Recently, the television series "Fast 'N Loud" featured a rebuild of a Pontiac Trans Am. They installed an overdrive transmission and lower gear set in the differential. Question: What, exactly does that do to the car in terms of performance?

3rd Gen already had overdrive in the 700R4. If it was 2nd Gen, I'd say the idea is to have more top speed if they dropped the 3 series gear to a 2 series gear.

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Lower RPM's at highway speeds = less strained engine sound and better gas mileage + faster top speed.

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Al

I'll second the motion.

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I'd dig a first gen Formula 455 with a 6 speed and a mid to low threes to one ratio....

In the case involved - the existing car was considered to be a bit of a lazy drive - not very fast. Because they didn't want to change the engine (despite rebuilding it twice - or so the story went) they decided to pep it up at the traffic lights by lowering the final drive ratio - but to counteract the peppiness at the lower speeds they regained some of the top speed ability with a top "overdrive" gear.

So in Mercedes diesel W123 terms it is a bit like having a turbo OM617 300D but you then fit a 240D 3.46 final drive differential => so now you can fly at the traffic lights. However to make the motorway bearable and to allow the motor to survive you manage to find and fit a 722.5 5 speed "overdrive" transmission.

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I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

I think that the Formula 455 was a second-gen creation, and would be a sweet car with the 6-speed. I would cheat a little bit on the final drive and go for at least a 3.73. I used to drive a second-gen Firebird Espirit back in high school; a nice handling car which just needed something a little more juicy than the 350-2bbl which was under the hood (which is probably why it didn't kill me).

I believe that ther term "overdrive" denotes that the top gear in that transmission is 'faster' than 1:1 (ex: 0.71:1 or 0.68:1)

Final drive ratios are normally 3:1 or 4:1. 3.2 is pretty fast/tall, and 4.11 is low/dragstrippy but first, Lets get the background


Assume you have an engine that revs from 1000 (idle) to 6000 (redline).
If you just hooked the engine to the wheels, the tires would spin at 1000rpm when idling. With a conventional tire (72" circumferance) the car would be travelling 68mph, at idle with the clutch out....so we need to add some gears to make things slower.
Between that engine and the ground you have TWO things that change shaft rpm - a transmission (with multiple gears) and a differential/rear-end/pumpkin/final-drive - which only has 1 set of gears.

As I recall my old watercooled VW stuff, 1st gear is about 5:1 (5 turns on the input shaft for every 1 turn of the output shaft) and final drive is 3:1 (3 turns in, one turn out). These ratios multiply so that at a 1000 rpm idle speed tire speed is 1/15th of that, or 66rpm, or 4mph...which is a good idle speed in 1st gear.
If I put "4.11 gears" (a common drag-race setup among muscle cars) in the rear end of that volkswagen (FWD notwithstanding) , idle tire speed in 1st gear would be 1/20th of 1000rpm, or 50rpm (3.4mph) - slower, but as you have seen, slower gears let you accellerate quicker. Hence all the drag racers that like a 'higher' final drive ratio.

I hate terms like slower, taller, faster, quicker, shorter...they are kinda misleading sometimes. Since these are all ratios, some are numerically 'smaller' than others (3:1 is smaller than 4:1) but people often call 3:1 "taller" than 4:1, becuase the 3:1 gearset will allow a higher top speed for the same engine. I try to skip that and just use numbers..

To answer your question, on older muscle cars:
1) Any 4spd automatic transmission is more gears than the 3spd that most came with.
2) an overdrive top gear gets revs down on the highway for less noise and better economy
3) The taller final drive lowers the gear ratio for 1st gear, giving better dragstrip performance (at the expense of top speed, but who runs a Trans Am at top speed anyway

-John

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oops, that's right. forgot about the pre 70s cars. second generation, horse collar grille...beastly engine and a few handling/brake upgrades...