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dom
12-01-2010, 05:18 PM
It's a ratio thing right?

grouch
12-01-2010, 08:27 PM
It's a ratio thing right?



Yep, your gears make the engine spin faster as you speed up. The output is the same speed and the drive shaft in direct drive (usually high gear) then you have over drive. In overdrive the rear is spinning faster than the output shaft of the transmission. This keeps engine speed down. You gain mileage but lose power.

DonTom
12-02-2010, 03:08 AM
It's a ratio thing right?Yeah, but to me, in a manual shift, it means use it mainly only on the freeway. IOW, the gearing is too high of a gear ratio for most other driving.

In many auto trannys, it means the 4th gear or highest gear (or a direct connect, bypassing the tranny) which will normally only be engaged at higher speeds and should NEVER be used when towing or going uphill. IOW, turn the OD off (shift back to "D" in some cars, switch in others) when up hill or towing.

It's bad for the tranny to keep on shifting in and out of OD.

BTW:

" Politics is like cars. When you wanna go forward, go for "D". When you want to go backwards, go for "R". "
-Obama

-Don Quoteman

Ken
12-02-2010, 05:46 AM
It's a ratio thing right?

Assume that your car is in top gear at, say 70 mph. At this speed, on a modern car the engine is probably turning at around 2500 rpm. The ratio of transmission output shaft speed to engine speed is probably in the region of 1:1. If overdrive is then engaged the overdrive gearing will drive the transmission drive shaft from the gearbox at a faster speed, say 1.2:1. This means that, for the same speed, the engine revs will drop considerably. This applies to both automatic and manual transmission cars fitted with overdrive capability.

The nett result is that the ride is quieter, smoother and, by virtue of the slower engine revs is more economical. Due to the higher gearing, however, will car will not be able to accelerate so rapidly for overtaking manouvres, nor will it be able to climb steep grades without losing speed. Older rear wheel drive, stick shift cars used to have the overdrive gearing built into the rear differential (think Austin Healey 3000) and overdrive selection (as on my Carina SE Auto) was by a switch on the shift lever.

All in all it is just a way of getting a long, loping gear ratio for steady, economical, quiet, sustained cruising speeds on open roads.

Ken.

dBrong
12-02-2010, 09:34 AM
It's a ratio thing right?

I drove 'over the drive' to park my car in the garage :)

chiph
12-02-2010, 08:01 PM
The 5-speed automatic in the Ridgeline (and probably the Pilot) is a double-overdrive. The top 2 gears have their ratio greater than 1:1 for fuel-economy reasons.

The downside to this is that it downshifts at the slightest hint of a hill...

Chip H.

dom
12-02-2010, 10:45 PM
The 5-speed automatic in the Ridgeline (and probably the Pilot) is a double-overdrive. The top 2 gears have their ratio greater than 1:1 for fuel-economy reasons.

The downside to this is that it downshifts at the slightest hint of a hill...

Chip H.

That is awesome. I could see a double-overdrive being useful in most vehicles.

Dave Brand
12-03-2010, 05:55 AM
An ideally-geared car would reach its maximum speed at the engine rpm in top gear at which maximum bhp is developed - in other words, the point at which engine power & resistances such as drag & rolling resistance balance out.

Any gear ratio higher (numerically lower) than this is an overdrive. It's fairly normal for a car not to be able to reach its maximum speed in overdrive, as the power at the lower engine revs in O/D is lower than required to overcome the total resistance. However, in normal driving O/D has many benefit - lower fuel consumption, less engine noise, prolonged engine life among them.