Gearing Selection for Ford 427 Small Block

[KauaiMK4]

Just about ready to pull the trigger on ordering my engine, trans and differential, and need some advice:

- Engine: new Ford Racing 427, 450 HP
- Transmission TKO 600, leaning strongly toward .82 OD gear
- Tire height estimated at 25.7"
- Cruising highway speed 55 mph
- 99% of the time Street vehicle

Would like to know the following:

- What is the minimum comfortable RPM in OD for this engine?
- Suggestions on rear end gear. Leaning toward 3.25 or 3.31

Thanks.

[2FAST4U]

I like the 3:31 gear

[mike forte]

Hi Kauai,
If you have 3.27 gear and a 25.7" tire at 3000 rpm you'll be at 70 mph. shift into 5th and you'll be a 2460 rpm. At 60 mph you're at 2108 rpm and 50 mph will be 1757 rpm.
If you have 3.31 gear and a 25.7" tire at 3000 rpm you'll be at 69 mph. shift into 5th and you'll be a 2460 rpm. At 60 mph you're at 2132 rpm and 50 mph will be 1777 rpm.
As you can see the numerical difference is nil between 3.27 & 3.31 gears.
Mike Forte

[KauaiMK4]

Thanks Mike.

Sorry, what I meant was I was leaning toward 3.27/3.31 range, but was wondering if 3.08 or 3.55 would be more appropriate for my set up and driving situation.

[Martin]

I'd stick to your range. i.e. 3.27. for street driving.

The 3.55 will feel faster, but 1st becomes a bit shorter.

(Note - I haven't actually got my car driving, but I spent dozens of hours with the calculators thinking this through).

Having read hundreds of threads on this, if you do track days etc., then 3.55 is the way to go. If you split your time between street use and track use - you are trying to meet two completely different requirements with one solution which is difficult - or at best gives you a compromised solution in both circumstances.

I ended up going for the 3.27 and a T56 with the 0.64. It is most likely I will only end up using the 6th gear when loping along on the motorway. If I was going to do more track days, then I would have just stuck with the TKO600 and stuck with the 0.82. Shorter rev range, but pulls better. Not many tracks will give you enough space to get up to 150mph where the longer gears are needed.

In the end, I think I thought about it too much and over complicated it.

Martin

[Jeff Kleiner]

Equally important is what's the first gear ratio?

Jeff

[mike w]

I have 3:08 gears. Initally when I bought the car, I was not happy with the way the car ran at low rpm and was thinking of increasing the gears to 3:55. After dialing in the timing and rejetting the carb, the car now runs great with the 3:08's. For street driving they work for me. It even gives you the chance to wind out 1st gear. If you are really looking for real qick "off-the-line" times, then you may want to consider 3:55's. But for a street cruiser, I found the 3:08's work fine in a car that weighs only 2200 lbs and 400+ HP. Here is a link for calculation different gear ratios for speed and rpm:

http://www.apexgarage.com/tech/gear_ratios.shtml

[CraigS]

I had 308s in my 400 hp 351 FFR and they were a good street gear. The one downside was that I have the tall (.64?) fifth which meant that 60mph was approx. 1500 rpm. I have changed to 331 for a little better 2nd gear accel for autocrossing. I got that and I also got a 5th that is usable more often. So I would also go w/ the .82 fifth that you are thinking off. In addition to the .64 being a little too tall, it is also a huge gap from 4th so there is an un natural feel to the 4th to 5th shift. Your engine will have more torque and hp than mine so I think a 308 would be a great choice. Also the 308 makes the short 1st gear more usable so you could stick w/ that ratio rather than the tall 1st that works so well w/ a 355 gear.

[KauaiMK4]

I have 3:08 gears. Initally when I bought the car, I was not happy with the way the car ran at low rpm and was thinking of increasing the gears to 3:55. After dialing in the timing and rejetting the carb, the car now runs great with the 3:08's. For street driving they work for me. It even gives you the chance to wind out 1st gear. If you are really looking for real qick "off-the-line" times, then you may want to consider 3:55's. But for a street cruiser, I found the 3:08's work fine in a car that weighs only 2200 lbs and 400+ HP. Here is a link for calculation different gear ratios for speed and rpm:

http://www.apexgarage.com/tech/gear_ratios.shtml


Mike,

Just curious on what is the minimum RPM you are able to run at in Over Drive to not lug the motor?

Thanks.

[Plebeian]

The 3.27's should work great for you, still peppy out of the hole with the torque you'll be making. I had 3.08's and really liked them, but was making more power. Cheers.

[68GT500MAN]

I still have the 2.73 gears from the donor. I will be changing them out this winter for 3.55's.
Doug

[mike w]

Mike,

Just curious on what is the minimum RPM you are able to run at in Over Drive to not lug the motor?

Thanks.

1600 RPM is a little painful..........1800 RPM seems fine.

[Bob Cowan]

Plan to do your most cruising at about 2,100 rpm or so. Sometimes traffic will slow a bit, and you won't have to be down shifting all the time.
0.82 OD + 3.31 rear gear = 62 mph @ 2200 rpm's, top speed =178
0.82 OD + 3.55 rear gear = 58 mph @ 2200 rpm's, top speed =166
0.82 OD + 3.73 rear gear = 55 mph @ 2200 rpm's, top speed =158

The TKO 600 has a 2.87 first gear. I prefer a shorter first gear - easier to cruise slowly in parking lots, load onto a trailer, get started on a hill, etc.

2.87 first + 3.73 rear gear:
-- final drive ratio= 10.7
-- Speed at 3,000 rpm = 21.4

2.87 first + 3.55 rear gear:
-- FDR = 10.2
-- Speed at 3,000 = 22.5

You might even consider the TKO 500, with a 3.27 1st. The shift points are so close to each other, that it really wouldn't matter. My current set up has a FDR in 1st of 11.6, and I really like that. Speed at 3,000 rpm is 19.7mph.

3.27 first + 3.73 rear gear:
-- final drive ratio= 12.2
-- Speed at 3,000 rpm = 18.5

3.27 first + 3.55 rear gear:
-- FDR = 11.6
-- Speed at 3,000 = 19.5

3.27 first + 3.31 rear gear:
-- FDR = 10.8
-- Speed at 3,000 = 20.9

If your brain isn' swimming already with all of these numbers, here's some more for ya, ;)

http://www.5speedtransmissions.com/5sp_comparison.html

And I have a spread sheet I can send you with some basic formulas you can play with.