Dumb car question...

[Dmitri]

I've had a stick shift (happily) for years now, and when I used to have automatics they didn't have the over-drive button. I'm borrowing someones car (older hyndai) for the past week or so and today drove on the expressway. It seemed like it did not want to shift into that 'last' gear, and the engine was revving like crazy just to get to 60mph... On the console was a light that said "OD off" (overdrive) -

So the dumb question:
Do I need to have overdrive on to get that last gear, and if so why the heck would they do that?

 

[thetrue]

Yes you do, although I have NO IDEA why they do that.

 

[KmH]

http://en.wikipedia.org/wiki/Overdrive_(mechanics)
An overdrive (OD) is a mechanism that allows an automobile to cruise at sustained speed with reduced engine RPM, leading to better fuel economy, lower noise, and lower wear.

They also do that for situations like climbing or going down a steep grade.

A truckers truism is '"Up high. Down low." Using a high gear going up a grade keeps the rpm's low which also produces the most torque. Using a low gear to go down a grade keeps the rpm's high which produces less torque.

Horsepower is an arbitrary value and is calculated from the formula - HP= [(Torque x Revolutions Per Minute) / 5,252]

Those of you familiar with math will see from that equation that at 5252 rpm torque and horsepower have the same value, and that above 5252 rpm horsepower will be greater than torque.

My shifter kart, Honda CR-125 engines have a 14,000 rpm limit. We change the final drive gear ratio for each track so the engine is right at the 14,000 rpm limit at the end of the longest straight. On short sprint tracks (1/2 mile or so) the final drive ratio may deliver a top speed of 80-85 mph at 14,000 rpm in top gear (6th).

On the longest regular road course straights, the final drive ratio will deliver a top speed of about 120 mph at 14,000 rpm in top gear (6th).

Being a chain driven final drive, it's easiest to just change the sprocket on the rear axle. More teeth on the sprocket give a lower final drive ratio, and are used for short tracks. Fewer teeth on the sprocket are used for longer tracks. However, sometimes we need to change both the rear axle sprocket and the transmission output shaft sprocket too, to get the final drive ratio we want.

 

[thetrue]

KmH I understand that, BUT it's a Hyundai, not a Peterbilt.

 

[Dmitri]

ugh, thanks guys. Good to know I wasted a buttload of gas today, on Long Island where there's already gas shortages... I miss my stick shift

 

[Derrel]

WHERE IS THE overdrive selector? Enable overdrive if you're revving too high at only 60 MPH!!!

 

[Big Mike]

I seem to remember that when I was test driving a Hyundai, the O/D switch was down on the console between the seats, or maybe at/on the shifter, which was down there. It seemed like a very odd place to put it.

Another possible issue might be that the car in question could have a CVT transmission, which has no 'gears' per say. These will rev higher as you go faster....it's an odd feeling.

 

[Dmitri]

I tested it out today and you're all right - with the overdrive on (button is on the shifter), it hit that last gear and was a much nicer drive. :\