Wheel and Tyre combo puzzle

FROGGY · 31st July 2008, 07:32

Looking in my Owners Handbook the Tyre Specification/Wheel sizes makes sense until you compare 17" and 18" where the tyre size quoted is the same (225/45), so surely the speedo/mileometer readings are going to be wrong for one of them?
Any thoughts anyone?
Mick

eurorover · 31st July 2008, 08:21

All depentand on tyre width and profile. try this: http://www.alloywheels.com/tyrecalc.asp

Nick999 · 31st July 2008, 09:50

How bizarre. Do you think a mistake was made when printing the handbook? Speedo will be way out with a 225/45/17 tyre.

T-Cut · 31st July 2008, 09:52

Quote:

Originally Posted by FROGGY

Looking in my Owners Handbook the Tyre Specification/Wheel sizes makes sense until you compare 17" and 18" where the tyre size quoted is the same (225/45), - - -

Not in mine (2004). The options for these wheels are:

215/50 R17
225/45 ZR18

TC

BT@Home · 31st July 2008, 11:35

Quote:

Originally Posted by Nick999

How bizarre. Do you think a mistake was made when printing the handbook? Speedo will be way out with a 225/45/17 tyre.

"Way out" is only a couple of mph. If anything it probably brings your speedo more inline with the actual speed.

Nick999 · 31st July 2008, 13:15

Quote:

Originally Posted by BT@Home

"Way out" is only a couple of mph. If anything it probably brings your speedo more inline with the actual speed.

Tosh!!!! More like 10 mph out at 70 mph!! An inch I think is rather a lot. Although would make the car nice and low. Much sportier looking.
I got the correct 225/45/18" fittement on my ZT. And my speedo is only a couple of mph out at even 100 mph compared with my sat-nav.

tigerchubs · 31st July 2008, 14:30

The differences between the 2 sizes you quote are massive in tyre terms. In particular if you consider the distance travelled per tyre revolution.

225-45-17 (1,930mm distance per rev) based on a new tyre

225-45-18 (2010mm distance per rev) based on a new tyre
this equates to a difference of 80mm shorter distance travelled per rev.

If you do the calculations again but compare a worn 17 against a new 18 the difference in the distance travelled is a massive 121mm per rev shorter

Result increased acceleration decreased top speed & inaccurate speedo reading. Oh and less miles per gallon.

Duotone · 31st July 2008, 15:16

In the brochure's for the mk.1 Rover 75 the 17" Meteor and Serpent wheels were supplied with 225x45xR17 tyres..

In the Facelift brochure the 17" Star Spoke, Meteor and the Serpents were all supplied with 215x50xR17 tyres.

The pre facelift MG ZT brochure does not show 17" wheels available.

The facelift MG ZT has a 17" 5 Spoke with 215X50xR17 tyre.

So maybe MG Rover thought there was a problem with the speedo readings and changed the profile of the 17" tyres tyres supplied when the facelift models were introduced.
Although it took five years of production before the profile was changed..

Dave T

BigRuss · 31st July 2008, 16:19

Mine was originally fitted with 15" Crowns and the speedo over-read by quite a bit even at 30 mph.
On fitting the 18" Straights due to the greater diameter of the 225/45/18 tyres the speedo still over-reads but only by 2mph at 50.

Russ

pondweed · 31st July 2008, 17:15

We were wondering earlier if the tyre combo choice might influence the programming of the dash ecu after or on the build line - ie. the speedo correction factor? Or would that have been just too much work?

31st July 2008, 07:32	#1
FROGGY I really should get out more....... MG ZT CDTI Join Date: Sep 2007 Location: 17500 REAUX, CHARENTE-MARITIME Posts: 2,749 Thanks: 2 Thanked 133 Times in 90 Posts	Wheel and Tyre combo puzzle Looking in my Owners Handbook the Tyre Specification/Wheel sizes makes sense until you compare 17" and 18" where the tyre size quoted is the same (225/45), so surely the speedo/mileometer readings are going to be wrong for one of them? Any thoughts anyone? Mick __________________ http://i200.photobucket.com/albums/a...ps628d4119.jpg IN THE BLACK NOW

31st July 2008, 09:50	#3
Nick999 Posted a thing or two Ex ZT160 ZS180 ZT190 Join Date: Jul 2008 Location: Bristol Posts: 1,018 Thanks: 0 Thanked 0 Times in 0 Posts	How bizarre. Do you think a mistake was made when printing the handbook? Speedo will be way out with a 225/45/17 tyre. __________________ [SIGPIC][/SIGPIC]

31st July 2008, 14:30	#7
tigerchubs Gets stuck in Rover 75 CDT+ in Yorkshire Gold Join Date: Dec 2006 Location: Sheffield Posts: 975 Thanks: 0 Thanked 0 Times in 0 Posts	The differences between the 2 sizes you quote are massive in tyre terms. In particular if you consider the distance travelled per tyre revolution. 225-45-17 (1,930mm distance per rev) based on a new tyre 225-45-18 (2010mm distance per rev) based on a new tyre this equates to a difference of 80mm shorter distance travelled per rev. If you do the calculations again but compare a worn 17 against a new 18 the difference in the distance travelled is a massive 121mm per rev shorter Result increased acceleration decreased top speed & inaccurate speedo reading. Oh and less miles per gallon. __________________ [SIGPIC][/SIGPIC]

31st July 2008, 16:19	#9
BigRuss Premium Trader 75 CDT Tourer,2.5 Launch Saloon, Omipro MG/Rover (T4) Join Date: Jul 2007 Location: Liversedge, West Yorkshire Posts: 5,405 Thanks: 1,105 Thanked 1,340 Times in 661 Posts	Mine was originally fitted with 15" Crowns and the speedo over-read by quite a bit even at 30 mph. On fitting the 18" Straights due to the greater diameter of the 225/45/18 tyres the speedo still over-reads but only by 2mph at 50. Russ __________________ Replacement Key Service http://https://the75andztclub.co.uk/...d.php?t=244732 Full T4 Testbook diagnostics available. Diesel ECU repair and replacement. Options enabled or disabled as required. Diesel X-Power 135 and 160bhp, Rover 1.8T 150 to 160 MG 160 V6 to 177 upgrades available P.M. for details.

31st July 2008, 08:21	#2
eurorover Posted a thing or two MG ZT Join Date: Feb 2008 Location: Swansea Posts: 1,231 Thanks: 0 Thanked 9 Times in 5 Posts	All depentand on tyre width and profile. try this: http://www.alloywheels.com/tyrecalc.asp

31st July 2008, 15:16	#8
Duotone This is my second home 75 Join Date: Nov 2006 Location: Leeds Posts: 8,432 Thanks: 176 Thanked 907 Times in 716 Posts	In the brochure's for the mk.1 Rover 75 the 17" Meteor and Serpent wheels were supplied with 225x45xR17 tyres.. In the Facelift brochure the 17" Star Spoke, Meteor and the Serpents were all supplied with 215x50xR17 tyres. The pre facelift MG ZT brochure does not show 17" wheels available. The facelift MG ZT has a 17" 5 Spoke with 215X50xR17 tyre. So maybe MG Rover thought there was a problem with the speedo readings and changed the profile of the 17" tyres tyres supplied when the facelift models were introduced. Although it took five years of production before the profile was changed.. Dave T

31st July 2008, 17:15	#10
pondweed This is my second home Waiter, Waiter! I seem to have a Roomster in my drive... Join Date: Nov 2006 Location: Sussex Posts: 3,268 Thanks: 0 Thanked 1 Time in 1 Post	We were wondering earlier if the tyre combo choice might influence the programming of the dash ecu after or on the build line - ie. the speedo correction factor? Or would that have been just too much work?