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View Full Version : I havn't been able to find a sure answer on this. Does a j30 vlsd affect speedometer?

240sxDann
08-05-2008, 09:36 AM
I just picked up a 93 coupe, and it has a j30 vlsd. Does this affect the speedometer? It does also have 18" wheels on it with 245/45 tires on it, so I know that affects it some. Main reason I'm asking is because either my speedo is acting funny (as in reading way faster then I'm actually going, which wouldn't make sense with bigger wheels, they would make it read slower) or this things a lot faster then I expected it to be. At about 6k rpm 4th gear I'm at around 100mph. (on a closed course obviously:squintd:) So my question is would the j30 affect my speed and make it read way off, or does that not make any difference?
fliprayzin240sx
08-05-2008, 09:55 AM
Shouldnt make any difference...
TTrx7Pete
08-05-2008, 10:37 AM
I just picked up a 93 coupe, and it has a j30 vlsd. Does this affect the speedometer? It does also have 18" wheels on it with 245/45 tires on it, so I know that affects it some. Main reason I'm asking is because either my speedo is acting funny (as in reading way faster then I'm actually going, which wouldn't make sense with bigger wheels, they would make it read slower) or this things a lot faster then I expected it to be. At about 6k rpm 4th gear I'm at around 100mph. (on a closed course obviously:squintd:) So my question is would the j30 affect my speed and make it read way off, or does that not make any difference?

Speed is read at the transmission. I'm not familiar with the final drive ratios of the factory 93 coupe and a J30. This can affect your speed, but not the reading of it. The speedometer gets it reading by the teeth turning on the transmission, from there an algorithm is performed which translate the speed of the transmission into MPH or KMH.

As for rim/tire size, there are some calculators online that you can input tire size from the factory and get a resulting tire size for different sized rims. These calculators pretty much allow you to upsize the wheel while maintaining an exact or close to exact overall wheel/tire diameter.
240sxDann
08-05-2008, 12:43 PM
Speed is read at the transmission. I'm not familiar with the final drive ratios of the factory 93 coupe and a J30. This can affect your speed, but not the reading of it.

But the speed it would be reading would be incorrect then right? The sensor says "the transmission spins this many times in x amount of time, so I must be going this speed" however, thats set up for stock gearing. With different gearing, the sensor reads the same amount of revolutions, and it assumes you have stock gears, so its gonna read the same, when in reality, the differential gearing is either making you go faster or slower at that given speed of the shaft the sensor reads off of, as compared to the stock gearing. Yes?:ugh:
projectRDM
08-05-2008, 12:49 PM
Ratio is the exact same, 4.08:1, unless you got a later model J30t diff, which is 3.92:1. Even that would only be off 1-2MPH.

Search would have told you that. The Q45 is the only R200 diff with a lower ratio.
240sxDann
08-05-2008, 03:14 PM
Ratio is the exact same, 4.08:1, unless you got a later model J30t diff, which is 3.92:1. Even that would only be off 1-2MPH.

Search would have told you that. The Q45 is the only R200 diff with a lower ratio.

I apologize, I wasn't sure what to search for. Thank you for the responses.