That was me! And I qoute me from vwbuge's previous thread:

"My vote: Always start at the odometer. Have you done a mileage check on the highway yet?

297,000 miles on an odometer -- might just be out of whack..."

and...

"no, i'm not referring to your tire/wheel setup. you might need to have your speedometer/odometer calibrated.

when i bought my wagon, the speedo/odo was (and still is) off by about 17 percent. makes my mileage seem really crappy..."

vwbuge,

You have clearly detailed all of the things you've done to squeeze every bit of mileage out of your car. Except for one thing:

Have you taken the car out on the highway yet and compared your odometer/speedometer reading to the mileage markers?

I realize that cahnging fuel filters will not increase my mileage. I just mentioned that I had 15,000 miles on them and that I thought it was time to change. i will check the odometer today.

__________________
'85 300SD (formerly california emissions)
'08 Chevy Tahoe
'93 Ducati 900 SS
'79 Kawasaki KZ 650
'86 Kawasaki KX 250
'88 Kawasaki KDX200
'71 Hodaka Ace 100
'72 Triumph T100R

Don't know about MBs, but many VW owners say that increasing wheel sizes can cost between 2 and 3 mpg. Wider and stickier meats can even cost more.

__________________
'91 300D 2.5 Turbo 330K
'00 VW TDI Golf, 190K
'67 BMW R50/2
'73 Norton Commando Interstate

Yeah, I took everything into consideration. That big of a jump in tire sizes might cost something. I didn't really see a big difference in my other car. 185/70-14 (stock) to 225/55-15. I checked the odometer today and it seemed to be on the money. I tried it a few times and there was no drastic difference. I was thinking that if it was off say....1/8 or 1/16 of a mile it may make a difference. For every 8 miles travelled I would be off a mile. But, after something like 160 miles I would only be off 20 miles. That is only like 1-2 MPG difference.

__________________
'85 300SD (formerly california emissions)
'08 Chevy Tahoe
'93 Ducati 900 SS
'79 Kawasaki KZ 650
'86 Kawasaki KX 250
'88 Kawasaki KDX200
'71 Hodaka Ace 100
'72 Triumph T100R

vwbuge,

The tire size increase, if you go to www.tirerack.com, from stock to the new size you cited, is nearly a 12% increase in diameter, which will become a 12% increase in circumference, which will read as a 12% reduction in your mileage (I used Michelin tires for the comparison, changing brands or using other brands could give you different results).

If you were going to be happy with about 24 mpg and you are getting 18, that explains a bit of it, which is not a true loss of economy. If you drive up hills and in the city, the effect will be more pronounced as you have effectively put a 12% overdrive on the car in all gears. On the highway, on level ground this will be ok, maybe even allow for better for lower fuel consumption per true mile driven. But up and down hills or in stop and go traffic you are working the engine harder - you have to get more torque to feel the car accelerate as you are used to, and that means you have to push the pedal down harder. More fuel gets burned. So, that might account for an additional loss of economy which is real.

The other "loss" due to the wheel turning fewer rotations per mile, or per fuel tank fill up and reading as lower miles is not a real loss of economy. Just a guage calibration issue. Knowing the actual diameter differences (visit tirerack, check the size and brand name for each tire by finding the tire and clicking on "Specifications" and then finding the exact tire you used) means you can adjust the miles recorded to be actual miles driven and fix the error.

Good luck, Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

According to http://www.miata.net/garage/tirecalc.html
I am nowhere near 12% difference in diameter. I am a little over 4 inches greater in circumference of the tire. The differences in tire sizes and facts illustrated by the above website would account for a noticeable difference n maileage, say, over 500 miles or so. But when I fill up with around 150 miles on the tripmeter it should not be that much of a factor.

__________________
'85 300SD (formerly california emissions)
'08 Chevy Tahoe
'93 Ducati 900 SS
'79 Kawasaki KZ 650
'86 Kawasaki KX 250
'88 Kawasaki KDX200
'71 Hodaka Ace 100
'72 Triumph T100R

Well, a 150 test run to check mileage isn't long enough to get any kind of an accurate reading. You need several tankfuls over 500 or more miles to get something close to accurate. Same with your mileage with your old tires. Take an average of many tankfuls to do a fair comparison.

I'm still not sure I saw an indication that your odometer/tripmeter is accurate WRT highway mileage markers.

Len

vwbuge,

I think the difference in our calculations rests in the stock tire size I assumed. The tirerack.com site has the stock tire size listed for your car as 195/70-14. The 12% increase is based on this tire size being the starting point for your drivetrain design and the odometer calibration. You cited 205/70-14 as the assumed starting or stock tire size for the system. A change from 195/70-14 to 225/60-16, according to the tables for Michelin tires available in those sizes from Tire Rack, yields about a 12% increase in diameter and circumference.

I do not have a manual for the 1985 300SD so I cannot verify the stock tire size. Maybe someone else on the site here can pipe up with the OEM stock tire size and resolve this issue. Hope this helps, and good luck, Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

that is probably where the difference in our answers are. I finally got around to checking my car today. It does indeed call for 205/70-14 tires.

__________________
'85 300SD (formerly california emissions)
'08 Chevy Tahoe
'93 Ducati 900 SS
'79 Kawasaki KZ 650
'86 Kawasaki KX 250
'88 Kawasaki KDX200
'71 Hodaka Ace 100
'72 Triumph T100R

Change the filters, P.E.H likes to blow a lot of smoke at people to make them think that changing their filters will not help their power or economy. It helped me a lot...so...thats just from personal experience. And in 15,000 miles, you probably want to change them anyway.

__________________
Nick
*****
2002 Sprinter 2500 140" High Roof :

Any chance it is related to airconditioning use increasing as summer comes on? We went from 9.8L/100km before we fixed the air conditioning, to 10.2L/100km after fixing the airconditioning (and using it a lot). At least, that is what I am attributing the difference to. It might even be worse with this last tank full.

This is interesting about the tire size. We had 205/70/14 tires on our wagon, but the manual says it should be 195/70-14. We had that size of tire on Mary (new!), so we switched them over today to Belinda (which we drive a lot more). The guy at the tire place said it might cause a decrease in fuel economy, but very slight. Is that right? It's only a 'calculated' difference anyway, right, not actual?

Anyway, your mileage is much worse than ours. There is a website regarding fuel economy that lists a number of things your can try to improve mileage. Try http://www.fueleconomy.gov

Natalie

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Natalie