When does a car reach topspeed in 4th gear?

[Jostyrostelli]

I know that a car with overdrive reaches it's topspeed in 4th gear and that the gearratio of the 5th gear is to save fuel.

But, when you take an ordinary car today (VW Polo for instance), can I try that on the highway? When it reaches it's topspeed of 170km/h, can I put it in 4th gear and try if it goes faster?

I know it works that way in cars which have an overdrive, but do normal cars with 5 gears come with overdrives too?

Edit: And I just thought of this, when you see a car on a dyno it always is in a certain gear before the guy floors it, could this happen to be the 4th gear (1:1)

[Buba]

Well, it can be the case... A buddy of mine can do that in his Honda... Stupid high revving machine for a small car with 90hp... haha...

Anyway, his Top Speed can only be reached in 4th Gear...

I would test it the other way round: Go in 4th until the limiter hits, then go for 5th and see if it gets slower or faster

Buba

[Jostyrostelli]

Yeah I thought it that, but I don't wanna thrash other peoples cars.

I always was told that it's really bad for a car to accelerate untill the revlimiter kicks in.

I don't know if I could test it in the Punto (with no revcounter)

[jneil]

I would think that it depends on the gearing, final drive and engine power.

I have a 6speed manual and my 4th is a 1:1 ratio, 5th (0.75:1) and 6th (0.5:1) are meant to be overdrives (I guess a 4+2 in reality), I would think that my top speed (of around 260km/h - 160mph) would be at redline in 5th. If I chucked it into 6th at that speed I would most likely start loosing speed due to the forces of nature acting on the car and 6th being too tall to continue to accelerate.

I haven't actually tested the top speed yet, but it seems to have plenty of balls (420+hp) to get me there quite quickly

[Jostyrostelli]

Assume you own a Holden Monaro?

(Australia)

[jneil]

Close!! It is basically a 4door Monaro (but with more poke!). It's a 420+hp 5.7 Litre (346ci) 2004 Holden Commodore VY Series 2 SS 6 Speed Manual



Check it, and my other/previous cars, out here

Jeff.

[Jostyrostelli]

Wow, so if you drive in 5ht or 6th gear do you have any power? Or do you rac in 4th gear only?

[jneil]

I am in 5th from 70km/h 95% of the time (just cruising around), however I can do 110km/h in 2nd if I really need to, just don't change up until 6200rpm rev limiter

Although I haven't done the 1/4 mile yet, It will probably be just on the limiter in 3rd at about 13.5seconds and probably 103mph (conservative figures based on others with similar mods)

It has got heaps of torque (although never enough!) so I can skip gears often too. 1st to 2nd to 4th or 5th usually.

And I think they use a 1:1 gear on the dyno's for simplicity's sake as no other calculations have to be done by the dyno to compensate for the gear box as it is already a 1:1. Not really sure on this one though.

Ohh, and to answer your question, 5th pulls fine, although obviously not as good as a lower gear, 6th is fairly useless apart from cruising at 100+km/h

[adrianpike]

Originally Posted by Jostyrostelli

But, when you take an ordinary car today (VW Polo for instance), can I try that on the highway? When it reaches it's topspeed of 170km/h, can I put it in 4th gear and try if it goes faster?
I know it works that way in cars which have an overdrive, but do normal cars with 5 gears come with overdrives too?

5th gear is usually an overdrive. A car will achieve maximum speed if you're able to be right in the thickest part of the powerband right up to the top speed.

Originally Posted by Jostyrostelli

Edit: And I just thought of this, when you see a car on a dyno it always is in a certain gear before the guy floors it, could this happen to be the 4th gear (1:1)

Generally 4th gear is used, as in most manual transmissions, 4th gear locks the input shaft to the output shaft, bypassing the layshaft, creating a 1:1 ratio, and having the effect of greater efficiency as well.
Also, "flooring it" on a dyno is generally a bad idea, I like to roll on and roll off the throttle to minimize shock to both the driveline and the dyno.
Plus, sometimes you can skew the power numbers with a sudden torque moment.

[Leppy]

My Lancer will do 176 km/h in 4th gear but will only do 150 odd in 5th. This is only because the gearing is too tall for the engine to go any faster.

If my car had Jniel's commodore engine in it, it would pull top speed in 5th.

Any car will pull top speed in its top gear (overdriven or not) if it has enough power/torque.

[jneil]

Actually coming home from work tonight I am doing 1750rpm in 5th at 80km/h. So if all is working the way it should, i would be doing 283km/h if I could get to 6200rpm in 5th (I THINK!). I doubt it'd get much over 260 (highest i've been so far is 220km/h), should try it one day

If, however, I could get to 6200rpm in 6th, i'd be doing 413km/h!!! (6th is 1500rpm at 100km/h).

[Jostyrostelli]

Haha

[Leppy]

Originally Posted by jneil

Actually coming home from work tonight I am doing 1750rpm in 5th at 80km/h. So if all is working the way it should, i would be doing 283km/h if I could get to 6200rpm in 5th (I THINK!). I doubt it'd get much over 260 (highest i've been so far is 220km/h), should try it one day

If, however, I could get to 6200rpm in 6th, i'd be doing 413km/h!!! (6th is 1500rpm at 100km/h).

haha 413 in a SS... I have a feeling it would be super light at that speed...

[Viper007Bond]

If you can redline in top gear, then you obviously won't go faster in the gear below it. Top speed is often reached in the gear below as the car is under powered and does not have enough power to turn the top gear at high speeds.

[freerider]

Originally Posted by Viper007Bond

If you can redline in top gear, then you obviously won't go faster in the gear below it. Top speed is often reached in the gear below as the car is under powered and does not have enough power to turn the top gear at high speeds.

You guys are all rigth, except when you have a (very) powerfull car. But a stock car will never reach redline in final gear(when not going downhill), but it will still go faster then in previous gear. It all depends on your powercurve and ratio's.

Greetz Johan

[chaos386]

Originally Posted by freerider

You guys are all rigth, except when you have a (very) powerfull car. But a stock car will never reach redline in final gear(when not going downhill), but it will still go faster then in previous gear. It all depends on your powercurve and ratio's.

Greetz Johan

Yeah, as an example, my friend's 4cyl Honda Odyssey has a four-speed automatic (so 3rd gear is 1:1 and 4th gear is overdrive), and he reaches his top speed in fourth gear. There's no set rule that says if you max out your second to last gear that you can't continue to accelerate in your final gear. It all depends on the engine, the gear ratios, the final drive ratio, the coefficient of drag, frontal area, and even the tire size.

[jneil]

I would suggest that some cars are geared so that top speed is in top gear at, or almost at, redline.

Probably most of the mass volume produced cars would be a compromise between, acceleration, speed and fuel ecconomy to cater for the most population. So they would include a fuel saving top gear where performance is limited somewhat, but it is good for highway speed crusing.

[Leppy]

Originally Posted by freerider

You guys are all rigth, except when you have a (very) powerfull car. But a stock car will never reach redline in final gear(when not going downhill), but it will still go faster then in previous gear. It all depends on your powercurve and ratio's.

Greetz Johan

3 on the tree 1970 odd Kingswood. They hit top speed in top gear.

[Cubits]

He said it won't hit redline, but he states they will hit top speed in top gear.

Generally that will be true, but never say never. I'm sure there is one or two cars that can punch the red in top.

[Leppy]

Yeah they hit redline... and then go past it. (no limiter)