using bigger wheels

liberated · Post by **liberated** » Wed May 04, 2011 10:03 pm

hello all, just a quick question has anyone had any trouble with their speedo after lifting and adding larger tyres or is it driven from the gear box?

suthol · Post by **suthol** » Wed May 04, 2011 10:22 pm

Check the change in rolling diameter because you could fall foul of radar traps depending on how much tolerance you get in your state or those that you visit.

Typically a Subie speedo will read about 7Kph fast at 100Kph which is OK under the ADR ( up to 10% ) but must not read slow at any speed, a larger tyre means you are going faster for the same revs.

Typically in NSW you get +10% on police operated radar and +3% on the private Macquarie Bank run radar. ( which is there to promote safety and not make a profit for shareholders )

90brumby · Post by **90brumby** » Wed May 04, 2011 10:42 pm

A thing called tyre calculator on Google will give u the spedo difference in a percentage all u need is new tyre and old tyre

Backyard_Brumby · Post by **Backyard_Brumby** » Wed May 04, 2011 11:04 pm

also if you have one of the gps units you can check the speedo while doing a highway run, I think from memory after putting 14s with bigger tires the speedo was reading under about 2 or 3 k's from the standard

YouTube · Post by **El_Freddo** » Wed May 04, 2011 11:16 pm

suthol wrote:Typically a Subie speedo will read about 7Kph fast at 100Kph which is OK under the ADR ( up to 10% ) but must not read slow at any speed, a larger tyre means you are going faster for the same revs.

I think you'll find that current ADR's stipulate that -10% +0% is the speedo inaccuracy that is approved for vehicles produced somewhere near 2006 onwards. The previous ADR's for speedo accuracy was the good old +/-10%...

Why we can't produce vehicles with accurate speed ratings in the 21st century is beyond me! I realise that this would change with owner modifications but from the factory I'm sure we have the technology for a spot on speedo reading with new tyres.

As for using bigger wheels - on my L with 27's it seems to be out about 15km/h at 100km/h and about 10km/h at speeds up to 60km/h.

Either way - running larger tyres, don't go doing anything stupid and you'll be right.

Cheers

Bennie

woody.t · Post by **woody.t** » Wed May 04, 2011 11:28 pm

i know my dad had his old triumph speedo cable adjusted in some way or another to have the speed read accurately, can this be done for the subi's aswell? specifically a gen 1 liberty.

sorry about jacking the thread.

d_generate · Post by **d_generate** » Wed May 04, 2011 11:40 pm

Here, type in old & new to get the difference.

http://www.net-comber.com/tirecalc.html

dobo · Post by **dobo** » Thu May 05, 2011 9:44 am

El_Freddo wrote: Why we can't produce vehicles with accurate speed ratings in the 21st century is beyond me! I realise that this would change with owner modifications but from the factory I'm sure we have the technology for a spot on speedo reading with new tyres.

I think its something to do with the car reaching its warranty quicker, my girls 2010 Dualis is -10% out, it really is ridiculous.

cheers

Venom · Post by **Venom** » Thu May 05, 2011 10:06 am

Any mechanical speed sensor is going to be flawed. Off the top of my head you could just consider tyre wear, which is say 10mm over the life of the tyre. Thats a 10mm difference to the rolling diameter and something in the order of 5% speedo error over the life of an average tyre. Then of course a vehicle manufacturer would want to be conservative in addition to that because who wants to be responsible for every speed fine inccured by the cars you manufacture and the potential litigation costs of that. Or someone saying i crashed because you car made me go around a corner 5km faster than i thought i was doing. Maybe its compensating for an idiot factor? Then as mentioned your speedo ticks over slightly faster, which could be signigicant when your add that up over the millions of cars you service. So to me it makes a lot of sense from a manufacturers stand point. Whats the benefit to them for giving you an accurate speedo/odo reading?

Matt · Post by **Matt** » Thu May 05, 2011 7:46 pm

And the best way to adjust it is adjust the needle on the speedo cluster carefully pull it off and set it 0km / hr point to just below 20 which should give you pretty accurate reading if your running 27" tall tyres on a MY wagon. You will need to check this with a GPS, etc.

d_generate · Post by **d_generate** » Thu May 05, 2011 10:57 pm

I actually put the face from my Liberty (220kph) in place of the WRX (260kph) and it made the speedo out by 3 - 6% instead of 20% after we put the 4.44 cw & p in the hi low 3.9 box......Though I have no idea what the trip meter is doing lol.

ScubyRoo · Post by **ScubyRoo** » Fri May 06, 2011 10:42 am

woody.t wrote:i know my dad had his old triumph speedo cable adjusted in some way or another to have the speed read accurately, can this be done for the subi's aswell? specifically a gen 1 liberty.

sorry about jacking the thread.

Gen 1 liberty are a cable speedo - you need to change the drive gear inside the gearbox to one from a gearbox with the same tire size / diff combination as you run. Painful. When I had big wheels on my lib, I made a table with 5km/h from 40km/h up that sat next to the speedo. In time I memorised the new speeds.

with gen 2 on (I think, but my car is JDM so it might be different for others) the speedo is electric, so you can get an interceptor box which changes the signal and makes your speedo accurate - about $35 from jaycar.

Cheers,
Owen.