pcutt

Now that gas prices are going up there is a lot of misinformation floating around in the news. In particular, one of the "helpful hints" that recurrs is that driving slower gives higher gas mileage.

Not so, I say!

IC engines run more efficiently at some speeds than others, and coupled with gearing, the result is that optimal MPG is not at the lower speeds (let's assume highway speeds). For example, I've heard anecdotal evidence that some cars get better mileage at 65 mph than at 55 mph.

So, for a 944 NA on the highway, what speed gives the highest MPG?

Scott at Team Harco

44 mph. Maybe 42. You need to keep the revs high enough such that you don't bog (lug) the engine. In the case of the 944 and its gearing this works out to just below 2000 rpm in 5th gear. Anyone that says you get better mileage at 65 than at 55 is full of crap!

sca037

I've noticed that in our '85.5, the engine seems happy to cruise at 2000rpm in 5th as long as you're not loading it (as in trying to accelerate, or climb a hill without downshifting). I hate lugging an engine....and figure I should get a diesel-powered car if I want to run at sustained very low rpm's
< engage rant mode >

Now, my winter ride and tow vehicle is an AWD Chrysler minivan which has an electronically controlled AOD transmission. It will not, no matter WHAT get into torque converter lockup until 45mph. I always try and run between 45-50 if I'm looking to save fuel, but there are many folks out there that think that driving their brand X SUV at 32mph in a 50 (or higher) zone is saving them gas....when I'm sure they haven't even locked into overdrive yet
It drives me nuts, but whatchagonnado?

< end rant mode >

I feel better now,
Brian

Scott at Team Harco

Good point Brian. All of my comments were in regard to a manual transmission where I have assumed the driver has selected top gear. All bets are off on auto transmissions. In that case, top gear must be engaged and locked to get maximum efficiency.

I kind of forget that a lot of people have auto transmissions. But I still stand by the 65 vs 55 statement. As long as top gear is locked there is no way 65 is going to save more fuel than 55.

Peckster

Damn that's precise, how did you arrive at those numbers?

tifosiman

0 MPH.

luckett

that would minimize your mpg. 0 mph=0 mpg

peterr

I agree that somewhere around 2000 RPM in 5th gear is correct. If that means 42-44 mph then so be it, although that seems slow to me (admittedly the amount of time i spend in 5th gear at 2000 RPM is near zero).

I find it hard to understand how 65 mph could be more efficient than 55mph just due to the effect of drag.

That being said I am often astonished at how fuel effiicient my 17 year old Turbo can be crusing at 85 mph.

I tend to think that how you drive in getting up to cruising speed, or scooting around in stop and go would make more difference than relatively minor variations in highway cruise.

Lawn Gnome

I doubt there is a constant speed you can stay at to optimize mpg unless the road is completely flat

GlenL

I saw a study where the average car did best around 50 mph.

As wind resistance and mechanical losses vary exponentially with velocity, going 75 instead of 65 really cuts the net efficiency.

http://auto.howstuffworks.com/question477.htm

Zporsche944

rolling down a hill with the car shut off lol. its 45mph for are cars u can get ~35-40 mpg in an N/A

pat944

Sitting at 3400rpm to 4000rpm on the highway, top gear running for about hour and a half, sees about 20-22mpg, this is for a Euro spec car however, guys in the states, YMMV.

bloodraven

in a turbo, wouldn't going at a higher RPM help with gas mileage? Maybe I'm wrong on this..but i know on my trip to ohio i averaged around 30mpg all highway through the mountains...

mgmarsh39

my 84 has that handy little guage on the dash above my speedometer telling me my mpg. dunno how accurate, but its kinda nifty

944na_whorhay

I know my 85.5 NA averages about 28 to 30 MPG when I make the 700 mile trip to my parents from Alabama to Ohio. It's mostly flat or slow grades and I probably average around 80MPH. My MPG gets considerably worse driving around town though as I like to drive like I'm going to a fire.