Mpg on your SL

[230/8]

When I got my 72 350SL 18 months ago it was returning about 10 mpg in town and 13 on the road. I changed the timing to 30 degrees at 3,000 (about a 12 degree advance from stock), checked all the vacuum connections, repaired a leaking fuel injector and made a very small adjustment of the MAP sensor (using a CO meter) and now it gives 13 in town and 16 on the road. Still not great but at least better. Some folks claim 18 on the road, but I doubt a real 107 with an automatic will do that without a tail wind. FYI, the one thing that seemed to improve the mileage was the MAP adjustment. The timing has definitely made it run better around town, but gave no significant mileage improvement that I'm able to see, yet.

230/8

[rzrob]

If gas keeps going through the roof, I'm going to have to brake out the bicycle every other day to get to work. Geez, it's $55 to fill it up and good for 3, or 4 days max. My guess on the mileage of my tidy '76 is around 10 mpg.

RZ Rob

[jjrodger]

Originally Posted by rzrob

If gas keeps going through the roof, I'm going to have to brake out the bicycle every other day to get to work. Geez, it's $55 to fill it up and good for 3, or 4 days max. My guess on the mileage of my tidy '76 is around 10 mpg.

Try paying nigh on $7.50 a gallon. I filled my SL last night at GBP0.839 a litre, which is roughly GBP3.80 a gallon. At today's rate, that is $7.00 a gallon. On the motorway yesterday prices were GBP0.919 a litre or $7.66 a gallon!

My tank full cost me GBP57.83 for 68.92 litres, equivalent to $107 for 15.3 gallons. I had done 360 miles since the last fill, which works out at 23.5mpg. Not bad for a 5litre V8! (My wife's beemer gets 37.1mpg, though.)

[drewmwillis]

Originally Posted by TheV12pwr

$120 for the pair...
I can get you one.
I ordered 2 pairs and will post some photos next week when installed.
It will look something like that....

Would those work for my 1991 SL500?
if so where did you get them?

Thanks,
~drew

[jjrodger]

Took the SL on a long journey this weekend past. I have adopted a monastic driving style due to the number of points on my licence (11 out of a possible 12, one more and I'm off for 6 months) which resulted in 29mpg. Over 450 motorway miles taken at 75 to 85mph I used 15 1/2 gallons. I was pretty pleased to get 450 miles from a tank.

Given the tank is 18 gallons, the reserve must be huge. The fuel guage was showing almost completely empty and the fuel low lamp had been on for 30 miles before I was able to stop to fill. Yet I could squeeze only 15 1/2 gallons into it. Maybe 500 mile range is attainable after all?

[Captain Scott]

My 1974 450 SL gets around 10-11 in town and I would love to try to increase the miles/gallon. With no CO meter, can you advise on how I might make the adjustment without fouling everything else up? Is the MAP, the control box up in the passenger footwell that has a ratchting plasitc knob? I'm also not afraid of pushing the timing back... I saw a post on putting hotter plugs in. Might that do more damage than good??
Scott

Originally Posted by 230/8

When I got my 72 350SL 18 months ago it was returning about 10 mpg in town and 13 on the road. I changed the timing to 30 degrees at 3,000 (about a 12 degree advance from stock), checked all the vacuum connections, repaired a leaking fuel injector and made a very small adjustment of the MAP sensor (using a CO meter) and now it gives 13 in town and 16 on the road. Still not great but at least better. Some folks claim 18 on the road, but I doubt a real 107 with an automatic will do that without a tail wind. FYI, the one thing that seemed to improve the mileage was the MAP adjustment. The timing has definitely made it run better around town, but gave no significant mileage improvement that I'm able to see, yet.

230/8

[230/8]

Ahoy, Captain...

I cannot recommend fiddling with the FI MAP sensor unless you have a good understanding of what you are doing and the proper tools, which in this case would be a CO meter. FYI, the MAP sensor is under the hood and on the driver side fender well. It as a large vacuum hose running to it and an electrical plug which connects to the computer box. Also FYI, that box above the passenger's feet, attached to the firewall is the computer. The knurled knob is for setting the idle CO, which again requires a CO meter to measure. The range of adjustment for the knob is about 10% so I'm told. It acts similar to the idle mixture screw on a carburetor except you are changing the FI fuel delivery at idle. It works backward from a carburetor...turning clockwise makes the idle mixture richer and CCW is to lean the mixture. It is a click-stop adjustment so is fairly easy to adjust and then return to the original setting, just count the clicks as you turn. You should try to achieve a smoothe idle with no miss.

Check your timing specs. I believe your engine is 5 degrees retard with about 18 - 24 degrees total at 3,000. If so, just cranking in another 10 degrees at idle should help your driveability. Try for 30-32 degrees at 3,000. Mark the pulley with paint to see the 30 degree mark.The retard on my 72 only works at idle, and is cancelled when above that speed. It also cancels when engine temp is above 100C or the AC is switched on. I think yours works the same way.

As to the MAP sensor, without a CO meter I'd limit my efforts to making sure tha vacuum is tight throughout the system. Check every vacuum connection for everything and replace any suspect items like cracked or hardened hoses. There are quit a number of vacuum attahcments to the engine and along the firewall under the hood and elsewhere. Trace them all and make all as tight as possible. Don't forget the reservoir under the passenger side fender. This vacuum tune-up may do more for you than anything.

Last note, check the MAP sensor. It should have a black plastic end cap on it. This denotes the self-compensating model which adjusts itself to changes in air pressure, i.e. changes in elevation (sea level vs. Denver). Mine has this and I suspect my improvement may have been simply reversing some previous adjustment made in the distant past. These are fairly simple systems compared to modern digital FI. They were never meant to be economy cars. I've accepted that and view 16mpg as a minor triumph for a car with such good all around character. I may even throw more fuel at it just to see how much power I can unleash.

230/8

p.s.: Nix on the hotter plugs, use NGK BP6ES, my personal favorite. Run great and last forever.

[Carlos Serrano]

Given the tank is 18 gallons, the reserve must be huge. The fuel guage was showing almost completely empty and the fuel low lamp had been on for 30 miles before I was able to stop to fill. Yet I could squeeze only 15 1/2 gallons into it. Maybe 500 mile range is attainable after all?[/QUOTE]

I find it interesting that my owners manual shows a 25 gal. tank in my 99 SL500 but the most fuel I'd put in it to fill up is 17.5 gals after driving aprox. 10 miles with the low fuel light on is that is the case the reserve must be close to 8 gals. of fuel.I get 270 miles + or - out of 17 gals or till the low fuel light comes back on that is daily city driving at 8 miles a day 5 days a week 15.89 miles per gallon I would like to know how much more I could get out of the whole tank but then again running the tank to low runs a risk of picking up debris and impurities from the bottom of the tank

[Steve Gutman]

I get between 16.5 and 18 MPG. I check every tank. Most common is 17.5. I think it's pretty good for this motor. It does as well as my 2.5 liter BMW 525 Wagon. Because of the high fuel prices I have been driving my wife's Honda Del Sol from time to time. It gets 30 to 34 MPG (city/Hwy). It handles great but has no power. I take a lot fo crap from my co-workers because it is kind of a "girlie car".

[abe g]

I get about 10mph in locl driving, up to 13-4 in highway driving. Have been told that my fuel distributor is at fault, but am doubtful that its just another $1000 sale for the shop. Am I wrong? What can I look for or as I suspect, these old 450 sl use a lot of gas? Regards, Abe G PS Have 126,000 miles on speedo.

[Strife]

Does anyone have a good technical explanation of why the gas mileage is so bad on a 560SL? I know that the car is heavy and the engine is large, but still, it has a 4-speed auto and fairly tall gearing. The highway mileage in particular is brutal. It isn't really any better than my 73 Camaro was with a pretty lumpy cam, 0 pollution stuff, headers, and a holley carb.

Does it have the aerodynamics of a brick? It can't be worse than a pickup truck, and yet they are getting pretty good MPG nowdays.

Would a lock-up torque converter have bought a mile or two highway?

Would more sophisticated engine management (i.e, digital computer with timing/mixture recalc between sparks) have helped?

Maybe the question really is not why this car is bad but why are cars so darned good NOW on gas? Seems 225 horsepower is "moderate" these days and yet cars like that are getting 24 MPG+ on the highway.

[Walrus]

I can't help wondering about Gas mileage on these cars, and the seemingly large range of numbers reported. I tried to start a thread to build a database to analyze the true range reported, but no one seemed intrested (however, this thread has now covered 2 pages).

One thing comes to mind, and that is how many cars have tire size changes from stock diameter/aspect ratio, and has the speedo been re-calibrated (either in the calculations, or actual re-calib of the unit). I can't imagine anyone putting larger diameter tires, but should it happen, the gas mileage would plummet due to less miles tabulated per tankful run. My 1988 560SL routinely gets 18+ with mixed driving, although most of the driving is highway (probably 60% or so to the next cruise in or car show...) I have smaller diameter tires, and have calculated the error to be 3%, based on test loops on the highway, and my gps unit compared to my odometer. If I did not back off the 3% error, I would be seeing artifically higher mpg due to more miles tabulated per tankful run... Looked good before I calibrated. Was at and one time above 20!!!!

Another thing to consider is the odo's on these cars (earlier 107's) are notorious for the odo gears to slip, not tallying up the true mileage.

Still, I am like strife in the regards to why don't the cars get more like 20+... Oh, well, I guess all things considered, my car returns to me so much pleasure in driving experience that the mpg is not to much of a concern.

Happy motoring...

[Steve Gutman]

Walrus, what size tires do you have? I think most of the users keep stock sizes. My 225/50/16s should be the same diameter as the 205/70/14s that came on the car. Is my mileage fooling me?

[Walrus]

Possibly... I can't remember right now what my tire size is, but my 107 has 60 series instead of the stock 65 series (although the aspect ration is closer because the tire width is wider...).

There are numerous sites online that can help you determine the actual diameter of your set-up, and the diameter of the original...

Check my math... I calculate the following:
225/50 16
sidewall height 4.429"
Total theoretical dia-24.8583"
Circumference- 78.055"
Revs/mile 811.7353

205/70 14
sidewall height 5.6496
Total theoretical dia- 25.2992"
Circumference- 79.4395"
Revs/mile- 797.5878

Looks like your current setup would lead you to believe your mpg is better than actual (ie showing more miles on odo than actually covering). Only a test loop calibration can tell you for sure. Run a couple of 10mile loops and double-check and let us know... My thoughts from these calculations lead me to believe your actual mpg is about 1.5% lower than you think (not very much and probably within statistical deviation of the instrument (odometer)error as manufactured).

Of course, I may have made an error in calculations

[cbdo]

Strife, what kind of numbers are you talking about for a 560SL? Mine does 15-16 in town, 18+ pure highway (especially if I keep it under the 80s most of the time). My wife's Bravada does about the same, perhaps not quite as good in town.
The real comparison is to the Saab SE wagon, which will do 22 overall and 27 or so highway. It's quicker and almost as fast, and a very nice car--but the Benz is my baby!