Transmission Gearing Question

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3,094
Location
Metro Detroit
I was looking at transmission specs on GM's web site and noticed that the Aisin 5 speed automatics used in the Chevy Equinox/Pontiac Torrent and the Saab 9-3 and 9-5 don't have an overdrive gear. They make up for it with a very low first gear (approx 4.7/1) and high axle ratio (approx 2.5/1). GM's 5 speed autos, however, use one overdrive gear and the 6 speeds have two with the typical axle ratios (3.05, 3.29, etc). What are the advantages either way? I'm curious about Aisin's reason for not including an overdrive gear.
 
Messages
736
Location
Illinois
The main purpose of an overdrive is RPM reduction, which is accomplished either way. I would say one advantage to an OEM is a smaller ring gear is possible with the numberically lower ratio, however, I don't know if that is necessarily the case in this application. I believe in most transmission assemblies a 1:1 ratio is most effecient for lower friction and lower heat generation, which a non-overdrive trans accomplishes in high gear. These are just my initial thoughts. Something may come to me later.
 
Messages
4,287
Location
Central Wisconsin
Only advantage I can see from a 5 speed auto, direct 5th, and a high differential ratio is that there MIGHT be a slight fuel mileage advantage. The indirect overdrive would use a small amout of drive power not there in a direct drive top gear. My 2¢
 

Kestas

Staff member
Messages
13,945
Location
The Motor City
Overdrives were incorporated into transmissions to reduce the overall package size of the transmission. It has little-to-nothing to do with fuel economy. It is the whole gearing setup - transmission and differential - that determines rpm (and economy) in final drive if everything else is the same. Ford did this back in '78. They introduced their FIOD (Ford integral overdrive) 4-speed automatic with overdrive. The marketing people really played up the "overdrive" angle during the second gas crisis. I did the math comparing the FIOD transmission with the 3-speed automatic it replaced. Because they fiddled with the differential at the same time, the final drive ratio DID NOT CHANGE, and highway fuel economy stayed the same. (City driving fuel economy did go up though). That said, there is a typical 4% power loss through the overdrive gearing when compared with direct drive.
 
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