One of the other posts got me to wondering about everybody's rate of RPMS at 70mph with different set-ups. I find myself wishing I had that fifth gear sometimes.

Mine: 235-60-14/3.23/4sp(m20)/3200rpm@70mph

Robert

Me too. 205-70-14, 3.23/4sp 3000RPM at 65MPH

of course , usuing an Automatic, but seems to be right at 2000 at 60, 3000 at 85-90...TH350, 245x 60x 14 " tires, 2.56 rear end

Good excuse to go out for a drive....:)

215 70x14, 3.23/4sp(M21) 2800 RPM @65 mph

Mike

215/70 r 14 around 3200 at 65 m.p.h. 3:90 rear gear with a wide ratio muncie.
Fun around town but not very fun on long trips.

Tires of X size (something normal, not all that tall)

Plus

2.78 rear + 700R-4 tranny = 2200 rpm at 90 mph.

How's that?

Tires of X size (something normal, not all that tall)

Plus

2.78 rear + 700R-4 tranny = 2200 rpm at 90 mph.

How's that?

Perfecto! Especially since you have a low gear off the line too. Doesn't that overdrive trans lower the first gear?

Yes, first gear is crazy-low at 3.06: http://www.smokemup.com/tech/700r4.php

Don't forget that a 2004-R is a nice alternative. You can see the ratios at the same link above. 1st/2nd are closer together, 4th is a smidge taller.

I've caught a lot of "feedback" from guys saying that the combo of 700R-4 and a 2.78 rear might be too optimistic concerning gas mileage. Guys seem to think I'm underreving the engine and the trans may not flow enough fluid since it is turning so slow.

I run a fairly large trans cooler and I have a lock-up torque converter, so I don't think I have heat as a problem so the slow turning tranny may not be as much of an issue.

IF you convert to an O/D trans definitely get a trans cooler, even in addition to the one in the radiator.

My two cents.

-Scott

great link Scott.

My momma's Buick Roadhog - er, Master revved pretty low at highway speeds, around 1600 rpm @ 70 mph from the factory, so I don't think fluid flow is a concern.

Wow Smitty. I knew the overs were good but that's rediculus. You must get like 50mpg after you get rolling.

Robert

Thanks TOHCan - jeesh 1600 rpm at 70 ticks slower than a clock! :-)

Well Robert, it's super-easy to assume that's the case but it's not. I barely get 20 mpg. I'm due for another test since I've recently had the carb/timing tweaked.

Talk to Bjorn and company. They seem to advocate turning slightly higher revs, and yet they see as good or better mileage than I do. There's chatting going around that the engine needs to turn faster than 2,000 or so to be optimally efficient. I'm not qualified to issue an informed opinion. Part of me says, "Slower is better" and part of me makes a reasonable assumption that there is a "sweet spot" in revs and throttle setting that might yield better mileage.

Where's a scientist when you need one?!? :-)

215 70x14 3.55:1 w/ T-10 4 speed 3000 @ 65 about the same when the M21 was still alive...

I drive a 5 speed Neon and found that I get 32-33 mixed driving when I drive it hard, 26-28 if it is accelerated gently and shifted at the lowest possible RPM. I think the total revs needed to get the car to speed at full throttle is fewer than lugging it.

My daily driver is a 2000 Civic Si. I seem to get great mileage pounding along at 85 mph (30 - 33 mpg). To get better than that I have to really cruise lightly. 65 mph, gentle throttle, even coasting down hills...

Excellent demonstrations of what is happening, Vikki and Smitty!

A couple of things at work here. The wider the throttle opening, the more efficient the engine is. Why? Because at low throttle, the amount of air coming in is also low and the effective (or dynamic) compression ratio is low. At high throttle openings, the cylinders are getting filled properly and the engine is running at a high dynamic compression ratio. The higher the compression ratio, the more efficient the combustion process.

At the engine's torque peak and full throttle, the cylinders are getting their maximum amount of air per cycle, and the engine will be operating at its maximum compression ratio. The power made with a given amount of fuel will be at its peak as well (BSFC). Based on this, some people advise that the car should be geared to cruise with the engine at its torque peak. However, this usually is too high a gear as the engine won't require much throttle opening, and the throttle opening has a much greater effect on fuel mileage than the torque peak because of the difference in dynamic compression. It's better to have the engine running at the lowest rpm that it will run cleanly than to worry about the torque peak.

The running clean also is important, more so for carbs than EFI. If you are running the carb on the idle and transition circuits (light throttle), the metering is usually richer and not as accurate as the main circuit. The atomization of the fuel as it comes out of the boosters is not quite as good either, so more throttle opening will be more efficient. However, getting into the power circuit will again cause the carb to run richer and decrease fuel mileage. If you know where your power circuit is calibrated to come in, you can use a vacuum gauge and run the car a couple inches of vacuum above that for the best mileage. That will be the "sweet spot" for your carb.

A Pontiac example: typical "hot" 400, ported #48 heads, RA IV cam and headers. I ran this with a 2.56 gear and a TH400 (2400 rpm at 70 mph) and got about 19 mpg (US), switched to a 3.23 gear and a 4-speed (3000 rpm - closer to the torque peak) and got 16 mpg. Ran at about 10-12" vacuum on level road.

Vikki, the best mileage will be had when you use full throttle and keep the revs close to the torque peak - as long as you don't have to brake for slower traffic - so pretty close to what you're doing! The worst mileage is light throttle and high revs.

I'm going to print that as an endorsement card for the next time my passengers tell me to slow down.

Looks like I found a scientist!
Well done TOHCan! :clap:

Now, if we could only get an endorsement from the gub'mint allowing us to drive this way in the interest of national security by reducing the need for foreign oil! Just think of the implications!

New testing, requiring drivers to show competence in attaining the best fuel economy by driving like Vikki(!), would sweep the nation, relegating those who drive slow and waste fuel and time to the public transit system. I'm sure that traffic accidents would drop dramatically and traffic congestion would disappear overnight - besides those of us out there having a lot more fun!

I think I'm going to try and remember this arguement next time I get stopped for excessive speed(kind of an oximoron, my opinion).
"It seems, officer, that the sweet spot in my throttle plate opening is at 5300 rpms. That's the reason I was going 115mph because that's where I get my best gas mileage."
Kind of a baffle em with bullsh!t approach. Sure that'll work.

Robert

"All in the name of national security, sir!"

Did you know that people who travel 10 mph under the speed limit are over twice as likely to get into an accident as someone traveling 10 mph over? And that getting rid of slow drivers would increase the efficiency of the road system by an average of 12%? And that cops get into 2 1/2 times more accidents off-duty than the same category of civilian drivers? Oops, better not mention that one when you get stopped!