rear end ratio questions

[jayh]

Just wanted some feedback on rear end ratios. I have a Factory Five Cobra that I would like to install positraction. While I'm at it I'd like to change ratio, currently it has a 3.27 rearend, thinking of changing to 3.73. At 60 mph rpm's just under 2000, at 65 just over 2000. Other details, Tremic 5 speed, 5.0 - 275 hp at the rear wheels. Just looking for your input on making the ratio change, positive and negative. Thanks

[Three Peaks]

Should liven up the response substantially.

[Ron61]

I had 3.27 gears in my other Cobra and with the large split 5th gear was almost unusable. Great for cruising if you could find a place with a high speed limit. But the .7 split made my 5th gear around a 2.10 which is really high. In the Coupe I have 3.56s which make it wind a little higher to 60 in 4th but I like them as they give a lot of acceleration and I still have a top end that will get me a ticket if I don't watch it. Also your tire size comes into play so don't forget to factor that in. If you are going to road race the lower gears are a definite advantage as with the 3.27 gears I had to drop to 2nd in some corners where with the 3.56 gears I only drop to 3rd and sometimes I stay in 4th. I had the Tremec 5 speed in the first car and you also need to know what your split between 4th and 5th is. In the Coupe I have the .6 split which also adds to the RPM at speeds but gives better acceleration and lets me drive in 5th most of the time when just out driving.

Ron

[1795]

Ron is correct in that you have to consider the entire package and what you want to do with it. Having lower gears, even slightly lower will improve your performance. Remember, there is a cost to every gain, so figure out what the cost will be when making changes to determine if the gain is worth the cost.

Jim

[Tommy]

I bet a Google search would lead to a calculator where you could input the transmission gear ratio, rear end ratio, and tire diameter to see the speed at any given RPM. Change the numbers to see the difference between 4th and 5th gear with each rear end ratio. Then decide which compromise best fits the way you drive where you are.

[Mark IV]

Use this calculator to figure out the optimum for your package:
Second Strike Gearing Calculator

[sdonnel]

I took my rear from 3.31 to 3.73. It woke the car up considerably. Cruises at 70 mph at 2,100 RPM. While 1st gear may run out a little sooner based on the calculators, unless you're at the drag strip, you will likely not notice. As stated, make sure the whole system works with the gearing. Enough HP, etc. If you know someone with the 3.73 in it, ask them for a ride so you might get a seat of the pants feel for the difference.

Scott

[bobcowan]

DO IT. The car will be so much more fun to drive everywhere except the interstate freeway at speeds over 75-80mph or so. But how much time do you spend there? This is a Roadster, a sports car, a fun car. It's not a grocery getter, a commuter, or a hyper miler. DO IT. You won't regret it.

BTW, you cannot install Posit-Traction if you're using a Ford 8.8 differential. Posi-Traction is what GM calls their limited slip differential. Ford calls theirs, "Traction-Lok".

[MaSnaka]

I have a 3.70 rear with Tru-Track. With a 4 speed toploader I wanted a taller rear because at freeway speed I was screaming RPM's 3000-4000 at speed. I went to a tremek tko600 with the taller 5th gear and the 370 is great. I run out of 1st a little quicker than I like but freeway is 1900/2000 cruising...quieter and better gas mileage. 1-4 are for fun and fast, 5th the party is over and its time to settle down unless you want to go extreme. If you want 5th to be a driving gear get the .82. But you will have higher rpms on the freeway. I believe I have the .64. I also have around 400hp at the wheels.

John

[eschaider]

The Second Strike Gearing Calculator that MarkIV over at Time Machines pointed you at will give you a good way to model engine speed and vehicle speed based on your transmission, tire and rear gear choices.

The choice of how tall or short a gear/ tire/ transmission gear set package you use in your car will largely be determined by how much torque your engine produces at off idle speeds. If you have a 275 HP engine (probably at 4400rpm?) you only have about 330 or so ft/lbs of torque at that 275 HP rpm.

Most engines will peak their torque before they peak their hp so lets say you are somewhere around 350 ft/lbs of torque at peak torque. Your off idle torque will be significantly lower and you will need to crutch that with mechanical advantage in your overall first gear starting ratio.

How you go about doing that will ultimately come down to personal preference. The wide ratio TKO gearsets will provide you the transmission component of the mechanical advantage you need and then you just have to decide where to put tire size and the ring and pinion to suit your preferences. This is where the Second Strike Gearing Calculator is so helpful. Of course alternatively you could also solve the problem using the close ratio TKO gearset and a different arrangement for the other components.

In the end there are no absolute answers there are only personal preferences. The Second Strike Gearing Calculator will help you translate those preferences into parts choices. I believe you will find that it is much less costly to make a mistake and correct your selections in the modeling software than in real life.

Also worthwhile noting is that, modeling software is sometimes a bridge too far for some folks. All kinds of reasons for that but for some it is simply a bridge too far. It turns out that is OK too. In that situation you just proceed according to your instincts and buy the parts you think are most appropriate for your usage, put them in, drive the car and make changes as you see fit.

Either way you get what you want, which is what you were originally looking for — I think.


Ed

[Snowsnake]

3.27 is Snap Neck Speed; 3.73 is Break Neck Speed. I run 3.27 for everyday and 3.73 at the track.

[joyridin']

Split the difference and go with a 3.55 and call it a day.

[D-CEL]

"Tremec 5 speed, 5.0 - 275 hp at the rear wheels."

4.10s and never look back...


Jason

[RallySnake]

Snowsnake, you make it sound like changing rear gears is easy?

Joyridin' I have to agree with you. I switched from 3.31 to 3.73 just before gas prices shot up to $4 here in California a few years back and was kicking myself. With the .68 overdrive gear in the TKO, the car runs great. But, after much carb tuning, gas mileage is close to 20 on the freeway and 10 in the canyons.

I wish I had picked the 3.55 gears.

RS

[D-CEL]

"mileage" vs. "smileage"
maybe 4.30s.........

[jayh]

Thanks to all for your responses. I guess I was looking for validation that I would get sufficient bang for my buck without any negatives. Thanks again