stevenb

I got a t5 and I am looking to upgrade to a bigger numerical gear. Not sure if I should get a 4.10 or a 4.56 but does it matter about the transmission?

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iTrader: (14)

the trans doesnt care, but you need to swap the speedo gears in the trans so the speedo will be right

Speed Racer

iTrader: (1)

I just put 4.11s in mine, and I love it. It's really awesome- especially if you have sticky tires. I got frustrated with the speedo calibration problems too, so I got an Autometer electronic programmable speedo. Once calibrated, it's perfectly accurate no matter what gear or tire size I decide to run. Good luck and give those deep gears a try. You'll really start to love stop lights!

AlkyIROC

Depending on the transmission, you can have too much gear in the diff. Running a 700R4 with 4.56 gears means you have a effective first gear ratio of 3.06 x 4.56 = 13.95. That's a 4x4 stump pulling gear. Your first gear would be useless on the street.

Guessing that your 87 T5 has a 2.95 first gear, that gives you an effective ratio of 13.45 with 4.56 gears. You'll probably still overpower any tire you put on the car and you'll hardly use first gear driving around. At least 4.10 gears brings it down to a slightly more reasonable 12.1

A good street gear is 3.73. It'll give a ratio of 11.0. It will still be fun to drive and won't run into the real high rpms at highway speeds. There's nothing really wrong with your 3.42's with a T5 and the 10.0 effective ratio.

racing geek

iTrader: (5)

So with a 700r4 with a first gear of 4.06 (is that right?) and 3.73 rear gears, I will have an initial first gear of 15.1438? Is that good..?
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AlkyIROC

700R4 has a 3.06 first gear.

The 700R4 is a great street transmission. Using a good highway gear set in the diff such as 2.77 or 3.27 gears, the 700R4 give a low first gear to get the car moving quick enough. Once into overdrive coupled with the highway gears in the diff, the engine almost idles going down the highway.

As a performance transmission, the gear ratios are all wrong. Putting a deeper gear in the diff makes first gear create a ratio that's too high. Having too high a first gear ratio is good for a very heavy vehicle but in a lightweight car, it means you'll be shifting out of first gear within seconds because the engine rpms will climb up so fast. The gear spread between first and second is too great for proper WOT shifts.

My powerglide uses a 1.76 low gear and I have 4.86 gears in the diff. That gives me a starting line ratio of 8.55 but I also have a lot more torque to multiply than a typical SBC.

antman89iroc

I agree with Stephen. I had a 700 and a 3.7 rear and 1st was usless. It would shift to second before the car moved 1 length. At the track (1/8 mi) it ran the same time with the 3.7 as it did with a 3.27.

I now run a T5 and 3.27 rear gear. Perfect combo for a daily driver that sees occasional track use. 1st gear still requires a bit of throttle control to keep the tires planted on the street. 2nd is your pulling gear where you can really lay the power down. I drive 30-40 miles a day and the engine runs around 1500 at 50 in 5th.

At the track I launch at around 3000 and leave easy. It pulls down to around 1500 and I cross the traps at the top of third (1/8 mi). With a mild build and a 200 shot of nitrous it'll run a 7.98 @ 93mph. Sould work out to a mid 12 second car. And with a modest rear gear I still have a daily driver with A/C and cruise. Getting around 17 mpg too.

stevenb

Which rear end gear do you recommend for my car? To be honest my gas mileage is already horrible and even throughout the summer its not a daily driver so I dont mind losing more mpg for better performance.

Oh, and also I don't believe the 3.42 is right...I have a thread about figuring out which one mine is...I'm leaning towards the 2.72

AlkyIROC

Providing the gears or diff is still original, a 1987 SC 2.8L was only available with 3.42 gears. The only way to know for sure is to pull the cover and count the teeth or get the numbers off the ring gear.

3.42 isn't a bad gear. 3.73 is a big better for performance. I wouldn't go to 4.10 unless you run a 28" tall tire and or have a shift point in the very high 6000 range. You could put 4.56 gears in it and it would be a fun stoplight to stoplight ratio but highway driving would be almost impossible.

tylersb350

ive got a 700 right now behind a 430 hp 355. the rear gear is 2.93 i want to put a 4.10 in it. i run a 275-60-15 which is 28 in tall. i can rev to 6000. how bad would my 1st gear be.

suh_wheat_86305

iTrader: (1)

i am looking for a nice 3.42 or even 3.73... but, i see a really nice 4.11 for sale, but that would be too much, yes? Mine is a daily driver and I enjoy acceleration, like with 3.73... see my sig, right now it's 2.77, which suck a**. If I could find a really nice 3.42 or 3.73, then i'd buy it.... I'm really sick of the 2.77... somebody talk me into the 4.11... mmmm (4.11 - 2.77)/2.77 = 48%, currently pulling 2200 rpm at 80, so 2200 x 1.48 = 3256 rpm... that doesn't seem too rediculous...? think about the stop lights... uuuhhhhggguuughhhh

kennn

FYI, I currently have a 2.95 T5 and 4.083 rear end. It's really annoying because the first 3 gears are really useless. The rear end is too short. I'm in gear for a second or two and then I bounce against the rev limiter.

My 0-60 time probably suck because I spend so much time shifting.

I'm currently getting a 3.133 rear ratio. I'm hoping it will be much better for me.

Take it for what it's worth. If you're thinking about going up in the rear, think carefully.

For reference, this is my torque curve.

racing geek

iTrader: (5)

So what is the ideal rear end ratio for a 305 TPI and a 700r4?
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antman89iroc

I don't want to recommend any ratio because so many variables come into play. What is "ideal" to one person may not be to another.

I am running a mild 5.7 with T5 and a 3.27 gear. It is perfect for everyday. At the track a 3.70 may be better. If I had to do it over and looking more street/strip I'd probably go 3.45.

Looking at your engine specs I think a 3.4x would be a good starting point. I had to try different ratios and use them in various operating conditions to really see what I liked/disliked about each. So far I've had a 2.77, 3.27 and 3.70 ratio. (My rear is a 9 bolt but the same basic ratios are available for the 10 bolt if that's what you have) Remember that low, low gears can actually be slower if you can't get traction. Also, bear this in mind. When driving around town if you have a 3.70 rear gear, for example, you'll shift out of one gear quicker leaving you accelerating in a higher gear. It "feels" the same as having a 3.4x ratio.

kennn

If anyone is interested, I can do a full writeup on the effects of gear ratio and torque to the rear wheels.

stevenb

Why would the 4.56 be better off than the 4.10?

kennn

It's fun for stoplight to stoplight because you have more torque multiplication to the rear wheels and the engine will reach redline sooner so you'll have to shift more within the same distance = more fun I suppose.

But you will be wasting time shifting, and you may overpower traction if you have a lot of torque.

You can run as high as 5 or 6 in the rear, but at that point you would need like 12 gears and you would waste so much time shifting that any benefit in torque multiplication would be diminished.


With my 4.083 I have to run 3rd gear all the way up to 4600 RPM to get 60mph. With my new 3.133 gear, I can hit 60 at the top of 2nd gear, thereby saving myself 1 shift.

You can have 1st run all the way up to 60mph. You don't have as much mechanical advantage but you save time on shifting.

Or you can redline every gear up to 6th to get 60mph. You have maximum mechanical advantage but you waste time on shifting.

The ideal situation is somewhere in between, where you balance torque multiplication with shifting time. So maybe 1 or 2 shifts are reasonable to get to 60mph.


It really depends on your torque curve and what you're trying to do. A big block monster may be better off running 1 gear or maybe 2 at most to get good 0-60 times, and an economy car may need 3 or even 4 gears to get the best 0-60 time.

Again, 0-60 is just an example. Replace with 1/8, 1/4, or whatever type of racing you want to do.

stevenb

But what is best for my setup performance wise? I want this car to be as fast as I can get it. I've put 2500 miles on this car in over a year...I don't drive it often and I don't care about MPG.

kennn

Answer these questions for me and I'll answer your question above for you.

1. What is your "setup"? What does your torque curve look like? What is your redline and rev limiter? How high are you willing to rev your motor? What ratios are in your transmission?

2. What do you define as "performance"? What kind of racing do you like to do? Do you like to drag, autox, or roadrace? Or do you like to do top speed pulls on the salt flat?


Your engine and transmission affect what kind of rear end you can put behind the car. Plus the type of driving you like to do also affect what you should use.

This is a hard question to answer without knowing all the facts. Even then it's still more art than science.

stevenb

I have not gotten a torque curve and I do not have a rev limiter and generally shift before 6500 rpms, which is my redline. I intend to do a bit of both roadracing and dragracing. I know nothing about my trans except that it is a t5.

kennn

I find that hard to believe but ok.

This means you can go a little numerically higher and won't run out of gear too quickly because your redline is high.

Hard to say without knowing if you have traction problems. Roadracing is totally different and depends on the course.

Can you use the T5 FAQ and figure it out? Or you can measure RPM and speedo to approximate your gears.

It's really hopeless without knowing your actual gear ratios. Otherwise it won't make any sense to haphazardly swap in rear end ratios.

antman89iroc

Do you know the duration @ 0.050 on your cam? If so, we could estimate an RPM range for your combo. That would certianly help. Then, depending on the intended use, you can pick a gear. If this is a street-street/strip car, one you drive for fun and occasional 1/4 mile blasts, I would start with a 3.7x and go from there. No one can tell you the "perfect" gear for you car cause no one but you knows what you seek. As others have stated, what I call performance may differ from someone else. Every decision you make when making modifications is a compromise. It helps in one area and hurts in another. It's ALWAYS personal preferance.

If you are going to run 1/8 mile drags and the lowest time is your 1st proirity, then you may want a 4.1x ratio. But the motor will be buzzing if you drive at highway speeds.

If you're going 1/4 mile, then a 3.7x may work better. It will be easier to live with around town.

Another thing to consider is this. It seems that an engine always makes a little less power, at a little lower RPM then one expects. Yeah your redline may be 6500 but are you making usable power there? You gotta have expreiance at the track and possibly some dyno time to really "know" what you need. So all we're trying to get you is to a starting point with what you say you want.

If it turns out that you do have a 3.42 ratio in the car now, by all means use it! Get some track time and see how you like driving it around. Then you can better answer your question for yourself than any of us can ever speculate. Sorry to dissapoint but there is no "perfect" gear ratio. It's your best, educated guess and trial and error.

zippy

Alot of good reading here. I'll post up my opinion for the hell of it. With a 700R4/4L60 I recomend a 3.23 gear for a daily driver, 3.73 for a weekend toy, and 4.10 gear for a mostly raced vehicle. This is for naturally asperated vehicles. The cam size really doesn't matter that much for the gear, but converter does when it comes to the cam. I'll show my example.

LG4 car
4.10 gear
25.7" tire
700R4
4650rpm@86.75 with TCC locked in 3rd
My LG4 trapped 87.xxmph with a 3.08 gear and shifted around 4600-4800rpm.

Same car with a mild cam and head change would net you around 95mph trap speed. That would put you at 5100rpm crossing the traps in third with the converter unlocked. a mild head and cam LG4 would likely be shifting around 5200 or so which means you'd be crossing at around the correct rpm.
As you raise the camshaft duration, you also raise the pwer level and hence the rpm that you'd be crossing the traps at which takes care of itself in the rpm department. From there you just change converter and tire height as needed.

Now for a street car it's important to not have too much gear to be shifting too often or just killing the tires. It also matters which trans you have as to how much overdrive the trans has. If overdrive is a .67 for example you could use a bit more gear and still get decent highway milage. With a .78 overdrive on the other extreme you would want a bit lower numerical gear to help the highway rpm out.

Boost and nitrous don't apply the same. The more boost or nitrous you use, the lower number gear you run.

kennn

I have a T5 and 4.083 rear gear. My torque curve is above.

It's absolutely horrendous in the first 3 gears. I may as well launch in 2nd and shift straight to 4th.

1st is absolutely useless because I'll spin horribly and hit redline in like 1 second. 2nd and 3rd are not as bad but still go by quite quickly.

4th is okay and 5th is unbearable on the freeway.

I don't have that much torque to the wheels so I can't imagine what a real "race" car can do with this ratio.

I'm going to 3.133 so I don't have to worry about wasting time shifting so much.

stevenb

Sorry I meant streetracing and drag racing

kennn

For you I would say try a high 3 first, like a 3.8 or 3.9 if you can find it, before going to a 4.1 or higher.

Like I said above, I have a 4.08 and it's really hard to get good acceleration because I'm always shifting. Gears run out too quickly.

Especially if you're on street tyres on the street. You can't hook as well and the extra tall gear will just make you spin more and bounce off the rev limiter.