Well, I thought I was getting great MPG, like 35 mpg by driving at 60....

well, I checked the odometer on the highyway...100 miles on the highyway markers was about 115 miles on the odometer, so about a 15% error....

Then I checked the speedo with my GPS....speedo shows about 5 MPH faster than GPS.

So basically I'm not going as fast or as far as the speedo/odometer is telling me.

I'm losing time, speed and MPG... So I've really been getting around 30mpg while doing about 55mph. Not that really sucks..

The tires are the correct sizr for the car according to the data plate in the door jam.

I'm guessing the odometer is slipping. It's probably unlikely that the rear end gears are wrong or the trans was swapped.

Any thoughts? Pull the speedo and tear it apart? The needle does not seem to bounce around or skip or anything that would look like a broken tooth or such.

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1987 300TDT
1981 VW MKI Caddy 1.6 diesel, waiting on engine swap
1983 D-50 Power Ram 4x4 "Mitsubishi" 2.3 turbo diesel
assorted gas powered crap and motorcycles

RIP: 1984 300TDT, 1982 300TDT, 1984 190D 2.2, 1992 300D 2.5, 1987 300TDT, 1982 Maxima LD28, 1983 Maxima LD28, Isuzu C223 P'ups X3, 1983 Holiday Rambler 6.2 Banks turbo diesel, 1984 Winnebago LeSharo 2.1 TD, 1985 Allegro 6.5

Not sure about your odo, but I was under the impression that most speedos are 4-5 mph off (normally over) straight from the factory. GPS can also be off about 2mph. Try checking next time you drive by a radar "your speed here" sign.

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85 300D 180k

Have you had the speedo out before for any reason? I was repairing my odometer once, and accidently knocked the speedo needle off of the post..I had to make a couple of runs to set it back where it should be (talk about a PITA!!)

I'd bet that your odometer is probably slipping a bit. It is quite common for them to slip / eventually just not work at all. It's a fairly simple fix. There are a number of threads on here about fixing the gear.

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Dan
2005 E320 CDI - 246k
1987 300SDL TD05-16g, Herlevi pump, Elbe manifold, 2.47 LSD - 213k
Past: 1987 300D - 264k

Guessing its not the speedo so much as a slipping odometer. Very common.
The speedo is mechanically driven by a cable, though the pin can wear/bend over time in the housing, giving you an incorrect reading. But much more frequently, it is an odo slipping on the 9s.

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Live with it; everyone else with a 70-80's era MB odo does.

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Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)

well...with 450,000 plus on the clock it could be wearing out and slipping. the radar speed signs always seem to be about 5mph slower than the speedo is showing, i just thought the law was being a little leniant...not.

I guess I'll play with the GPS some more and see what the trend is. I did the 100 miles test with highway markers a couple times on long sections of straight interstate...all about 115 on the odo.

I would think the speedo and odo are directly connected, so they both err the same...

__________________
1987 300TDT
1981 VW MKI Caddy 1.6 diesel, waiting on engine swap
1983 D-50 Power Ram 4x4 "Mitsubishi" 2.3 turbo diesel
assorted gas powered crap and motorcycles

RIP: 1984 300TDT, 1982 300TDT, 1984 190D 2.2, 1992 300D 2.5, 1987 300TDT, 1982 Maxima LD28, 1983 Maxima LD28, Isuzu C223 P'ups X3, 1983 Holiday Rambler 6.2 Banks turbo diesel, 1984 Winnebago LeSharo 2.1 TD, 1985 Allegro 6.5

I guess I could leave it alone but would rather have it be correct...

now if it was under what I'm actually doing I guess I would not care since I'm getting "extra", lol

__________________
1987 300TDT
1981 VW MKI Caddy 1.6 diesel, waiting on engine swap
1983 D-50 Power Ram 4x4 "Mitsubishi" 2.3 turbo diesel
assorted gas powered crap and motorcycles

RIP: 1984 300TDT, 1982 300TDT, 1984 190D 2.2, 1992 300D 2.5, 1987 300TDT, 1982 Maxima LD28, 1983 Maxima LD28, Isuzu C223 P'ups X3, 1983 Holiday Rambler 6.2 Banks turbo diesel, 1984 Winnebago LeSharo 2.1 TD, 1985 Allegro 6.5

Would not a slipping odometer show less miles travelled? Certainly never more. With your 15 percent overage maybe someone has changed the rear end or speedometer out sometime. With your milage indicated I would slide under the car and check the rear end ratio. It is stamped on the housing.

Or look on the bright side. The car might have less actual milage if someone switched out the original speedometer head when the original failed.

There is unfortunatly a flip side to that as well of course. You may be in the million mile club and just unaware of it. Did you have to remove a slew of badges off the grill when you aquired the car to let the radiator get air?

hmmm, well the car was a one owner and very, very well cared for. I would think that if the speedo was ever replaced it would have been with a correct one and probably the mileage set correctly by the dealer or very legit MB shop. But most likely it's original I think.

the speedo needle did bounce a little on the way home, guess i really never noticed it. and when it was bouncing the cruise would not hold and kept kicking off.

But I think you are right...a slipping speedo would show slower and less distance than actually traveled...so the mystery deepens....

__________________
1987 300TDT
1981 VW MKI Caddy 1.6 diesel, waiting on engine swap
1983 D-50 Power Ram 4x4 "Mitsubishi" 2.3 turbo diesel
assorted gas powered crap and motorcycles

RIP: 1984 300TDT, 1982 300TDT, 1984 190D 2.2, 1992 300D 2.5, 1987 300TDT, 1982 Maxima LD28, 1983 Maxima LD28, Isuzu C223 P'ups X3, 1983 Holiday Rambler 6.2 Banks turbo diesel, 1984 Winnebago LeSharo 2.1 TD, 1985 Allegro 6.5

Your MPG doesn't "suck", you are just getting closer to a more accurate reading. Reference the thread a week or two ago about MPG. All of those high MPG claims you see are almost all flat out wrong. The people aren't getting good measurements (multiple tanks of fuel with long round trips with the same wind conditions, speed verified by GPS and odometer compared to maps).

well, the math was consistently showing 35 mpg for me using the odometer and fuel reciepts...and i was religious about maintaining 60mph.....

but then when miles driven is corrected for what the highway markers verses odometer shows, it drops to like 30-31 mpg...which is still above the official ratings but not as good as i hoped. I know my MPG doesn't really suck but it still sucks that i'm not getting the 35 i thought i was, haha

__________________
1987 300TDT
1981 VW MKI Caddy 1.6 diesel, waiting on engine swap
1983 D-50 Power Ram 4x4 "Mitsubishi" 2.3 turbo diesel
assorted gas powered crap and motorcycles

RIP: 1984 300TDT, 1982 300TDT, 1984 190D 2.2, 1992 300D 2.5, 1987 300TDT, 1982 Maxima LD28, 1983 Maxima LD28, Isuzu C223 P'ups X3, 1983 Holiday Rambler 6.2 Banks turbo diesel, 1984 Winnebago LeSharo 2.1 TD, 1985 Allegro 6.5

If its a car you want in pristine condition have the odo recalibrated. These folks rehab mechanical and electrical units. They also sell parts.

http://paspeedo.com/about.htm

__________________
Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)