BennyLava
07-06-2009, 11:33 AM
I am having some trouble finding the right transmission to put in my 1987 Chevy Silverado that I am restoring. I have the one that came in it, and it works fine. It's known as a 700R4. It is a nice medium duty 4-speed automatic that has a good overdrive for the highway. However, I am trying to do anything and everything I can to improve my fuel mileage, as are many of you here. So I thought to myself, hey, if i had one or two more gears in that transmission, I could simply change the gear ratio in my rear-end differential (as many power seeking gearheads do) from a 2:73 to 4:10 and get the best of both worlds. If i was able to do this, i could drastically increase both my city and my highway MPG. Well, the only thing that would matter when you get right down to it would be the last gear (the overdrive gear) in the six speed automatic transmission, and the 4 speed automatic transmission. They are nearly identical. That is to say, the ratio of the overdrive gear in the 4 speed is 0.70, and the six speed has a 0.67.
That translates to almost exactly the same RPM's at the same speeds. If i was doing 70 mile an hour with the 4 speed, i may be at 1950 RPM. If i was doing 70 miles an hour with the 6 speed, i would likely be in the 1900-1925 RPM range. So what gives? Why can't they give us transmissions that have taller overdrive gears, that way our highway MPG would go way up, and at the same time you could put gears in the differential (that would be rear wheel drive vehicles only) that were very short and would make excellent city MPG. I think this is some kind of rip off. All they would have to do is put a little bit different gears in there, and they could increase both highway and city MPG by like 3 MPG. Possibly more in the little economy 4 cylinder honda's and toyotas and the like. Anyone have any thoughts or opinions on all this? I would love to hear from you.
That translates to almost exactly the same RPM's at the same speeds. If i was doing 70 mile an hour with the 4 speed, i may be at 1950 RPM. If i was doing 70 miles an hour with the 6 speed, i would likely be in the 1900-1925 RPM range. So what gives? Why can't they give us transmissions that have taller overdrive gears, that way our highway MPG would go way up, and at the same time you could put gears in the differential (that would be rear wheel drive vehicles only) that were very short and would make excellent city MPG. I think this is some kind of rip off. All they would have to do is put a little bit different gears in there, and they could increase both highway and city MPG by like 3 MPG. Possibly more in the little economy 4 cylinder honda's and toyotas and the like. Anyone have any thoughts or opinions on all this? I would love to hear from you.