brillo

I was playing around with a automotive calculator trying to see what RPM you would be cruising at when going 70mph on a fwy with a slightly lower ratio of say 0.77 or 0.75 for our 6th gear. My reason for doing this is that stock, at 70mph, I'm doing about 3500rpm, and at 65mph I'm doing about 3250rpm, and this is the difference between about 25mpg and 27mpg on the highway. a slight decrease in the 6th gear ratio would put the car at 70mph in the current 65mph zone.

what I'm getting at is it would appear that Mazda could have improved the hwy mileage with a different gear ratio for 6th, without having to sacrifice acceleration (ie, keeping the 4.44 final drive), but I can't figure out why they didn't.

Now I know that there would be virtually no passing ability if you went lower, not thats there is much now, but it would mean you'd have to upshift to 5th to get any kind of power. Is there a safety law or issue at play here? This seems like a no brainer.

just curious.

KYLiquid

my guess : with the lack of tq in the motor, they wanted to try to keep the RPM up to avoid people lugging the motor trying to pass?

r0tor

iTrader: (1)

all the data i logged shows the engine is actually most efficient in the 3200-3500 rpm range... thats looking at bsfc curves and raw instantaneous mpg readings at different rpms vs load/tps

i posted it sometime ago...

beachdog

Traditional close ratio gear box design puts the ratios spacings closer and closer the higher the gear. If the 6th gear ratio was a lot taller most people would complain that the shift from 5th to 6th feels wierd.

tuj

Because Mazda was stupid on picking the 6th gear ratio. The other gears are decent compromises between being usable for a fuel-efficient shifting strategy and for performance driving. However, the 6th gear was not chosen well, at least not for the European / American markets which routinely see crusing speeds over 80 mph.

If you lived in Tokyo, you'd spend all your time in first gear away.

wedge357

I heard the reason Mazda picked the 6th gear ratio in the RX-8 because they tune the car to run most efficiently at 62mph, which translate to 100kph (Japan speed limit?).

beachdog

They could have done that with a 5 speed and saved the expense.

r0tor

iTrader: (1)

yes, its completely stupid to pick a 6th gear ratio that gives you the best gas milage possible for the (pretty much) country wide speed limit

psbjames1970

Perhaps the RX8 should have 7 gears... (Just kidding... sort of)

I find 5th less useful than 6th and 6th a bit too tall. If 5th was made closer to 4th and 6th lowered, it would be the ideal and still achieve 4.44

tuj

I don't want this to become a 'speeding is ok or not' debate, but lets be honest: in America and Europe, we drive significantly faster than the posted limited. Its not unreasonable to go 80-90 mph for long periods of time. Especially when you consider the the nature of the car as a performance vehicle. Check out the average speeds on most highways, especially of sports cars; depending on the area, they are probably 10 to 40 mph higher than the posted limits.

And the autobahn is now largely 130 kph (80 mph) limit. Sorry, but at least 50% or more of the target market for this car doesn't live in Japan. There are significant differences in our public driving cultures, which should be taken into account when designing a car for our market. They don't give us the more edgy JDM ecu-tune; why not give us a better trans at least.

I can achieve the best possible gas milage rpms (3.5k or whatever) at virtually any speed under 70 or so. Its not like gas milage at 65 would be so terrible with a different 6th gear.

therm8

Just for reference the auto's 0.69 4th gear puts you at approx 3300rpm at 80mph (on the speedo), and still has plenty of pull for passing but not enough to maintain speed on a 10% grade at 2500ft

PaulieWalnuts

7? Yeech. I only use 3 of the 6 most of the time as it is (1-3-6)