Quote:
Originally Posted by GMU-BRZ
I was wondering about this. I'm not planning on tracking my car, just having it as a daily driver. I was wondering if I could make 5th and 6th, or maybe even just 6th, a little longer to make it about even with the AT. Probably get 35mpg highway vs 30 currently. I guess I'd have to consider cost, potential warranty issues, etc.
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On the surface, it certainly seems like turning fewer rpms would yield fuel economy benefits on the highway. And it
might in certain situations. But I'm not sure where you get an additional 5mpg from--where did you get that number? Do you really think that, if 5 highway mpg (even 2) were achievable with no drawbacks from something as simple as a gear ratio change, that Toyota wouldn't have done it?
Here's what you have to keep in mind. It's a relatively high revving 2 liter engine with limited part throttle, low rpm torque. It's not a GM LSx V8 engine. If you put the thing in top gear and accelerate at Interstate speeds like on the US06 drive cycle used for certification, you are going to load the engine down further.
Not only could the vehicle feel more sluggish than if it had shorter gearing, the exhaust temperatures are going to increase from the additional load. Then the stock ECU would probably enrich the mixture to reduce exhaust temperatures so the cat, exhaust manifolds, exhaust valves, etc will be protected. So there goes part of your fuel economy benefit. From Toyota's perspective they could have trouble meeting CO emissions standards from the enrichment that results from lugging a high revving, small displacement engine.
Remember that due to taxation the manufacturer already has an incentive to get higher fuel economy for the vehicle as long as that doesn't cause other problems.