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30 inch tyre's and speedo
https://auszookers.com/forum/viewtopic.php?f=37&t=53944		 Page 1 of 1
Author:  Yake [ Sat Feb 25, 2017 3:26 pm ]
Post subject:  30 inch tyre's and speedo
Hey all just wondering if someone had a rough idea how much running 30's effects your speedo reading? everything else is stock and its a sj70 (next upgrade will be reduction gears for sure :roll: )
Cheers.

(I'm aware you can use a GPS speedo app but just thought I'd ask :) )
Author:  greenzook89 [ Sat Feb 25, 2017 8:33 pm ]
Post subject:  Re: 30 inch tyre's and speedo
when I had 30s on my coily it was roughly 10% slower, so at 100 on the dash I was actually doing 110
Author:  bumstein [ Sat Feb 25, 2017 10:19 pm ]
Post subject:  Re: 30 inch tyre's and speedo
There is also plenty of converters available to see very accurately what the difference is. There's a heap of smartphone apps like TireCalc that do the same thing.

Plug in the standard tyre size and the size of the tyre you want to use and it will give you the % increase on circumference (as well as other info). That percentage is close to the increase in speed over the speedo reading.
Author:  alien [ Sat Feb 25, 2017 11:27 pm ]
Post subject:  Re: 30 inch tyre's and speedo
stock tyres are 26" (roughly) and a 30" tyre equates to approx 15% increase in size.. therefore the speedo is likely +15% out of whack... ie: 100km/hr on the speedo means you're really doing 115!

DEFINITELY use a gps (ie: your phone) to track and map out your true speeds. If you're really pedantic, you can find a shop that will modify your speedo to get it reading right again. I did this with mine and it cost $110. Basically they added a bit of extra material to the spring (though the bloke did far more calculations in order to know how much to add) and mine now reads about 2% over actual speed. ie: 100km/hr = ~98km/hr.
Author:  bumstein [ Sun Feb 26, 2017 12:15 am ]
Post subject:  Re: 30 inch tyre's and speedo
Spot on Alien, 14.3% difference. I plugged in the calc (based on rough conversion for imperial 30" to metric size.

Image

Where did you go to get your speedo recalibrated?
Author:  alien [ Sun Feb 26, 2017 8:04 am ]
Post subject:  Re: 30 inch tyre's and speedo
http://www.appliedinstruments.com.au/
Author:  bumstein [ Sun Feb 26, 2017 8:49 am ]
Post subject:  Re: 30 inch tyre's and speedo
Cheers [emoji4]
Author:  Yake [ Sun Feb 26, 2017 9:44 am ]
Post subject:  Re: 30 inch tyre's and speedo
Wow thanks guys for all the responses :peaceout: awesome that helps heaps appreciate it! also add on question I've noticed a lot of people run stock sized tyres as spares when they are running 32 inch plus tyre's on the car, just wondering what peoples thoughts on that are? is it safe to use such a small tyre if you do get a flat? thanks
Author:  bumstein [ Sun Feb 26, 2017 9:55 am ]
Post subject:  Re: 30 inch tyre's and speedo
Best bet in the event of a flat is keep a tyre plug kit and air compressor with you. Fix it on the car (quicker and easier than changing tyres) and keep going.

The chances of you ever having an irreparable flat on a Sierra is slim (some will argue that it will never happen, but common consensus is that it's on the upper end of unlikely).

So!!! Standard size spare is ok as a last resort, large tyres are difficult to put on the tyre carrier and there's near Buckley's chance of really needing it.
Author:  alien [ Sun Feb 26, 2017 12:28 pm ]
Post subject:  Re: 30 inch tyre's and speedo
Larger tyres also weight a hell of a lot more and the tailgate is prone to cracking with the stock spare attached, let alone adding more weight and leverage with something bigger.

I don't run a spare at all on mine.
Author:  Yake [ Mon Feb 27, 2017 8:31 am ]
Post subject:  Re: 30 inch tyre's and speedo
Sweet was thinking along those lines as well I'll just make sure I keep a plug kit and compressor on board :)
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