Speedo Gear Impacting Mileage

[wildbill]

This is probably a dumb question and I think I already know the answer, but does an incorrect speedo gear also impact the mileage recorded on the odometer?

 

[GeorgeSully]

Yes


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[TJ_2000]

It will give you incorrect mileage readings

 

[wildbill]

Yes


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Thanks. Figured as much but wanted the confirmation.

 

[GeorgeSully]

Thanks. Figured as much but wanted the confirmation.

No problem, that’s why you should always check when buying jeeps with big tires how long they’ve been on there.


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[Davidtj]

There are a lot of great apps out that will give you actual speed.

 

[dbbd1]

It will also skew your gas mileage numbers. Unless you compensate for the actual vs shown miles.

 

[Chris]

Yes, it can. Though usually it's pretty negligible depending.

Not that TJs get any sort of decent mileage to begin with, haha.

 

[jazngab]

So would bigger tires without changing the speedo gear make the mileage off higher or lower?

 

[dbbd1]

So would bigger tires without changing the speedo gear make the mileage off higher or lower?

You will be going faster than your speedo indicates.

 

[dbbd1]

Yes, it can. Though usually it's pretty negligible depending.

Not that TJs get any sort of decent mileage to begin with, haha.

True.
It's just that when people scream "why is my TJ only getting 6mpg?" This is usually the reason.

 

[jazngab]

You will be going faster than your speedo indicates.

So the odometer reading would be reading lower miles than it actually is, correct?

 

[wildbill]

So the odometer reading would be reading lower miles than it actually is, correct?

I think it would read more than the actual. If your speedo is reading that you're going faster than what you really are, I would think the computer would interpret that as you going farther than what you really are... I could definitely be wrong in my logic. Maybe someone more mathematical than me can chime in. lol

But your question is actually the premise of mine. I was trying to calculate the ending mileage for my 500 mile break-in for new gears. My speedo was off by 30%. If my math is correct, I needed to travel 650 miles per my incorrect odometer.

 

[wildbill]

You will be going faster than your speedo indicates.

this wasn't the case for me. My speedo was reading faster than my actual.

 

[dbbd1]

If your system thinks that it is turning 30" tires that go X distance per revolution that is what it will report, even if your larger tires are now going X-plus distance per revolution.

 

[Kiwi TJ]

I'm thinking it will put fewer km (miles) on your TJ.
The odometer measures the number of revolutions per mile and with a bigger tyre the revolutions will be less as it has a bigger diameter or circumference

 

[wildbill]

If your system thinks that it is turning 30" tires that go X distance per revolution that is what it will report, even if your larger tires are now going X-plus distance per revolution.

Wouldn't the gearing also play into the calculations? I changed tire size and gearing. Went from 3.73 to 4.88. Now that my axles are spinning faster wouldn't that also impact the reading? That would seem to make sense because my speedo was reading faster than my actual.

 

[dbbd1]

Wouldn't the gearing also play into the calculations? I changed tire size and gearing. Went from 3.73 to 4.88. Now that my axles are spinning faster wouldn't that also impact the reading? That would seem to make sense because my speedo was reading faster than my actual.

You contradicted yourself.
Yes, gearing will make a difference on actual speed, not indicated speed. Unless, you change the speedo gear (non-rubi).
But, your axles are actually turning slower now, with the higher numerically gears. Which is why your speedo is reading faster than actual.

Gearing, tires both play parts in actual speed.
The speedo gear (non-rubi) tells the PCM what speed it "thinks" that you're going.

 

[wildbill]

You contradicted yourself.
Yes, gearing will make a difference on actual speed, not indicated speed. Unless, you change the speedo gear (non-rubi).
But, your axles are actually turning slower now, with the higher numerically gears. Which is why your speedo is reading faster than actual.

Gearing, tires both play parts in actual speed.
The speedo gear (non-rubi) tells the PCM what speed it "thinks" that you're going.

OK. Thanks. I thought I had read a few times on some of the driveline posts people referencing the axles spinning faster after regear. Thanks for the clarification.

 

[dbbd1]

Your driveshaft and things forward of the diff will be turning faster to go the same speed as before. That's why your RPM's are so much higher to go 60mph than before you re-geared.