Ricardo Vega

Wanted to see what you guys are averaging for city and road trip miles for each gallon. I'm at 19.3 for city, occasionally pressing on pedal for a good acceleration in 1st gear(auto).
Any good lows and highs for our cars?

Ricardo Vega II
115,955
1987 928 S4
Plum Red/Black

Ducman82

I'm sitting at about 12-13 in town with the way i drive in town here in the LA hills.... about 21-23 highway.

The Forgotten On

I can get about 20 when I drive at the speed limit and about 18 with the AC on when I don't.

I only have a 3 speed auto. If I had a 4 speed I would expect to be in the high 20's.

MainePorsche

I'm about 18-20 mpg with cruise set at 70-75 mph on the highway. Like Blake, a little less with AC on.

SwedeInSiam

I count SPM instead. Smiles Per Miles

NoVector

Related, at a buck forty on the A6 autobahn last weekend my computer showed 5.9 mpg. Has anyone else looked during high speed runs? My buds with modern M3 and RS5 still get ~16 mpg at those speeds. So I don't know whether to write it off as my car's just old and has crappy fuel management or if there's a problem.

The Forgotten On

Probably the amount of fuel getting dumped into your engine. I get about 14 mpg near those speeds in my 81 Lets just say I wanted to turn a 5 hour trip into 2.5.

You only have 1 litre of displacement more than me so it shouldn't be that drastic of a difference. Especially considering you have an overdrive gear.

FredR

In simple terms you need power to overcome the rolling resistances from the wheels [r1] and aerodynamic drag [r2]. The relationship between speed & drag is squared- thus whatever fuel you are burning at a constant 75 mph to overcome drag resistance you will need 4 times that amount to support 150 mph all things being equal [which they seldom are].

The resistance due to the rubber gripping the road [as I am aware] is a linear relationship. Thus total rolling resistance takes the form: R= r1 x V + r2 x V**2. When the total resistance equals the power developed you reach terminal velocity. Thus wider wheels [like mine] have a negative impact on top end and if you have seen the video of Al Holbert at Bonneville his rubber looks "suspiciously skinny" for a S4! He also complained that he could have gone faster had his rear wheels not been slipping!

Considering the above it is not at all unrealistic that fuel consumption will drop into single digit figures when warp drive is engaged. At regulation speeds [75 mph] the S4 will return a fuel consumption of about 13 litres for every 100km [approx 62 miles] covered. Engage warp drive and the same journey will consume about 40 litres- in about half the time! Do not ask me how I know this!

With modern performance vehicles like the BMW 3 series family, I rather suspect there is no meaningful comparison. M3 owners typically report 16 mpg when giving it some welly and some variants have very impressive Cd values [circa 0.3]. The early 928 models had a Cd of 0.4 and the S4 had 0.34- very good in its day and still not bad today.

So, 5.9 mpg in a GTS sounds a bit excessive to me but not outrageously so- perhaps 9 to 10 mpg is more the norm for such conditions but...? The stock tune at full load burns rich for starters [about 12.5 AFR] compared to stoich at 14.7 so that does not help. Maybe the figure quoted was more an instantaneous value rather than an averaged value over some time?

Rgds

Fred

Dictys

I've had 5.4 mpg whilst north of 160mph and average 19.4 mpg on a motorway cruise at 75 to 80, lower the speed to 60 mph 25mpg. My overall average is 20 mpg.

FredR

Sheikh M,

Good to see you are still alive and well- trust the S4 is still in one piece!

Your consumption at warp speed [and other speeds] seems reasonable but do you remember what your consumption was at the speeds No Vector quoted in post No 6? Pretty sure they will be in the region of 10 mpg or more.

One factor that folks often forget is the impact of the wind at high speeds. 140 mph into a 20 mph headwind takes almost the same amount of power as 160 mph with no headwind. Trouble is in most cases one has no clue what the wind situation is without getting out of the car and measuring such.

Flying England to Dubai I have completed the leg in 5 hours 50 minutes on the A380 but in the opposite direction it typically takes about 7 hours 15 minutes [prevailing winds are from the west].

Rgds

Fred

worf928

Modern engine management allows near-stoichiometric operation at very high loads. Our 70s-80s era systems just go pig-rich at medium-high loads to keep from blowing up.

kmascotto

My digital dash (instant consumption readout) once indicated 79 liters per 100km...I was just coming out of a hairpin...and heading for a straight stretch

prescott

18 to 19 MPG city (light foot, no AC)
22 highway (around 85 MPH)
around 10 @ sustained 100+
Mine's an 88 5 spd.

ThetaTau87

I track my fuel economy with the Fuelly app. My best tank was 21.2 MPG and the worst was 15 MPG. My overall average is 18.2 MPG with most of my driving done on short trips around town of 10 miles or less that hurt fuel economy because so much time is spent with a relatively cold engine.

Adk46

Two tenuously relevant bits:

1. I found my other Porsche (Cayman S) on top of a list of fastest cars that get 30 mpg. It really does get that mileage when my wife is in the car. It's sorta like a fast Prius, except for ... nearly everything.

2. To expand on Fred's post, I happen to be the operator of bikecalculator.com. It's for nerdy cyclists who want to estimate the power needed to ride at various speeds and conditions. The nerdy car guy can abuse one of the calculators in the "Old Stuff" section, where the parameters of any vehicle can be entered numerically. It's left as an exercise for the student to convert kilocalories to gallons of fuel (the calculator assumes a 25% conversion efficiency).