Mileometer over-recording

bluejon

bluejon

Club Member
I'm discovering that the milometer on my car is over-recording mileage to the tune of about 15%. The car thinks it is going further than it actually is. I haven't tested to see whether the speedo is doing the same but I would guess it is.

I'm running 14" wheels with a 195/60 profile tyre, so would that be contributin versus OEM? Could it be a cable issue (hmm maybe but low likelihood?). Or is it rooted where the speed/distance is being read from the drivetrain? By what mechanism does this take place - gearbox/diff?

Note that the gearbox and diff are not the original (4spd -> 5 spd with R200 diff). If this is the likely cause, what is the solution to recalibrating ?
 
S

Stockdale

Club Member
bluejon said:
I'm discovering that the milometer on my car is over-recording mileage to the tune of about 15%. The car thinks it is going further than it actually is. I haven't tested to see whether the speedo is doing the same but I would guess it is.

I'm running 14" wheels with a 195/60 profile tyre, so would that be contributin versus OEM? Could it be a cable issue (hmm maybe but low likelihood?). Or is it rooted where the speed/distance is being read from the drivetrain? By what mechanism does this take place - gearbox/diff?

Note that the gearbox and diff are not the original (4spd -> 5 spd with R200 diff). If this is the likely cause, what is the solution to recalibrating ?
Click to expand...

Not sure of an original tyre size for a 240Z. For a 260Z I gather the size fitted when the car was new was 195x70x14. that would give a circumference (new tyre) of about 1980mm. A 195x60x14 when new will have a circumference of around 1850mm. The difference is almost 8%. As tyres age and approach their minumum legal limit the circumference on these tyres will probably be reduced by 30-40mm .

I gather that there is a very considerable manufacturers permitted tolerance (up to 110% of actual speed + 6.25mph) on speedometer readings and I would imagine that, depending on the mechanism driving the mileometer, similar margins exist in recording distance :unsure:.
 
jonbills

jonbills

Membership Secretary
Site Administrator
my 14" tyres are 195/70 and speedo/mileage are more or less spot on compared to gps speed
 
niroshann

niroshann

Club Member
I have some issues with my car which is a 280zx. The readings are understated on speedo (3-5mph less) and mileage since I did a manual conversion so I need to use the correct gear at the speedo cable pickup - I am still using the diff from the auto setup. Wondering if the diff and transmission setup in your car is matched - assuming that tyres are all ok as per earlier comments.
 
A

andrew muir

Club Member
The cause will be the 60 section tyres to a lesser extent.
The main reason will be the diff each diff has a different colour of speedo drive gear.
they could be white blue or black depending on which diff ratio you have installed.
I am pretty shure yours is not matched correctly.
You need to find out what diff is fitted and then check the speedo drive gear at the gearbox.:thumbs:
 
bluejon

bluejon

Club Member
Good comments so far, thanks guys. By my calcs, running a 195/60 gives a rolling circumference 4% smaller than a 195/70 tyre. This is contributing a portion of the error then - the wheel is travelling less physical distance per revolution relative to what the car believes is the case. And I agree with Andrew that the diff/speedo drive gear is chief culprit. Can anyone show what this looks like if I get under the car ?
 
AliK

AliK

Vehicle Dating Officer
Staff member
Club Member
You'll struggle to see it as it's inside the gear box on the side. Off side iIRC. You'll have to take it out to see what colour it is.

It looks like this when out ...

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a7c44e06f663a23a368e67526699e3d3.jpg
 
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