Rear end gear reccomendation

tabester · #1 (**permalink**) 03-21-2003, 06:41 PM

Is a 3.27 gear usable for a slightly overstock 302. I won't be doing any racing just want to be respectable.
Kirk

Steve R · #2 (**permalink**) 03-21-2003, 07:49 PM

Forget respectable. You want a nice low milage duel ovoid type with the single thong option.

kountzecobra · #3 (**permalink**) 03-21-2003, 08:31 PM

3.50 without overdrive and 4.11 with overdrive works well.

tabester · #4 (**permalink**) 03-21-2003, 08:48 PM

With a Cobra I sholdn't have a problem with the thong wearing type. Right? Just wasn't sure what the 3.27 would do. Does that equate to low speed or high RPM's. I do have overdrive though.

Kirk

cabo · #5 (**permalink**) 03-21-2003, 11:13 PM

I have a 302 that puts out about 290 HP. I have a T-5 transmission with a 3.73 rear end. Works really well. But that is because the 5th gear is like an overdrive. I agree with David
Shelton; however if you want more acceleration go with the lower gear. You have plenty of power in a very light body.

Bob In Ct · #6 (**permalink**) 03-22-2003, 06:42 PM

It's always nice to be able to cruise down the interstate at 65MPH turning about 2000 RPM.

Bob

Ron61 · #7 (**permalink**) 03-22-2003, 07:22 PM

Kirk,

I am running the 3:27 gear and have no problems at all with it. A friend of mine with a slightly built 302 is also running it and he likes it. My ony problem is I messed up my transmission split and got a .6 and shoud have gotten a .8 as the .6 kicks the ratio up to 2:10. I can cruise at 70 MPH at 1900 RPM but can't get past the wall of air at 110 to even get into my power band. These cars are not aerodynamic and a big block can push them through that wall of air. I have had mine just over 100 in 3rd gear and it was still pulling. I chose the 3:27s because of the cars weight and my power and it has worked out great on the street and track as I never use 5th on the track. I also don't know what I could top out at but it would be around 145-150 max. So unless you are really interested in that quick acceleration and can get the power to the ground, for just normal driving I think the 3:27s are a good choice. I skipped the 3:55s because with that low of a gear my 1st and 2nd gears would be worthless unless I could really get the tires to hook up coming out of the corners. I still break loose in 3rd on most corners if I get into it to hard. The posi-trac is a must for me. It helps straighten the car out under throttle.

Ron

Cobra20646 · #8 (**permalink**) 03-24-2003, 06:42 AM

Kirk,

Ron61's advice is more on target....... it's not just the rear end gearing, you MUST also consider the transmission gear ratios and spread of the gears, also refered to as wide or narrow.
I remember that years ago, we used to look at an overall ratio of about 9.0 to 1 for first gear/rear end gear. For example, a 4.10 rear with a tranny first gear of 2.2 would equal 9.02. When that number starts getting larger, your first gear starts to wind up to red line real quick...... a lot of guys running a t5 with 3.7 gears don't even use first.
- Jim -

klayfish · #9 (**permalink**) 03-24-2003, 07:36 AM

As everyone said, the trans gear ratios are just as important.

My car has a stock carb'd 302, with a T5 trans. When I bought the car (it was already built), it had the stock 2.73 Mustang gears in it. It was dreadfully slow in 4th gear, and 5th was useless below 75 mph. I put 3.73s in the car. Yes, 1st gear now winds out pretty quickly, but I like the acceleration it gives (I have sticky 275 17" tires on the back, so wheelspin isn't much of a problem). 5th gear is now useable, and I can cruise comfortably on the highway all day long. But my 5th gear is a big overdrive gear. If you don't have overdrive, you may want to think in the 3.27-3.50 range. I'd go for 3.50 just for the extra kick when you hit the pedal. You'll also find that the 3.27s and higher will really lose acceleration in the high gears. Even now with my 3.73s, my acceleration in 4th and 5th isn't what I'd call supercar fast. Damn aerodynamics!

Steve

Junket · #10 (**permalink**) 03-24-2003, 08:50 AM

Kirk If you know your trans ratio try this site you can plug all the # in and http://www.smokemup.com/ will give all the RPM & MPH Something more to look at if you dont have the trans. # have the site for that some place also just let me know.
Ken

wyocobra · #11 (**permalink**) 03-24-2003, 10:43 AM

Being very poor at math, I finally went to the "pros" Bought my kit with 3.73s in 9inch ford. Actual driving with Chevy Turbo 400 automatic behind 454 Chevy. What a rush! Unreal acceleration, but tachs at 5100rpm at 75mph. Currie Enterprises put my numbers in their "tach computer" and recommended 3.00s for me. Should yield 2150rpm @ 75. Haven't tried the refit yet but should work for my daily cruising needs. If in doubt, give them a call with your particulars.
(714)528-6957. They were a great help for me.

bmercer · #12 (**permalink**) 03-24-2003, 12:13 PM

wyocobra

My math skills aren't too bad and my instincts are telling me there is something wrong here. Let's, for ease of calculation, say that your tires are 26 3/4" tall. This would make their circumference very nearly 7 feet. 75 mph works out to 6600 feet per minute, and if you divide that by your tire circumference, you will get about 943 rpm. This is the speed the wheels need to turn to go 75 mph. Take into account the original gear reduction of 3.73:1, and multiply this by the wheel speed and you will get the driveshaft speed, which, in this case, is about 3520 rpm. Assuming that your transmission is 1:1 in top gear, that should be your engine speed. Your 5100 rpm would suggest to me that you're not getting into top gear. Is this a possibility?
Also, with the change to the 3:00 gear, your engine speed can be calculated in the same manner. 943 rpm wheel speed multiplied by 3.00:1 will give you about 2800 rpm.
Tire size will obviously have some effect on this as will other variables such as transmission slip, tire slip, tire growth at speed etc.
3.00:1 should still be fine in a light car like a Cobra and your top speed should be frightening.

Ron61 · #13 (**permalink**) 03-24-2003, 01:08 PM

Steve is correct about losing acceleration in the top gears with the 3:27s. However I have enough power to pull very strong all the way through 4th. What killed me was I chose the wrong trans. overdrive split which effectively made my 3:27s 2:10s in 5th gear. And at about 3300 in 5th which is 115 I can't push through that huge wall of air. Aerdynamics, What areodynamics ? These cars are as aerodynamic as a brick.

Ron

flipper35 · #14 (**permalink**) 03-24-2003, 02:16 PM

Changing to a 3:50 wouldn't change your cruise rpm as much as you think. There are several auto math calculators that will let you compare different ratios side by side. Smokemup.com has one for instance.

Your engine configuration will play a smaller role, but it should be considered also.

Hal Copple · #15 (**permalink**) 03-25-2003, 05:23 AM

here is Mike Stenhouse's gearing calculator:

http://www.secondstrike.com/Technical/GearCalc.asp

I agree that one has to think carefully about the intended use and expectations of their Replica. I think a lot has to do with camming. Unless you are gearing for "racing", then most likely you will want some sort of reasonable cruise RPM. I have a stroked Windsor, and can comfortably cruise with all sorts of hill-climbing & passing torque at 1800-2000 RPM.

My car also slows in acceleration once i shift into my .68 Tremec's fifth gear, but since this is about 135 mph at @ 6000 RPM, it isn't something i experience on the "street." I am happy to have the relaxed cruise RPM i do.

Remember that more "power" is just a downshift away. In a "bigger block" engine, i would suggest gearing so one has about 2000 RPM at cruise (60-70 mph), and in a small block, ie, a 302 CI car, something more like 2500 rpm in fifth.