Is it possible for the speedometer in my 420sel to be correct and the odometer/trip meter to be incorrect? I ask because I noticed i was getting 100 less miles per tank than before. I decided to experiment with a handheld gps and they are almost spot on with speed, but as i drove more and more the difference between the distance covered on the gps and car got greater and greater. The gps showed more mileage driven than the car. The only thing is that i cant really tell if im spending more on gas before because of actual reduced economy or just a faulty reading because i moved to a different house and the landscape and drive times are different. would new gears in the cluster fix this?

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1984 300SD Orient Red/ Palomino
1989 560SEC
2016 Mazda 6 6 speed manual
1995 Ford F-150 reg cab 4.9 5speed manual