t4 vs t5 trans.

[shanebo]

I am in the process of trying to scora an 81 sx4. It has a 4 speed. I was just curious as to how much more top end the 5 speed gives you.

[Sunny]

It depends on the gear ratio and power of your vehicle.
Theoretical increase in top end depends on gear ratio, actually getting there depends on the power of your vehicle.

Because the T5 is an overdrive transmission, it's 5th gear is less than 1.00. So it effectively lowers [numerically] your gear ratio in your differentials while in fifth gear, vs a 4 speed where the final gear is 1:1.

Say you have 3.73's, and you have a 4 speed.
With a 5 speed with a 5th gear ratio of say.. 0.73 [T5 ratios have a huge final gear range depending on application] You would have the theoretical top speed as if you were running 2.72's. Which is a huge drop.

A good way to figure out exactly would be to use something like this http://www.catherineandken.co.uk/sti/tyres.html as I don't know your tire size, what T5 ratio's you'd be looking at, your diff ratios, etc.

Being that Eagle's generally have lower [numerically] gearing to begin with, I don't think you'd see much benefit to a 5th gear if top speed is your concern, without a huge boost in horsepower. The vehicle is going to have to work that much harder once you go under a 1:1 ratio. My ex's Honda wouldn't even move in 5th gear, even with the pedal to the floor. It was basically a "economy" gear for the highway. If you wanted to speed up you'd have to downshift.

[Whuntmore]

didn't know the T-5 was an overdrive tranny, but that makes sense.

Probably whatever rpm you're running at in 4th gear, say 75 mph, then you're gonna drop quite a few rpms in 5th.

My grand am at 60 mph, will go from about 2100 rpm in 4th, to about 1700 in OD.  At 75 mph, it's just slightly under 2000 rpms in OD.

big saving on fuel on the highway. 

I have no idea how much 'top end' you'd get.  By 'top end' I'm guessing you mean 'top speed' you can get outta the car?

[captspillane]

In 81 the T4 wasn't available. That came around in late '83 I think. That SX4 has an SR4 transmission. Its externally similar, has the same input shaft and bolt pattern, and is made by the same company. The difference is the shift lever and weaker torque rating.

If you buy an Eagle with an SR4, immediately scrap the transmission and replace it with a T5. I've broken 6 of those shift levers. They will break on you and leave you either stranded or on the side of the road trying to limp it home with a big screwdriver. The T4 and T5 shift levers will not break because they have a robust ball where the SR4 has a fork with half its thickness notched out on one side.

The good news is that the Jeep T5 is the cheapest and most plentiful transmission out there. They're easy to rebuild and the three I've bought cost me less than 150 dollars off craigslist. You should never spend more than 200 on a T5, unless its been professionally rebuilt. 600 is a reasonable cost for a rebuilt T5. Thats approximately half the cost of any other option and the T5 is strong enough to live behind a 304 V8 or fuel injected 4.0.

If the Eagle was a 6 cylinder from the factory it will have obnoxiously high gears in its axle. The four cylinders have the ideal and very rare 3.54 ratio. If you tell me if its a 6 or 4 cylinder and what tire size you want (I use 235s, most people use 215 or 205s) I have a spreadsheat I made that shows exact speed vs RPM numbers for comparison.

At 2500 RPM in 5th gear and 6 cylinder 2.35 rears, 215/75 R15 tires, you will be going 98.82 mph. On level ground the 258 does not have enough power to ever realize that speed. In 4th gear you will be going 84.98 mph. At 2500 RPM in 5th gear and 4 cylinder 3.54 rears, same tires, you will be going 65.41 mph.

Changing to my favorite 235/75 R15 size gets more favorable shift speeds of 68.20 mph versus 65.41 mph with the 215s.

By the way T4s and T5s both have the same gear ratios 1 through 4th with 4th being 1.0 direct drive. The T5 adds a 0.86 overdrive.

I drive between 75 and 80 mph consistantly. The 258 needs to be above 3000 rpm to have enough power to maintain those speeds. I chose 2500 rpm as the comparison RPM because that is the lowest speed you can shift into that gear while the engine is under load. In other words, 65 is the speed I shift from 4th to 5th and then the engine is happy in 5th at 75 mph. A minimum speed of 98 mph for 5th gear is ludicrous, but that's exactly how my 83 SX4 came from the factory. AMC must have chosen those gears as standard to complement the standard T4, and the T5 upgrade was an afterthought.

[shanebo]

Thanx for the input fellas....Ive never had the liberty of owning a standard eagle...I never put much thought into them until now...All I need now is for my truck to sell so I can buy this sx4.

[BenM]

The T5 has several 5th gear ratios available, from about .83 to .72, so you do need to watch that, but close to 2000 RPM seems to be a good spot for economy on a carburetored 258.  That's usually a 3.54, 3.31, or a 3.08 axle ratio depending on tire size and overdrive. I've heard the 5th gear swap isn't so bad.

3.31 is really hard to find, but you could get by with a 3.08 on a smaller car. If you don't enjoy shifting, go to the 3.54. With the Pacer I can squeal the tires with a 3.08 and 3:1 first, and the T5 is close to 4:1, so it's going to go.

A lot of standards came with a 2.73 ratio, and those can be swapped to a 3.08 or lower. I think that 2.73 is good with an SR4 or automatic, but difficult with an overdrive.

I like This Calculator for playing around with the numbers.  If you compare against the Cherokee with the stock 3.54, 225/75r15, and an AW4 or 3.08, 225/75r15, and a T5; you can good comparison.

(Note: depending on rounding, printed ratios can vary .01 or .02 from each other)

[shanebo]

Eggs for an awesome link Ben....thats really cool...Makes the math of this stuff alot simpler