Odometer/Speedometer 10% off?

I've got an '05 Outback XT - I've been a bit disappointed with the mpg's I've been getting, so I've started to pay attention to my mileage. My rough calculations make me think my Odometer (and maybe Spedometer) is off by about 10%.

 

The two ways I calculated:

1) Using the mile markers on the Highway, after 50 miles (marker to marker) my odometer shows 49.5 miles.

2) GPS - I see that on my spedometer I'm going "60" when my GPS shows 55ish. Unfortunately, I wasn't paying attention to how many miles the GPS said I traveled vs. odometer milage.

 

I know these are very rough, but 10% is large. Is that normal? What led me to this is I'm only getting around 23mpg on long highway trips (200+ miles) averaging around 65-70mph. Somewhat disappointing, I think.

According to my GPS, my LGT wagon is around 1-3% off. When the speedometer says 70, it's more like 69, and when it's 80, the GPS reads 78. Haven't done the mileage calc yet though..

I wonder if Subaru is aware of this. It would only require a tweak in the ECU to fix.

If you showed 49.5 miles on a 50 mile course that is only a 1% error, not 10%.

 

Also, GPS ground speed indication is not always very accurate, so I wouldn't rely on it.

 

Usually the spedometer will have about the same amount of error as the odometer as they both come off the same sensor.

 

Most cars have a spedometer/odometer error, but they generally read on the high side. If you changed tires or your stock tires are worn, that is enough to cause a change in the accuracy of the readings.

 

Using the mile markers on the Highway, after 50 miles (marker to marker) my odometer shows 49.5 miles.

 

 

 

That is a 1% difference, not 10%. I get about 23 mpg with my Legacy GT with about 80% highway driving.

 

Tom

 

edit - sduford beat me to the math

I meant to type 45.9 on a 50 miler. Stock tires/stock everything. I'm going to check my tire pressure in a few. What should they be? They don't look low.

According to my GPS, my LGT wagon is around 1-3% off. When the speedometer says 70, it's more like 69, and when it's 80, the GPS reads 78. Haven't done the mileage calc yet though..

 

That's exactly what I'm reading on my GPS on my GT wagon.

 

When I set my cruise to exactly 70mph on the speedo, my Garmin Quest reads 69mph.

Most cars have a spedometer/odometer error, but they generally read on the high side. If you changed tires or your stock tires are worn, that is enough to cause a change in the accuracy of the readings.

 

I have a friend who used to work in instrumentation for GM cars. He told me that gauges are purposefully conservative to save the consumer's butt. They get fewer complaints when "E" on the gas gauge really means there's a gallon or so left than if it meant "truely empty". Also, the coolant temp gauges aren't a linear scale either (even if they are marked with degree measurements).

Come on guys - why complain about 1 mph at 70? That means at 140 it's off by only 2 mph so how bad can that be??

 

FWIW, mine's been dead on with one of those roadside "Your speed is" radar signs.

how would a dealer test this. GPS or radar?

Yes my GPS read the same thing. The speedo is off a little.

Actually, I think it is the tires. I have a buddy at the Subaru Dealership and he said the tires on the USspec and JapSpec are different by a little. However, they did not change the speedo cluster or speedo sensor. So, in theory the odometer and the speedo are off by a little too.

Post scans of your tickets for speeding twice at 70 and 140 in a 45 so we can get this over.

 

On a more serious note, this thread is not worth the disk space it's stored on.

Don't be hatin'!

 

Post scans of your tickets for speeding twice at 70 and 140 in a 45 so we can get this over.

 

On a more serious note, this thread is not worth the disk space it's stored on.

I have tested the speedo on all my veicles for about the past 12 years with GPS, and radar prior to that. Most were off by a little bit (all reading high). I had a 79 Fairmont and 91 4Runner that were both reading by 5 mph. Both were factory stock (who would put a nickle into moding a 79 Fairmaont lol)

Drop a 429 in that Fairmont, Auto tranny with a 3500 stall and a posi rear...You would have a street sleeper that looks good enough for your granny to drive!