see those until late spring)

At this point, I'm reasonably confident that those mods will make the GT into a reasonably decent-handling car.

 

[mike keirstead]

Dont worry about all those calculations. Just get 3.73s and be done with it. Get the calibration gear. It wont be that bad. My car is street/strip. I drive it from Canada to Florida all the time for work. I have 3.73s and I cruise on the highway with a Z-spec t-5 and 275/50/15 tires.

It revs around 2300-2400@ 73-74 mph.(autometer tach)

Smaller tires would raise yours rpms a little. I still get 25 mpg on the highway.

Why get anything lower than 3.73s.

I had 3.55s for a few years and I was very happy when I got the 3.73s.


Like I said my car is a street/strip car but I drive it all over the country and I have no complaints.

If you are worried about top speed the 3.73s will pull 5th awsome.

My speedo is snap on. I dont know how fast my car will go but I know I can bury the needle way down. 150-155 I am guessing. It wasnt done pulling. I just got off it because I had SKINNY DRAGLITES in the front and it was starting to wander a little.

Just get the 3.73s. You will be happy. 3.55s are still a little sluggish in 5th even with good power trust me.

How fast are you going to drive up and down the highway.....1500000 mph.

Leave that to the Porche's,Viper,Ferraris, of the world. Mustangs are not meant to be driven over 160 or so. I dont know if you have ever been in a stang at 160+ but I have on the highway and it's not exaclty a smooth ride.

Dont get the 3.27s.....dont get the 3.55s. The 3.73s are an awsome well rounded gear. They will give you lots of top speed. If you motor pulls to 5500 then thats plenty to pull 5th hard.

Put the calculator AWAY.

 

[TheFleshRocket]

Dont worry about all those calculations. Just get 3.73s and be done with it. Get the calibration gear. It wont be that bad..

I checked out your page. Your Stang has almost the same mods as mine (not the exact same parts now but the same parts have been upgraded) and if you're turning a 12.58@108 with 3.73s in a GT, then my heavier convertible with 2.73s should be good for low 13s as is. That's good news.

I wanna know how you get 25mpg on the highway--that's better than the factory's 17/24 EPA rating, and a good bit better than the 20mpg I see when doing 75-80mph with my long 2.73s.

Since you're so gung ho about the 3.73s, I might give 3.55s a chance instead of 3.27s.. but I'm sure 3.73s will be too short for my tastes. With the 3.55s, I'd be looking at a redline-limited top speed of right around 160mph. I don't want to cut my top speed any more than that. If I'm wrong and the 3.55s aren't short enough, hell, I can just sell them and get 3.73s anyway.

 

[mike keirstead]

I was running 12.58@108 with stock heads and a gt-40 intake. I expect Low 12s with the combo that is posted on the site. My car is set-up for drag racing. The chassis is done up right. Its set-up for the 10s right now.

I have no idea why my Mustang gets 24-25 on the highway....but it does. I guess its tuned really good. Its also lighter than it was from the factory. It weighs 3050. So that must have something to do with it. Its 200 pounds lighter than it was from Ford.


Having a car that will do 160+ on a CALCULATOR and having a car that will ACTUALLY do 160+ is to different things.

Like I said my speedo is correct and my car will go over 160. I had it going 150-155 and it was STILL PULLING.

HAVE YOU EVER HAD YOUR MUSTANG GOING THAT FAST. Or do you just want to tell people it will do 160+.

3.55s will leave 5th still a little sluggish. TRUST ME I HAD THEM. My car pulls alot harder in 5th now than it ever did.

Get the 3.73s and a little higher tire and you will go as fast as you ever need to.

 

[TheFleshRocket]

Well so far I've only hit about 140 and that was a short run on a deserted 4-lane at 2AM. I do intend to get it up to 160+ but yeah, I wouldn't mind that for bragging rights either.

5th gear with 3.55s is like 4th gear with 2.73s, and I've been more than happy with how hard fourth pulls. In fact, I've seen over 5000rpm on the tach in 4th and the car was still pulling strong. With 3.55s, I'd almost be at redline, with no more gears left. Of course, I've heard that the factory tachs often read high, and I doubt mine is any different. I guess I'll have to get an aftermarket tach so I know what the engine is really doing before I come to a conclusion.

 

[mike keirstead]

Yes the stock tachs are notorious for reading high in the upper rpms.

You will have all the bragging rights you want with 3.73s. And your car will scat through the gears. Your car may not pull 5th with 3.55s. Trust me I had them. My car will go everybit as fast now as it would then. And it gets there quicker.

 

[TheFleshRocket]

Your car may not pull 5th with 3.55s. Trust me I had them. My car will go everybit as fast now as it would then. And it gets there quicker.

If your car's top speed isn't redline limited with 3.73s or 3.55s, then yes, I'll agree that the 3.73s will definitely get the car to its top speed quicker.

Let's assume that my GT is making 300rwhp at 5500rpm. Which would equate out to around 350hp at the crank. (The actual number should be very close to that--no less than 280rwhp IMO.) That's enough to easily push the car to 160mph, assuming gearing that puts the car close enough to the power peak without going over.

Using the calculator again (even though you don't like it ) shows the following:
3.55s
160mph = 5784RPM

3.27s
160mph = 5328RPM

3.73s
157.9mph = 6000RPM

The 3.27s are a tad bit close to the theoretical 5500rpm power peak. Keep in mind that most engines' powerbands fall off pretty quickly once you get beyond the peak. So the engine is probably making more HP 200rpm below the HP peak than it is 200rpm above it.

In addition, if I do add a blower to the car, the engine will have enough legs to reach 180mph at 6000rpm in top gear with 3.27s. With 3.55s, the engine will be out of room when it hits 6000rpm at 165mph.

I suppose what I will probably do is go for 3.55s now, and if I end up supercharging the motor, I'll go with 3.27s so that I can make use of the higher top speed.