Tyres (again)

[The Jackel]

Just had the first service done and a comment was passed on my rear tyres being to large

I have 245/40/18's on the rear.

I can't see a problem with this as it seams many Monaro's are now running the same.

Thought it was very strange please can someone put my mind at rest

[V8HSV]

I think they are being cautious that your speedo will be out by 2-3%, so if the indicated speed is 100, you will be doing 103 mph.

I have run 245 section knowing that it stops kerbing of the rims without any problems at all, use RA2 for accuarate speed.

[booster]

I'm completely non-technical but was told that a 40 profile will affect the gearing? ~10%

[stevieturbo]

There is no way it will affect the gearing by 10%.

Mate is running 255/40x18 on his with no problems.

Try any wheel/tyre combo in this gear ratio calc and see.

www.geocities.com/z_design_studio/transmission_z28_6.html

Your diff is 3.46, tyre size is obvious.

Your gear ratios are slightly different, I dont have them off hand. But 4th is always the same, so easy to use it as an indicator.

>> Edited by stevieturbo on Saturday 26th November 22:16

[V8HSV]

Right,if you fit 245 section over standard 235 section your speedo will read 1.2% slow, so at 60 MPH you will be travelling at 60.7%

Sidewall 3.7" 245's 3.9"
Radius 12.7" 245's 12.9"
Dia 25.4" 245's 25.7"
Circ 79.8" 245's 80.8"

10% indeed!

[PCV8]

Ask yourself the question "how informed was the person that commented the rear tyres were too large?"

This comment is based on visualy comparing the apparent tyre size front to rear because you still have the original bridgestone potenza re040 235/40x18 fitted on the front.

The wheel rim is 55mm from the road on 235/40x18
The wheel rim is 65mm from the road on 245/40x18

while 10mm looks a large difference v8hsv correctly points out that the tyre circumference is the important point, being only 1.2% different.

I also use a RA2 which proved the speedo was 1-2% high on original 235/40x18.

Since fitting 245/40x18 on the rear the speedo now matches the RA2 perfectly!

[caspy]

The garage have a duty of care to point out potential issues with your car that fall outside the std factory settings. They would have someone check the car and report all visual 'faults' as they saw it.

They have reported, correctly, that your tyres are too large for std fit items. If u had had tyres fitted somewhere else and they had put the wrong size on, u may nit have even known. The service garage are being diligent and reporting.

[booster]

Well I put 255/35's on in the end and they are awesome.