RPM at 70 mph

Synchros shot? Weird noises while shifting? Not sure what needs to be replaced?
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ClutchFork
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RPM at 70 mph

Post by ClutchFork »

Ok so here is what I have:

Wife has 2008 Mazda 5 with an automatic transmission.

Son has 2009 Mazda 5 with manual transmission (5 speed).

Both cars essentially the same in all other respects including both have 2.3L 16-valve DOHC four popper.

The automatic turns 2700 rpm at 70 mph.

The manual turns 3000 rpm at 70 mph (this in overdrive 5th gear).

I am surprised that the gearing on the manual is so deep as to run that high of rpm on the freeway. Why you think they did it that way?

BTW my 2008 Fusion manual also turns 3000 at 70 mph in 5th gear, but i once had a 2001 Ranger pickup with same engine and 5 -speed and it turned 2700.
Stick shiftin since '77
theholycow wrote:Why in the world would you even want to be as smooth as an automatic? Might as well just drive an automatic...
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Re: RPM at 70 mph

Post by Johnf514 »

Mazda 5 automatic 5th gear ratio is .69, while the manual is .75. They probably figure the manual is bit more enthusiast oriented and gives it more pep overall. Or the automatic can decouple and rev up a bit on the highway when flooring it. Honestly no idea, just picking at straws here.
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ClutchFork
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Re: RPM at 70 mph

Post by ClutchFork »

Well I would have guessed a different final drive ratio as first gear is pretty low in the manual. I don't even need first if I am even rolling.
Stick shiftin since '77
theholycow wrote:Why in the world would you even want to be as smooth as an automatic? Might as well just drive an automatic...
Rope-Pusher
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Re: RPM at 70 mph

Post by Rope-Pusher »

Picking the ratios, and hence the cruising speed engine RPM, is kinda like picking the best vaccine for next year's flu season.

You've got your window sticker fuel economy ratings to consider - and those are based on running tests on a chassis dyno that, for the most part, don't feature brisk acceleration.

Then there is the consideration of Amish vs Slushbox. If the Amish gearing ends up requiring a downshift at the slightest uphill grade, some drivers will be annoyed. The Slushbox can unlock the torque converter clutch and/or downshift all on its own volition and they driver may either not notice, consider it right and proper, or consider it to be "Too Busy".

Then there is the idea that mebbe the Amish version is being marketed more to performance enthusiasts, so they might prefer better throttle response in top gear over lower cruising rpm.

Then you might consider the wind resistance, the rolling resistance, the typical cargo load, and if it is likely to trailer tow. Any of those consideration may tend to bias the cruising speed engine rpm to be higher.

Now, if a 6-speed is made available, there can be a greater span from the 1st gear ratio to the 6th gear ratio, or smaller ratio steps between the gear ranges. It opens up the possibilities.

Remember too, that 10 % lower cruising rpm does not equal 10 % lower cruising fuel consumption. Depending on what rpm you are starting from, the gains will likely not be significant.
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ClutchFork
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Re: RPM at 70 mph

Post by ClutchFork »

Rope-Pusher wrote: Tue Oct 19, 2021 9:06 am Remember too, that 10 % lower cruising rpm does not equal 10 % lower cruising fuel consumption. Depending on what rpm you are starting from, the gains will likely not be significant.
Thanks for the comprehensive answer. On that last point, it makes sense. If the engine is running faster, that is compensated presumably by the throttle plate not being open as wide and so the fuel consumption difference between the two may be a wash. Oh, speaking of wash, did you know (of course you do) that gasoline was the parts cleaner of choice for many home DYI knuckleheads in the 1960s. It also made good way to kill weeds in cracks of the driveway. But then the EPA came along and spoiled all the fun.

So wifes Mazda 5 has not bested about 29 mpg, but Clutchdisk's manual Mazda 5 got 31 once, and that with the plastic belly shield removed.

I do appreciate that I don't have to downshift to speed up on the freeway and can anyway if I really want to speed up fast. Car is quite peppy with the gearing and the only drawback is sometimes I mindlessly try to grab 6th gear, which being that it does not exist, quickly becomes apparent and I sheepishly put it back into 5th.
Stick shiftin since '77
theholycow wrote:Why in the world would you even want to be as smooth as an automatic? Might as well just drive an automatic...
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Re: RPM at 70 mph

Post by Rope-Pusher »

ClutchFork wrote: Tue Oct 19, 2021 11:42 pm
Rope-Pusher wrote: Tue Oct 19, 2021 9:06 am Remember too, that 10 % lower cruising rpm does not equal 10 % lower cruising fuel consumption. Depending on what rpm you are starting from, the gains will likely not be significant.
Thanks for the comprehensive answer. On that last point, it makes sense. If the engine is running faster, that is compensated presumably by the throttle plate not being open as wide and so the fuel consumption difference between the two may be a wash. Oh, speaking of wash, did you know (of course you do) that gasoline was the parts cleaner of choice for many home DYI knuckleheads in the 1960s. It also made good way to kill weeds in cracks of the driveway. But then the EPA came along and spoiled all the fun.

So wifes Mazda 5 has not bested about 29 mpg, but Clutchdisk's manual Mazda 5 got 31 once, and that with the plastic belly shield removed.

I do appreciate that I don't have to downshift to speed up on the freeway and can anyway if I really want to speed up fast. Car is quite peppy with the gearing and the only drawback is sometimes I mindlessly try to grab 6th gear, which being that it does not exist, quickly becomes apparent and I sheepishly put it back into 5th.
My Dodge omni GLH Turbo was like that. It was geared for max top speed in 5th. Between the gearing and the engine's mid-range torque, "By Feel" it felt like I was cruising 1 gear down and I sometimes subconsciously grabbed for that next gear range.
'08 Jeep Liberty 6-Speed MT - "Last of the Mohicans"
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