Speedo error... is it linked to the odometer?

[Slim]

I've been assessing my fuel economy over the last 4 tanks of fuel and have been a little bit disappointed with the 14.6(ish)L/100km i've been getting around town.

Last night it suddenly dawned on my that my odometer may not be reading correctly. I've always been aware of an error in my speedo, but i naively assumed that my odometer was correct.

What's the likelyhood that my odometer has a similar percentage error that my speedo has?

example... at 100km/h (GPS verified speed), my speedo shows 106km/h.

if my odometer has the same percentage error then the 14.6L/100km i thought i was getting is really ........?

It's going to be even worse economy isn't it?

[midnight_purple_GA70]

I've been assessing my fuel economy over the last 4 tanks of fuel and have been a little bit disappointed with the 14.6(ish)L/100km i've been getting around town.

Last night it suddenly dawned on my that my odometer may not be reading correctly. I've always been aware of an error in my speedo, but i naively assumed that my odometer was correct.

What's the likelyhood that my odometer has a similar percentage error that my speedo has?

example... at 100km/h (GPS verified speed), my speedo shows 106km/h.

if my odometer has the same percentage error then the 14.6L/100km i thought i was getting is really ........?

It's going to be even worse economy isn't it?

I Would assume if your speedo is out then your odo will be out by the same percentage...

i had this problem with my car after changing from a 4.11 diff to a 3.7diff

only mine is way more out than yours... at 100km/h im only doing 80 according to my dash and as a result i have gone coffs to brisbane and its said it was about 330kms... when in actual fact its closer to 400

[JustCallMeOrlando]

Yeah, likely they're both out if one is. Unfortunately for you, it means your fuel economy is actually worse than you originally thought

[spectral]

Fuel economy is worse, but you'll save that money on speeding tickets that you won't get

Brand new cars are usually reading a bit high. Could be from a safety perspective, but the marketing cynic in me says they do it for :
* Owner thinks they are getting better fuel economy
* Owner thinks their car accelerates faster
* Car needs servicing more frequently, so more $$$ at the dealership

[Roundy]

Whilst the speedo on new cars does seem to read high, from those odometer checks that are provided on the side of the road the odometer seems to read spot on.

And yes as stated it does affect the odometer, with my GT4 i had some larger wheels on it that brought the speedo to basically bang on accurate, but the odometer wound up reading about 5% short (4.75km per 5km)

[JustCallMeOrlando]

Cars as far as I have known have always read high. They're only accurate to a point, and they have to read higher than lower by law.

[Billzilla]

Every time I've checked in a car the error has been much the same for both the odometer and the speedo. But FWIW on my bike the speedo is a full 10% out but the odometer is pretty accurate.

[timbosaurus]

Whilst the speedo on new cars does seem to read high, from those odometer checks that are provided on the side of the road the odometer seems to read spot on.

And yes as stated it does affect the odometer, with my GT4 i had some larger wheels on it that brought the speedo to basically bang on accurate, but the odometer wound up reading about 5% short (4.75km per 5km)

Yeah, in new cars (say, post 2000ish), we tend to differentiate the odo and the speedo. People want their odo's to be spot on (+/- a little bit), yet speedo's have to be set to deliberately read over by a fair amount so that even if all the tolerances stack up against it, it STILL reads high (ADR requirement).

But Slim, you're car would predate such stupidity, and would most certainly be linked. So, looks like you need to install a new ECU to lean it out a bit...

[CrUZida]

FYI, cars with speedo cables have the odometer driven by the same cable, so the speedo and the odo will read the same.

Cars with electronics senders (ie, most Toyota's from 1990 onwards - keyword *most*) have the odo and speedo separate.

[Slim]

But Slim, you're car would predate such stupidity, and would most certainly be linked. So, looks like you need to install a new ECU to lean it out a bit...

I know, that's where the powah's at. House mods are more important than car mods tho. So, not for a while.

Thanks for the replies guys.

[o_man_ra23]

Vital point has been missed... and that is how the ODO works compared to the speedo.

Once the speedo cable enters the back of the speedo clustre, it drives a rotating shaft. This shaft has 2 outputs. One is directly coupled to a set of gears, which drives the Odometer. The odo can be inaccurate if your tyres change diameter (different sized tyres, different brands, different treadwear), if you change your diff ratios, if you change the speedo drive gear ratios in the back of the gearbox, or if there is a tooth missing off something.

The second output is coupled to the speedo dial itself, which has a spring trying to pull it to zero. As the speedo gets older, this coupling and spring combination can vary tremendously due to mechanical wear. So even if your speedo is spot on, your odometer can be way out, and vice versa.

Testing your odometer against one of the sets of check posts is a good thing to do, and will give you a better idea of its accuracy rather than guessing that the speedo and odometer are out by the same amount.

[CrUZida]

You are assuming it runs a cable, and that both the speedo and odo are mechanical.

Don't forget the are cars with no cable, but mechnical speedo and odo, ones with mechanical speedo and electronic odo, cars with cable but electronic speedo and mechanical odo.
All combinations are possible.

Not all are 100% mechanical.

[Slim]

Well, i tried to do a test this evening. I turned on my GPS and zeroed the trip meter on it. Then took note of the cressida's trip meter.

When i got home, the car's trip meter had changed exactly 40.0km (114.4km to 154.4km)
The GPS trip meter showed 39km.

Unfortunately, as soon as i'd travelled more than 10km the GPS started rounding the distance to the nearest kilometre. So to get an accurate reading i'll need to do a much longer trip (ie: 39km could be anywhere from 38.5 to 39.4km).

[allencr]

The odometer is directly linked to the turning of the tire(can't speak for electric), so its accuracy is strictly a matter of gearing & tire circumference, it works right or it don't, nothing variable.
The speedometer has a magic spring loaded needle to spinning magnet setup, no direct connection really, that can do almost anything anytime, especially if the cable doesn't spin smoothly from lube or kink.