Increased tyre size

Submitted: Wednesday, Jun 16, 2004 at 08:40
ThreadID: 13813 Views:1798 Replies:5 FollowUps:2
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I have a 100 series 4.5 auto petrol LC. I have recently upgraded the tyres from the standard 275/7R16 to 285/75R16. This has caused the speedo to be out by approximately 5%.

I have read other posts on this forum about speedo correction. These have mainly been to do with diesels and manuals. I have also read on here that Toyota can make the adjustment (but at a price) and that there is also a kit from Marks 4wd adapters (I have confirmed this is at $175).

I have also heard that the speed sensor in the auto petrols controls more than just the speedo. My questions is, what have other 100 series owners done in a similar situations and why.

Thanks
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Reply By: rolande- Wednesday, Jun 16, 2004 at 08:46

Wednesday, Jun 16, 2004 at 08:46
Cammo,
Have you checked manufacturer's innacuracy. Most speedo's already set too conservative at 100km/h, usually between 5 - 10 % to stop any legal action against them by customers. You may find your speedo is now accurate. Others would have checked using GPS or time over distance measures.
Rolande
AnswerID: 63368

Reply By: GaryW - Wednesday, Jun 16, 2004 at 13:19

Wednesday, Jun 16, 2004 at 13:19
I have 285/75/16's on the LC100 T/D Auto. On the trip this w/e to the Grampians we went through 2 of the speed check thingos. Both times I had the cruise running at 99kph on the speedo. First one read 101kph second one read 105kph. Go figure.

If my maths is correct the difference in tyre circumference is 5%. ie. speed would be different by 5%. As mentioned by others, speedos out of the factory usually show speeds higher that actual speed, so changing tyres sizes up should bring it back towards accuracy (or over compensate).

I'm currently working on the basis that I need to take off 5% of the speedo reading to get to actual speed. A few more trips under the speedo checks - or a dyno reading is on the short list. BTW I was told there was no fix for this so I will be following up the Marks 4WD lead ASAP.

Gaz
AnswerID: 63420

Follow Up By: Cammo - Wednesday, Jun 16, 2004 at 13:24

Wednesday, Jun 16, 2004 at 13:24
Gaz

Here is a response I have just received from Marks 4wd adapters

Dear Cameron
Thankyou for your email.
The ECU and Speedo on the 100 series use the same speed sensor as the input
sensor.
If you have fitted larger tyres not only will the speedo be out but the
signal to the ECU will also be altered.
Our electronic ratio box part no. MFK1180 will correct this.
If you have any further questions do not hesitate to contact me.
Best regards
Rob Martin

I suppose the questions is, how much will a 5% difference affect the engine management system.
0
FollowupID: 324671

Follow Up By: Wazza (Vic) - Wednesday, Jun 16, 2004 at 15:10

Wednesday, Jun 16, 2004 at 15:10
Gary,

I have been through those speed traps and at 110km/h on the GPS they always read higher (they say you are doing 113km/h for example on the Hume Hwy north of Kalkallo). They are being conservative and slowing down the general public by 3km/h. (Mr Public thinks he is doing 113, so he slows down by 3km/h)
0
FollowupID: 324680

Reply By: Member - Captain (WA) - Wednesday, Jun 16, 2004 at 14:06

Wednesday, Jun 16, 2004 at 14:06
Hi Cammo,

Gotta agree with Rolande here, my speedo was way out.

With my original 265/70R16, the speedo read 106 km/hr at a true 100km/hr (Garmin GPS V). When I changed to 285/75R16's (increase of 7%), the speedo now reads 99km/hr when doing a real 100km/hr, much more accurate (-1% error compared to +6%) IMHO.

As my engine management also reads of this sensor (so I am led to believe), the larger tyre size actually makes it more accurate.

So, before spending money on a speedo recalibration, check out what the real speed reading is. You may find you have made it more accurate.

Cheers

Captain
AnswerID: 63429

Reply By: Member Eric - Wednesday, Jun 16, 2004 at 16:59

Wednesday, Jun 16, 2004 at 16:59
Not to sure on some of the replies , I have fitted 315/75/16 to a 100 series , the speedo is around 8ks out . Vehicle seams to not have suffered any ecu problems .

The reply I am talking about is the one where the speedo read more acuratly
AnswerID: 63461

Reply By: Croozer - Wednesday, Jun 16, 2004 at 23:53

Wednesday, Jun 16, 2004 at 23:53
I have a new 100 series TD to which I have just fitted 285/75 BFG AT's. With the tyres brand new on a recent trip across the nullabor the gps indicated a steady 104 k/ph with the cruise set on 100 k/ph on the speedo.

Stuart
AnswerID: 63523

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