Relative Fuel Economy

[hans611]

We also drive a lot and use a lot of gas, so we get a better "deal" .... and our government has a low tax on gasoline compared to other nations, like the UK...

 

[JC_451]

I have a 2019 GTI with a manual transmission and I typically run regular gas. Logically, I understand that the car will get less MPG cruising at 75mph compared to 65mph, but I'm curious about just how much of a difference that is. My gut feeling is that it's very minimal - maybe a half mile per gallon. Does anyone know for a fact how much the differential is based on some published source? Thanks.

This is totally incorrect.

At higher speeds the amount of air you need to push out of your way becomes exponential, the drag is vastly greater at 70 mph vs. 55 mph.

Take the car up to 100 mph, you can actually see the gas gauge going down (probably should watch the road tho)

 

[bentin]

Take the car up to 100 mph, you can actually see the gas gauge going down (probably should watch the road tho)

I used to work in Germany and at 250kph (155mph) you could literally watch the fuel needle move down. My normal rental car was a 525i wagon that got about 12 mpg at those speeds, so in the few spots that you could maintain vMax, you had a little under an hour and half of gas. Worth noting, gas in Germany was expensive, but it's comically more expensive in Switzerland, so the gas stations on the border were always packed on the German side and empty on the Swiss side.

 

[Strange Mud]

Slight derail....the Comet is like my old world truck in that you can always put $40 in it

<10 MPG.

at least it's fun to cruise in

 

[tigeo]

Why not just collect your own data to answer this?

 

[back2brand]

Again, thanks for all the responses. While collecting my own data to answer my specific 65mph vs 75mph highway cruising question is certainly possible, it's complicated by the fact that like most people my driving is a mix of highway and local. My local driving also routinely includes elevation changes of several thousand feet, complicating things further. I thought the folks on this forum probably already knew the answer, and you did. Thank you.

 

[shovelhd]

One thing that you can do is calibrate the average fuel mileage readout in the MFI in your dash. Use VCDS or OBDII to adjust the offset to match your calculated mileage at the pump. Once calibrated, the MFI readout will be pretty accurate if you drive conservatively. If you drive aggressively, not so much.

 

[Strange Mud]

Many thanks for the responses - I guess my gut feeling was unduly optimistic. With $5 gas, I may give up some of that extra speed.

do a bit of made up math. How many miles per year x 4 mpg x $5.00 per gallon that is how much you can save. If you normally drive like a race car driver use a larger number than 4 mpg.

I tend to drive conservatively, I speed but I enjoy momentum driving. Long term I get 33 mpg (real number on regular)

I'll report myself for getting that good a number

Mud

 

[ElectricEye]

I've gotten into the habit of using cruise control a lot on the highway.
Most of my highway trips are east bound from Manalapan NJ, to Avon/Bradley Beaches - and the return trip home.
Most of that's at a speed limit of 65mph.
I set the cruise control to about 68, and stay in the right lane.
That route at that speed gets me about 36 miles per gallon.
At times like this, I'm so happy with the mileage this car is capable of if driven easy.

 

[MonkeyMD]

I get 7.5 mpg












On track

 

[tigeo]

I get 7.5 mpg












On track

Same.

 

[kevinkar]

Doesn't running 87 cut timing and result in reduced efficiency? I'd be curious if the reduction in efficiency offsets the savings per gallon? I'd guess it's probably about a wash.

I'm pretty sure the timing is only cut if the sensors detect knock/pinging so, if you're not mashing the pedal to the floor to induce that, it won't retard timing and you won't notice a difference. I regularly put both 89 and 91 in my R and I notice no difference in MPG around town nor when I go on long trips regardless of which I choose and I record all my data so I have decades of fuel usage data on all my cars to back up my average MPG.

My recent experiment has been to drive around in D rather than S which we know is a different shift map and the car short shifts like there's no tomorrow. It's possible the knock sensors are detecting ping I can't hear but the overall effect of changing to D is that my MPG has increased about 4-5 MPG around town. That's an extra 70 miles per tank at around-town speeds and, for me, an extra 9 or 10 days before full ups. That's significant and I'm running 89 lately - it's $0.20 or more cheaper per gallon.

Extrapolating that up to 65 or 75 MPH, I have a feeling (without taking the time to calculate it) the wind resistance between those speeds have more to do with MPG than octane by a wide margin.

Lastly, my historical data shows that long trips on the I-5 in California gets several miles per gallon more going 72 than going 76. On an 18 gallon tank in my Corvette, I could go an extra 70 miles or about an hour of driving more by dropping my speed only 4 MPH. Whether that's significant enough for anyone here is up to them. But for me, it was worth the slightly later arrival time and a few bucks more in my wallet.

 

[Ghawk]

No one mentions the origin of their numbers. I have found that the mileage gauge in our cars, as well as ever other car, is optimistic by 2-3 mpg.

 

[mrmatto]

You can save a lot of gas slowing down 5-10mph on a long drive. At the same time, for a really long drive, that could add 30 min or more to your trip. I often start out thinking I want to save the gas, then as the hours tick by, I decide "to hell with the gas" and punch it.

 

[Strange Mud]

I adjusted my with OBD and it's correct. I found in My Mustang (only other car I've owned with numbers) was accurate