RPM at 70 mph

[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.

[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.

[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.

[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.

[ClutchFork]

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.

[Rope-Pusher]

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.

[Stelcom66]

Doing some mpg comparisons online brought me to this post, I know it's from 2021. Years ago, Consumer Reports would provide very detailed statistics of the vehicles they tested. Among those, was rpms at a highway speed. I believe every manual transmission vehicle's rpm was at least a few hundred rpm higher than the automatic. Some, much more so like the Subaru Outback and Honda CR-V. The 5 speed manual Outback I had was about 3,200 rpm at 65 mph, the automatic Outback I had was 2650. Reading Honda CR-V member's posts the 5 speed manual (available up to 2006) was also around 3,200 rpm at 65mph, but only 2,000 with the automatic, like the 2012 I had.

Except for the Honda, the EPA's ratings of the highway miles per gallon is either the same as or slightly better than the automatics. Years ago I thought fuel economy correlated only with rpm, of course it does not. These comparisons I'm referring to are for geared transmissions, not CVTs which supposedly do better all around than manuals. Then again, when reading comments on the Honda Fit forum, many manual transmission owners claim at least matching the CVT's fuel economy numbers. The Honda Fit, like the CR-V is up around 3,200 rpm at 65mph. Many complaints about that, then some were glad to heard that the 6 speed manual was being introduced. It ended up with pretty much the same final drive ratio, with very little difference between 5th and 6th. I do wonder why the engineers did that.

I did consider a Mazda 5 because surprisingly there were some available in my area with a 5 speed manual, and they got decent reviews. With the very old CR issues, it was interesting to see the rpm of the typical non-overdrive (or maybe even non torque converter lockup) 3 speed automatic transmissions. Much higher rpms than the manuals and worse fuel economy.