[Nitebird]

I have a 1979 Trans AM SE 400 4speed car. I built a Pontiac 455 and put the numbers matching 400 and 4speed into storage.

The engine

1975 Pontiac 455 .030 bore with factory crank .010/.010
TRW flat top pistons at zero deck
Old Faithful cam 570/574 and 236/245 all at .050
Harland Sharp roller rockers 1.65
Edelbrock RPM intake with Holley 950 carb
Edelbrock D port heads cleaned up but basically out of the box
Dougs 1 7/8 headers

The Transmission

Borg Warner T10 that has been rebuilt. It has the 3 rings on the input shaft so it has the 2.64 gears.

The Rearend

Factory rear disc brakes with 3.23 gears and TA engineering differential cover


Would you guys do a rear end swap or go for a transmission swap?
I want to be able to do at least 80mph or higher and stay low in the rpm range but still be able to rip off some good quarter mile times of hopefully in the 12s range.

[TheSilverBuick]

I run a TKO-600 with a .64 overdrive gear with 3.89's out back and a short 26" tall tire. I cruise at 75-80mph at 2,400-2,500rpm and run mid-12's with a 2.0 60ft and shifting like an over the road trucker.

For sustained over 70mph driving, I wouldn't go over 3.23 gears on a short tire.

[JLMounce]

Unless you add overdrive gearing this is one of those areas where you can't really have your cake and eat it too. Depending on the car and tire size you're probably going to want a healthier gear for track use. That will make driving on the highway a bit more burdensome. If you gear the car for the highway you lose the torque multiplication of the shorter gears.

If the funds are available, an overdrive transmission can get you to a happy medium.

Others talk about going with a low 1st gear set transmission and taller gearing in the rear. This accomplishes some of the same by giving you an adequate total final drive in 1st gear while giving you better roadgoing final drive in your road gear.

The problem with that approach is that the low 1st gear creates a large ratio difference going from 1st to second. Big 455's tend to make enough torque everywhere that this isn't a massive issue, but if you're falling out of your power band, you're losing performance and possibly anything you gained by having the lower 1st gearset.

I have a 3.73 ratio in my '69 Firbeird with an engine similar to yours. KRE Heads and a cam that's not quite as large as the old faithful. The gearing is fun, but it's a stretch on the highway. 65 is about the limit that I really want to keep the car at for any period of time. That puts the rev's at about 3000 rpm.

I went with the shorter gears because I wanted to do a lot of auto-x and a bit of drag racing. I find however that I do that rarely and instead do mostly driving around in the city. If I could do it again I'd have done something like a 3.08.

[John62]

Would a Gear Vendor Overdrive work in this situation?

[JLMounce]

Gear vendor is always an option, but at about $2800.00 about the same cost as doing a true OD manual or automatic.

Since the OP already has a 4 speed car, a TKO600 is a near drop in for him and would cost about the same for a brand new transmission. Less if he found a good used unit.

[rexs73gto]

There is some slight mods to the floor for the TKO600 to fit & with the Gear Vendors unit all you have to do is bolt it up & get a different (shorter) driveshaft. Either on is about the same cost & the Gear Vendors will take a bit more abuse then the stock TKO. Having a 455 you already have the TQ you need to carry over if you lose some grunt down low. Keep the 3:23'a & install the G.V. unit. If it's not what you like you can always go a bit higher on the rear gears to get you off the line better. I run 3:73's but I don't go out of town much , but am going to get the G.V's unit next year to help on the highway.

[Tom Vaught]

I run a Doug Nash 5 Speed trans with a 9" Ford (2.75) rear gear.

The Trans Ratios are: 3.27, 2.14, 1.57, 1.23, and 1.00 so

in 1st gear
at 23 mph I am doing 2500 rpm

in 2nd gear
at 35 mph I am doing 2500 rpm

in 3rd gear
at 47 mph I am doing 2500 rpm

in 4rd gear
at 61 mph I am doing 2500 rpm

in 5th gear
at 75 mph I am doing 2500 rpm

The car/engine gets good gas mileage at 2500 rpm and 1st gear works well for the 25 mph streets, 2nd gear works well for the 35 mph streets, 3rd gear works well for the 45 mph streets, driving in Ohio where the speeds are 60 mph a lot, 4th gear works well, and in Michigan on I-96 I can cruise from the house to Lansing Mi at 75 mph and stay ahead of the trucks and buses. The "rabbits are doing 95 mph on I-96. (;>)

This is with a 27.5" diameter rear tire.

Tom V.

ps At 4500 rpm, I can run 135 mph and the engine is not unhappy at that rpm for a short period of time.

["QUICK-SILVER"]

Originally Posted by Tom Vaught

I run a Doug Nash 5 Speed trans

The Trans Ratios are: 3.27, 2.14, 1.57, 1.23, and 1.00 so

.

Hey Tom V.
Have you got a part number or link to info for one of these?
I remember when the DN2's first came out and you could get one for around $1,500. I was worried about the torque rating on them behind a 455.
Thanks in advance
Clay

[dmac]

I know you can get a decent drop in rpm with a taller tire. How about two sets of tires for the rear. Wallace racing has rear ratio/tire size/mph/rpm calculators, if I remember right

[HoneyHush]

With that much engine you could just put in a 3.08 or maybe higher. TA man on here is in the low 11s with 3.08s and a similar build. Where in the twelves would you like to be? 12.01? Lol or 12.80s?

[i82much]

I vote 26" slicks and 28" street tires and stick with your current gear and trans.

Do you really drive 80 mph that often? I live out west and thought this was a capability I really wanted. However, I have found that when I drive my GTO I tend to stick to the back roads, cruising at 60 or so. I don't mind my 3.55's at all.

Of course, everyone's use is different, just my 2 cents.

[Nicks67GTO]

Originally Posted by Tom Vaught

I run a Doug Nash 5 Speed trans with a 9" Ford (2.75) rear gear.

The Trans Ratios are: 3.27, 2.14, 1.57, 1.23, and 1.00 so

in 1st gear
at 23 mph I am doing 2500 rpm

in 2nd gear
at 35 mph I am doing 2500 rpm

in 3rd gear
at 47 mph I am doing 2500 rpm

in 4rd gear
at 61 mph I am doing 2500 rpm

in 5th gear
at 75 mph I am doing 2500 rpm

The car/engine gets good gas mileage at 2500 rpm and 1st gear works well for the 25 mph streets, 2nd gear works well for the 35 mph streets, 3rd gear works well for the 45 mph streets, driving in Ohio where the speeds are 60 mph a lot, 4th gear works well, and in Michigan on I-96 I can cruise from the house to Lansing Mi at 75 mph and stay ahead of the trucks and buses. The "rabbits are doing 95 mph on I-96. (;>)

This is with a 27.5" diameter rear tire.

Tom V.

ps At 4500 rpm, I can run 135 mph and the engine is not unhappy at that rpm for a short period of time.

I run the Richmond version of the same transmission and the same Ford 9" 2.75. My 455 loves it. I run 80mph all day long. It's also seen lots of drag, Auto X and road race abuse. I've been very happy with it. There are better options now. The LGT700 is supposed to be the good one now. If you don't mind hacking go straight for a silversport t56 magnum. Best tranny I've ever driven hands down.

[Tom Vaught]

Gale Banks ran the NASH 5 speed with a OD unit in a 3rd Gen Trans Am at Bonneville.

Twin Turbo BB Chevy with NASH trans Pic Attached. If you don't screw up on the tire it will handle any street driving situation.

Tom V.

[azbirds]

I ran a 69 Bird with the same five speed, but 2:41 gears. 400 with KRE D's ran a 13.01 at 107 when it was 105 at the track. I forgot to take the tires out of the trunk, and I had the A/C on full bore. That trans has a good gear split and 1-4 are low and fifth is 1:1. Had 3:42's in it, and taking off in third was easy with them.

[leeklm]

Originally Posted by i82much

I vote 26" slicks and 28" street tires and stick with your current gear and trans.

Do you really drive 80 mph that often? I live out west and thought this was a capability I really wanted. However, I have found that when I drive my GTO I tend to stick to the back roads, cruising at 60 or so. I don't mind my 3.55's at all.

Of course, everyone's use is different, just my 2 cents.

valid point. I know I get caught up in some of these scenarios and then have to ask myself, how am I really going to use the car?

Sure, I missed my 3.08 gears last week on a 300 mile round trip weekend, but in reality, that was the only long trip of the summer. Rest of time is within a few miles of home, or on the trailer to the drag strip a few weekends. For that type of use, something in the range of 3.23 to 3.55 works quite well. If I was inclined to put 10,000+ miles per year on the GTO, that would be a different story