rear end gear ratio

jpfletch1 · #1 (**permalink**) 06-12-2003, 01:00 PM

What is a good rear end gear ratio. I will be using a 351W engine with about 400hp. I would like to smoke the tires a little bit but mostly street driving.

klayfish · #2 (**permalink**) 06-12-2003, 01:09 PM

What trans are you going to run? That will make a big difference. But if you're running a 5 speed with similar gears to a T5, you may want to go in the mid 3's. I have 3.73s in my 302 and am just about to put a very similar 351W to the one you have in my car. I'm staying with the 3.73s, but that will probably be the upper end of how far you'd want to go.

Steve

Excaliber · #3 (**permalink**) 06-12-2003, 01:26 PM

400 hp? You will be able to smoke the tires a LOT with just about any gear ratio.

ME? 5 speed with 3:55 ratio works nicely.

Ernie

SSS · #4 (**permalink**) 06-12-2003, 01:45 PM

My car is being built with 3.07, with the option of upgrading as high as 3.55. I will probably never have beyond 300 HP, and will have the standard T-5. My understanding is the higher ratio you have, the less street-friendly the car is.

What difference in driveability would I see from 3.07 to 3.27 to 3.55?

Thanks.
Steve

Mr.Fixit · #5 (**permalink**) 06-12-2003, 01:50 PM

3.73's with a tremec or T-5 just lets you really use 5th gear for cruising on the freeway. Anything less (3.23's) makes the required freeway speed too high to grab 5th for cruising, you just won't have the RPM's. I don't like a lug along at 1700 rpm in fifth gear, motors don't like it too much either. I also think 4.10's without overdrive is just fine for around town driving, just don't get on the freeway. I have a car with 3.73's and no OD, runs 3500-4000 rpms cruising on the highway. I like to run as much rear end gear as I can, 4.56's are a bit much for a daily driver though.

HighPlainsDrifter · #6 (**permalink**) 06-12-2003, 02:34 PM

Hi,
You 1st figure out what trap speed you want[top end speed at end of 1/4 mile]. Say that is 125 mph in 4th gear(1 to 1 ratio }.

Then with a tire/gear/speed calculator work backwards to see that at 6,000 rpm with a 26" tall tire you will need 3.50 rear gears.
If you want to road race and go to 140 mph in 4th, then it will be a 3.25 rear gear, etc.
Good Luck,
Perry.

turnpike boy · #7 (**permalink**) 06-12-2003, 02:47 PM

Put 4.09's (Jag rear) in my Unique, with XB-3 302 crate motor. Still a leftover from drag racing youth, I guess, but I can cruise with great comfort at any speed up to 60 in the first 4 gears - 60 in fourth is 3000 rpm. Have a 3550 w/.68 fifth gear and love it - 80 mph is about 2500 rpm.

Wheelspin nonexistant unless you drop the hammer; othewise just flat digs in and hauls a**. Very easy to bounce off the rev limiter in first (set @ 6200), it winds out so quick. I think most here would consider 4.10's useless...I think 3.23's are useless. 99,99% of all your street driving is up to or around 80-85 mph, and the acceleration factor with steeper gears has to be experienced to believe. 3.23's, 3.08's great if you think you'll spend lots of time up around 150-160 mph; don't have too many roads, around here anyway, that will permit that.

Mebbe that 351 will smoke the tires all the time with 4.10's - mebbe not. But I stone-cut, iron-clad guarantee you that NOBODY will outrun you in the stoplight drags.

jpfletch1 · #8 (**permalink**) 06-12-2003, 03:17 PM

I forgot to mention I will be using a Tremic 3550.

Bob In Ct · #9 (**permalink**) 06-12-2003, 04:56 PM

The 3.73 with the Tremec makes first gear very low. With 275-60-15 BFGs I turn about 1850 RPM at 60 MPH. If I had it to do over again I would use a taller gear in the back.

Bob

Lubrecon · #10 (**permalink**) 06-12-2003, 07:28 PM

A 3.89, and a 5 speed with a .68 5th is the best of both worlds. Shelby's '65 and '66 GT 350's had 3.89's, only with a 4 speed though.

George Snyder · #11 (**permalink**) 06-12-2003, 08:41 PM

I agree fully with Bob in CT.
You could use 3:73s but, I think you would enjoy driving the car more with the 3:55 during every day driving.

The piece of the equation that is missing is your cam. What is the maximum operating range for it. 5500, 6000,6500?

An example would be a Ford B303 is 5,300. That comes pretty quick when when you consider it in combination of a 3550 and it's ratio + 3:73 gears + 400 hp 351.

A Tremic 3550 ratio is 3.27-1.98-1.34-1.00-.68.

3:73 is a nice street performance ratio from years past, but most cars were then using transmissions with ratios higher also.
An example would be a:
A close ratio T-10 is 2.43-1.61-1.23-1.1 a lot higher.

I have a TKO and a 3:15 rear. The car is certainly very quick. If I were to change, I would limit it to 3:55 at the max.

Also consider fuel milage with today's gas prices.

The higher rpms constantly is harder on the engine also. You have a sizeable investment, why not try and make it last a little longer.

bobl · #12 (**permalink**) 06-12-2003, 09:42 PM

JP...if you will go to out website www.427cobras.com and select "contact us"...go ahead, dont be bashful, you will receive an Ebrochure. Then select the "gearbox" option. Viola! There is a table for a 351 Windsor 385 H.P., TREMEC 3550 series, with a 3.50 / 3.73 / 4.10 ratio. Should empirically show you abour everything you want to know. Personally speaking I like the 3.50. Couple of days ago we were putting a new car through its pre-delivery shakedown. I wans't paying attention ( can't believe that I admitted that ) and came off a light in 4th. Just hit the clutch friction point, bring it up to 1000 rpm from 775 idle, and motor on off down the street. It's quite surprising that you could drive the car in one gear ( ALTHOUGH WE EMPHATICALLY DONT RECOMEND IT ). 2380 lbs isnt hard to move with 400+ ft/lbs of torque. If you choose to NAIL it, it's a WHITE KNUCKLE RIDE in any gear. Be careful out there!

For "normal" street driving, higher ratios just tend to jack the RPMs up which burns gas and wears engines. We dont supply less than the 3.50 and caution against anything higher than 3.73.

So! There's our penny's worth. All the best and enjoy your ride!!

Mr.Fixit · #13 (**permalink**) 06-16-2003, 04:44 PM

"The higher rpms constantly is harder on the engine also. You have a sizeable investment, why not try and make it last a little longer."

I don't think that many high perf motors enjoy having a load on them as they putt putt down the highway in OD at 75 mph at 1600 rpm. I think your motor would prefer staying over 2000 rpm if it is under load. Look at what your rpm's will be at the cruising speeds you anticipate in fourth and fifth, fifth gear is a pretty big drop in rpm with a tremec, with 3.73's you have to be cruising pretty fast to want to be in fifth, otherwise the RPM's will be so low as to hammer the rod bearings. (Depends on your engine's operating range)

George Snyder · #14 (**permalink**) 06-16-2003, 05:42 PM

Here are the numbers.
Using 3.55 rear/275/50/15 tires(25.9") / trans ratio of .68

75 mph= 2349 rpms
70 mph = 2192
65 mph = 2036
60 mph = 1879
55 mph = 1722
50 mph = 1566 Now this is where you get into 1600 rpms range.
Personnely speaking, I would stay in fourth below 55 mph.

And for the record.
I do have another car that I run with 3:90 gears on the street. I tach 2950 at 60 mph using a Close ratio Super T-10. When I made the statement about beating the engine my comments were sincere. 9 mpg using 94 octane pump gas. 3500 lb car that needed a little boost from behind.
Bud
http://www.richmondgear.com/101032.html

Jack21 · #15 (**permalink**) 06-17-2003, 04:58 PM

The real toss up here is 3.55, or 3.73. The decision matrix gets back to the trans gearing, and from it, what you want your cruise RPM to be.

Engine is a 351W, and it pulls like a diesel from 1200 RPM up.

The trans is a T5Z with 2.95 first, and .80 optional fifth retrofitted. Rear is 3.55. On the highway, I can put it in fifth from 40 MPH up. Cruising at 75 MPH is 2500 RPM. With a typical .62 fifth gear in Tremec's and T5's, the 3.73 rear might be a better choice, particularly with a 302. Or, don't plan on using fifth until your highway speed is higher to use it.

Frankly think the Tremec's are geared all wrong for a Cobra, and T5Z's with better gearing are underrated. A T5Z, particularly with the close ratio fifth, is great for a street driven Cobra with anything from a mild 302, to a stroker 393. In a 2300 - 2400 pound car, you can't put enough torque through the trans to damage it if you're running street tires. Even the super sticky 17" "Z" rated tires will smoke before the trans breaks.

Fred · #16 (**permalink**) 06-20-2003, 01:55 PM

I just purchased a set of 4.10's to replace the current 3.55's. I installed the 3.55's when my motor was stock. Over the years I have made several engine modifications and the happy zone for the motor is now a higher rpm, thus the 4.10's. The cam smooths out and the supercharger starts moving significant amounts of air around 3200 rpm. The top speed in 4th should be about 130 at red line, while freeway cruising at 70 in 5th will be about 2400 rpm. I will let you know how I like the 4.10's next week.

Fred

jpfletch1 · #17 (**permalink**) 06-20-2003, 02:02 PM

Thanks for the info Fred. Have a great weekend.javascript:smilie('

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