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I supported Toymods
Location:
Epping, Sydney
Registered:
May 2002
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speedo is 10-15% out - why?
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Sun, 02 February 2003 05:18
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I had an unpleasant suprise over the weekend. Heading up the coast for a weekend with some mates, I found they kept lagging behind. I was only doing 115, so I slowed down.... and had to keep slowing down to about 90 for them to catch me.
Now, I figued the guy was distracted by his girlfriend or some such, but when I got up there he demanded to know why I'd done 140 the whole way up. whoops.
we matched speeds on the way back, and my speedo reads 95-98 when the car is actually doing 110. what could be causing this? I know one problem could be total wheel_tyre diameter.
My wheels are the stock mk2 supra ones, with P235/60R14 S96 rubber on them. how do these compare to the stocker supra dimensions?
also, any other ideas on what the problem may be? I dont actually know where the speed sensor is, so maybe the new gearbox could be causing issues?
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Location:
Brisbane
Registered:
May 2002
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Re: speedo is 10-15% out - why?
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Sun, 02 February 2003 06:26
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Factory rubber is 225/60x14, so your overall diameter is slightly larger which would have a small effect on the accuracy of your speedo... not that much though. The speedo is connected to the output shaft of the gearbox, so if you've swapped gearboxes that could definitely screw up the speedo.
There are little nylon gears in the box which drive the speedo cable, and they have a ratio which matches the diff ratio and the wheel diameter; if your box came out of a car with a different diff ratio and/or wheel diameter, the speedo drive will be incorrect and thus give inaccurate speedo readings.
If this is the case, the fix is pretty easy - just get the correct speedo drive gears and swap em in. There are a wide variety of ratios available and they're easy to swap, but I think you have to remove the gearbox to do it unfortunately.
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I supported Toymods
Location:
Epping, Sydney
Registered:
May 2002
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Re: speedo is 10-15% out - why?
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Sun, 02 February 2003 12:51
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aahhhh crap
I had a nasty feeling that may have been the case. One more thing to add to the ever growing "to fix/improve" list 
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Location:
Cabramatta, NSW
Registered:
May 2002
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 Re: speedo is 10-15% out - why?
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Mon, 03 February 2003 00:25
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Draven, welcome to the club! 
You're off by pretty much the same ratio as me. I drive 110km/h down the freeway and people tell me that I'm doing close to 130.
95 on my clock matches 110 I think.
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Location:
Brisbane
Registered:
May 2002
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Re: speedo is 10-15% out - why?
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Mon, 03 February 2003 00:51
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Would it be possible to make up a little converter box to put in just before the speedo?
It would help if you had the diff ratios and the number of speedo gears, then just make a step down gear set? Probably more work than its worth tho!
Cheers
Wilbo
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Location:
Perth
Registered:
July 2002
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Re: speedo is 10-15% out - why?
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Tue, 04 February 2003 15:48
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Autospeed had an article on speedometers last week. This bit would be relevant to you:
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Recalibration to Suit Gearing Changes
If the speedo unit itself is functioning accurately but you've altered the car's overall gearing (diff ratio or wheel diameter), you need a ratio box. The first step is to drive the car with a pulse counter plugged in to measure the number of speedo shaft revolutions over a measured distance (say, one kilometre). This number is then cross-referenced on a special chart and the right gear for the job is chosen from the list. It's then just a matter of putting the right gear set into the small ratio box housing and installing it to the cable between the gearbox and the speedometer input. These mechanical ratio boxes generally cost around $110 retail.
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Source: http://www.autospeed.com/A_0346/P_1/article.html
Regards,
Liam.
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Location:
Brisbane
Registered:
May 2002
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Re: speedo is 10-15% out - why?
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Tue, 04 February 2003 23:38
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That would work, but it sounds a lot harder and more expensive than doing it the right way...
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I supported Toymods
Location:
Perth
Registered:
May 2002
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Re: speedo is 10-15% out - why?
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Wed, 05 February 2003 00:14
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Which method are you saying is the "right" way Norbie? I'm confused 
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Location:
Brisbane
Registered:
May 2002
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Re: speedo is 10-15% out - why?
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Wed, 05 February 2003 01:09
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| Norbie wrote on Sun, 02 February 2003 17:26 |
If this is the case, the fix is pretty easy - just get the correct speedo drive gears and swap em in. There are a wide variety of ratios available and they're easy to swap, but I think you have to remove the gearbox to do it unfortunately.
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I'm pretty sure a little reduction box would be easier than removing the gearbox! I agree though that it would be better to fix the problem correctly in the first place. But a reduction box does make sense as well, its easy to get to and it should be cheap to recalibrate if you make a modification that changes the effective ratio again (change diff, gearbox, tires). You would only need to change one cog in your little box and it would all be good again!
Cheers
Wilbo
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Location:
Sydney
Registered:
September 2002
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Re: speedo is 10-15% out - why?
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Wed, 05 February 2003 01:20
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I remember reading that you could use one of those bike speedos, like you can get from dick smith. All you have to do is chuck in the pulse reader in the wheel, and plug in your wheel's circumference.
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Location:
Brisbane
Registered:
May 2002
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Re: speedo is 10-15% out - why?
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Wed, 05 February 2003 02:50
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| wilbo666 wrote on Wed, 05 February 2003 11:09 |
I'm pretty sure a little reduction box would be easier than removing the gearbox!
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True, however this situation usually arises when you're changing the gearbox from the factory item to something else, and in that case it's easy to swap the speedo gears before you install the box.
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Location:
Brisbane
Registered:
May 2002
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Re: speedo is 10-15% out - why?
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Wed, 05 February 2003 04:57
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Agreed, if at all possible (i.e. the gearbox is out) change the speedo gear then for sure!
Cheers
Wilbo
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I supported Toymods
Location:
Epping, Sydney
Registered:
May 2002
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Re: speedo is 10-15% out - why?
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Sat, 08 February 2003 05:10
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unless you're stuck in my situation where the gearbox is already in and the car working.
I'm not too keen on taking it all out again, if it can be avoided at all.
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