I'm going to be swapping in a 350/th350 in place of my 2.8/t-5 setup(doesn't run) into my 88 firebird and im fairly new to these cars since this is my first. I was wondering where to look to find out what gearset it has and if it has a posi or not. Also what luck have people had with setups in the rear. It will get a positrac eventually and im looking for a sort of balance of street/strip, but with more emphasis in the street category.
Thanks

 

Gear ratio in the diff doesn't really care what tranny you use. The gear ratio can be just about anything depending on the vehicle's use. For overall daily driving, something in the mid to low 3.xx range is good. 3.73 offer good performance but fuel mileage will suffer on the highway. 4.10 or 4.56 may be best for dragstrip use.

All gear choices depend on rear tire size and maximum engine rpm. Typical street tires are 26" tall. Using them with 4.10 gears and an engine that doesn't see much past 5500 will work but will be a poor combination.

The 88 Firebird with a 2.8/T5 came with 3.42 gears. You'll have to look for the RPO code G80 to see if it has a posi or not. You could also pull the diff cover and look to see if everything is still original inside.

 

[quote=Stephen 87 IROC;3894729]Gear ratio in the diff doesn't really care what tranny you use. The gear ratio can be just about anything depending on the vehicle's use. For overall daily driving, something in the mid to low 3.xx range is good. 3.73 offer good performance but fuel mileage will suffer on the highway. 4.10 or 4.56 may be best for dragstrip use.

All gear choices depend on rear tire size and maximum engine rpm. Typical street tires are 26" tall. Using them with 4.10 gears and an engine that doesn't see much past 5500 will work but will be a poor combination.]


hey i got 3.27s in my 87 iroc with the original engine and auto trans. Car has 110700 miles on it and i hate going down the high way because at 60mph its running 2000 rpm, which aint so bad. When i hit 70 - 80 though (depending on the highway) i'm running 3,500 rpm on the highway for prolonged periods of time. I feel like this is bad for the engine is this just me being paraniod or what?

 

It never stopped millions of cars for decades. I drive a 91 454SS pickup. With 4.10 gears in the diff, even will the overdrive, the engine is around 3000 rpm going down the highway at around 70 mph. I did a 16 hour trip about 7 years ago with no problems. Many performance cars had 3.73 or 4.10 gears long before overdrive transmissions were available.

People get too hung up and wanting to drive around at low rpm. The only benefit to low rpm is less fuel consumption. That's why overdrive transmissions are so popular. The OD gears offer no performance but do lower engine rpms at high speeds for better economy. I'd be panicky if I was cruising down the highway to 4500+ rpm.

It's unlikely you'll have a car with 26" tall tires, non OD transmission and 4.10 or deeper gear making a long highway trip. Although it can be done, it wouldn't be very economical.

As for needing a deep gear for performance, my car also originally had 3.27 gears in the diff and I got it into the high 11's on the 1/4 mile before finally changing the diff and gears. I was also spinning the 383 I was using to 7000 rpm.

 

Sweet thanks for the info ill check to see if it's a posi but i think itll do good with the factory 3.42's and some taller tires. Do any of you have an fuel economy estimate? thanks again

 

lol im getting 15 -18 mpg... since gas prices went up i'v e been getting 18. before then i nvr got better hen 15. I got an 87 iroc w/ 350 an 3.27 gears. Also 1107xx miles and its my daily driver.

 

that sounded about right, i figured driving it and my manual v6 ram i shouldn't do too bad on gas, thanks again