Rear end gear ratio

Sorry in advance for this being long, don't want to leave any details out.  I have a 76 TA with a 400/4spd/3.08.  I was thinking about changing the ring and pinion for a little deeper gear.  I have a 400 bored .60 over with 6X-8 heads, 068 cam(215/225 @.50), edelbrock performer intake, holley 4175 spreadbore 650, headers, 2 1/4 exhaust, k&n air filter, i also was told that the heads were ported.  I got a chance to make a pull on a chassis dyno last year and it made 255hp at about 5200 rpm(rev limiter) and 337tq (pretty flat curve).  At the time the car had a slight stumble and a vibration.  Over the winter I fixed the vibration(stripped pressure plate bolt) and sent the distributor out for a recurve(the curve wasn't all in until 3800rpm and only 11 degrees mechanical and the distributor was pretty worn).  I ended up buying a new custom curved HEI from Larry Rowe(who i sent it to) and i bumped the timing up and now have 36 total degrees in by about 2400(noticeable improvement).  I also plan on tuning the carburetor a little better and adding roller rockers.  Finally, what gear ratio should I switch to to get more acceleration, but without losing cruisability(being able to run 65mph without lots of rpm)I was thinking 3.73 but is that too deep or should I stick with the 3.08 or go with something else?  

3.42 or 3.55 is plenty for your combo and you will still be able to cruise with it.
Steve  
Trying to help... 'cause we don't all have to learn the hard way!

would changing from a 3.08 to a 3.42 be worth the swap or will i definitely notice a difference in acceleration?  Also will this improve economy at cruising?  My friend has a 94 camaro LT1 that came with 3.23 and he changed to 3.73 and he gets better mileage with the deeper gear because he is closer to peak torque at cruising speeds.  Will a 3.42 put me closer to peak torque or will a 3.08 be closer to peak torque?  

The 3.42 will be noticable over the 3.08 but the 3.55 will work better as far as acceleration is concerned.  
professional hi-performance engine builder

Horsepower sells engines and torque wins races

I can only find 3.08, 3.23, 3.42, 3.70, 3.73, 4.11, etc for the 8.5 10 bolt.  I guess I will plan on going with the 3.42.  Will the 3.42 provide better economy by bringing the peak torque closer to cruising speeds?  

The 3.42 should work just fine.
I don't think you will get better fuel economy.  
professional hi-performance engine builder

Horsepower sells engines and torque wins races

No you won't get better mileage, due to the higher revs unless you are using an overdrive trans. Your buddy's 94 has overdrive and the change in rear gears only affects it slightly due to the speed reduction of the od. With a 26" tall tire your TA will run approximately 3000rpm at 65mph. This is the combo I run in my Formula and it gets a bit old after 1/2 hour on the highway. If you really intend to drive on the highway alot, keep the 3.08's. If mainly around town, go with the 3.42's.
Steve  
Trying to help... 'cause we don't all have to learn the hard way!

I usually drive in town and on country roads, but occasionally it does see the interstate for 20-30 minutes at a time.  I have heard that sometimes a slightly deeper gear will give better mileage even though the revs are a little higher just because it is closer to peak torque and peak manifold vaccum, but you're saying that this isn't true?  I'm not that worried about it as this is not my daily driver but if it does improve it slightly or no change that gives me another excuse to upgrade. If I won't see a increase in acceleration or mpg though I will just leave the 3.08's and concentrate more on trying to squeeze more power out.  

Going from the 3.08's to 3.42 will definitely improve your launch and around town driveability. Generally speaking, the faster you spin a big carbureted V8 to achieve a given road speed, the worse your mileage will be. I understand what has happened with your friend's LT1, the actual rpm change of the lower gears, combined with the overdrive has moved the engine into a more fuel efficient rpm range. You will likely experience slightly worse mileage with the 3.42 gears, due to the direct 1:1 ratio of your fourth gear but the performance will make up for it. Going from 2.73 to 3.42 in my 79 Formula (75 350 Pontiac/4bbl/TH400) was the best single mod I have made to the car.
Steve  
Trying to help... 'cause we don't all have to learn the hard way!

that makes sense.  thanks steve i appreciate your input, but one more question.  If the LT1 could be moved into a more fuel efficient rpm range by going up in cruising rpm, why couldn't a pontiac motor do the same with a slight increase in cruise rpm(several hundred rpm)?  

I run 406 w/ ported no. 48's, Holley 750 v/s on an e-performer, Crane 284H cam w/ 1.65 roller rockers, Hooker comps, 2 1/2" duals, TH-350 w/ 2200 stall, and a 3.08 posi- my 'Bird runs 13.0x's and tops out at about 140-145 mph. Just from your numbers alone I'd say you need more cam and carb to make more power- if your 6x-8's have a good port job it will respond to the increased duration and lift with more power and faster acceleration.

3.42's and 3.73's will give you better acceleration, but at the expense of being able to drive on the freeway- not a good trade-off IMO. Having a combo with great midrange and top end and the gearing to use it makes for a ride that accelerates well off the line w/o alot of tire spin, and has the ability to run hard up to top speed.

Geno  

What it boils down to, is with the overdrive (and that modern miracle, fuel injection) the rpm on the highway maybe went from 2000rpm at 65 mph to 22-2300rpm at 65 mph. This may well have given a slight increase in mileage. Without overdrive, using 3.08 gears and assuming a 26" tall tire, you are already turning between 2600 and 2700 rpm at 65mph. Swap to 3.42 gears and you add another 2-300rpm to that figure and now you are spinning it way faster than it needs to for optimum efficiency. Now remember, the more rpm that 400 is turning, the more fuel it will burn and our old carbs are not as efficient as EFI. Peak torque is not necessarily the point of peak efficiency. When we measure torque and HP on a dyno, it is under full throttle conditions, not part throttle which is how you drive on the highway. So peak cruise efficiency is typically the speed at which you obtain the highest steady vacuum reading from your motor. Try it sometime, hook a vacuum guage to a full time vacuum source on the carb or manifold and go for a drive in top gear and see what mph the highest vacuum reading occurs and note the rpm. Then you can figure out what rear gear and overdrive combo would give you the best mileage. I can guarantee it won't be 3.42 with a th400! Personally I would go with the 3.42's and a 200-4r trans swap, it will drop your cruising rpm down to the same as if you had 2.28 rear gears and you get the launch of the 3.73 in 1st gear due to the 200-4r having a 2.75:1 1st gear as opposed to the 2.48:1 of the th400 (second gear is slightly lower in the 200-4r than the th400's 1.48:1 as well). There is a kit available thru Bowtie Overdrives to convert your factory shifter and kits to properly install the TV cable to your original carb as well, which is very important.
Steve  
Trying to help... 'cause we don't all have to learn the hard way!

I have run 3.73 for 15 years in my 76 TA. With a 550hp 455 I get 10mpg on the highway. When I had a 400hp 400 before the 455 I was getting 13mpg or more. On the highway with a 28" tire it cruises at 2800rpm and I have taken vacations that involve driving 3000 miles in a week. It is very hard to notice any gear change of less then .5 as it will only make a difference of 2-3 tenth of a second in the quarter mile with the power you have. 3.73 will get you a noticable change in performance. The trade off with 3.73 is RPM and not very good fuel economy.  

OK, last summer i installed all new autometer gauges in my dash, one of them is a vac gauge so i will do what steve suggested, drive around and see what the highest vaccum reading is  while cruising and note the rpm.  I will probably still go with 3.42, i don't feel comfortable with the rpm that i would turn on the highway with 3.73 but i could live with 3.42.  I am not that concerned with fuel mileage but i figured if i could get more fuel mileage it would just be another good reason for the swap.  I will not change the transmission, i currently have the factory bw super t10 4 speed and in my opinion, a performance car with an automatic just isn't nearly as fun, i will never have an automatic in my TA as long as i own it(don't plan on selling it).

Thanks for all of the info, opinions, and suggestions.  I really appreciate everyone's input.