Transmission Gear Ratio Question

I'm hoping one of you technical guys (or girls) can answer this question regarding transmission gear ratios. On the 2nd gen Tacoma, the manual has a 1.00 5th gear ratio and a 0.85 6th gear ratio. The auto has the same 1.00 4th gear ratio and a 0.72 gear ratio. My question isn't about why the manual doesn't match the automatics, but why can't vehicles have 2 overdrive gears. Maybe something like:

4th - 1.00
5th - 0.75
6th - 0.65

I would think with the HP today's engines can make, they should be more than capable of having 2 overdrive gears with the second one being really tall. This would be able to give the driver better fuel economy on flat ground yet downshift to a reasonable overdrive on any incline. Thoughts?

David

 

Anybody???

 

I don't know I think Toyota would have done much better with a taller overdrive. .85 isn't enough. But I think .65 would be pretty crazy... we don't quite have that much horse power

 

0.65 was just a number I threw out there. But then again, on a nice flat or downhill section, maybe it might be possible. After all, the auto version uses a 0.72 gear so 0.65 doesn't see that far off.

I would think that some engineer could figure that out though.

David

 

A bit of a thread necro but it seems like an interesting question. There's no physical or engineering limitation that prevents a transmission from having more than one overdrive gear. For example, this 8 speed auto that appears in the Dodge Ram and other vehicles has 2 overdrive ratios http://en.wikipedia.org/wiki/ZF_8HP_transmission.

It boils down to a trade off between having a limited number of gears and the purpose of the vehicle. In automatics, more gears means more complexity, weight, and cost. It isn't necessarily simple to just add another gear. This is also true of manuals but I think more importantly, no one wants to have to row through more than 6 gears.

I suspect that because the Tacoma is rated to tow up to 6500lbs there isn't room for two overdrives. It needs to be geared appropriately to haul that load around without obliterating the clutch or otherwise being useless on hills. People complain about the extra short 1st gear on the 6 speed and I suspect it is designed that way to make it easier to get a maximum load going on an incline. On the automatic they can get away with a taller 1st gear because the torque converter will absorb some punishment during dead-stop acceleration with a max load as long as it has a transmission cooler.

Even with those constraints in mind, I think there is still room for improvement on the 6MT gear ratios. I'm interested to see how things shake out in the next generation. also try reading this thread for more exciting gear ratio discussion. https://www.tacomaworld.com/threads/rear-end-gear-ratio.272346/

 

I had an 00 WS6 TransAm with a MN6. 6th was very tall turning only 1300 RPM at 70. It had gobs of low end torque which allowed it to accelerate up hills in 6th or maintain speed with cruise control on. My truck hunts on small hills in 4th. I don't think these trucks produce that type of power down low in the rpm range to really take advantage of such a tall gear. That and they are about as aerodynamic as a barn door.