What size? - The Unofficial BMW M5 Messageboard (m5board.com)

g boyd · 3rd September 2002, 13:04

Question for the EINSTEIN'S

Changed my 17 ins standard for 18 in M parallels on my 1992 M5
The standard F 235/45 zr 17
R 255/40 zr 17

Now F 245/40 zr 18
R 245/40 zr 18
The rolling circumference of the tyres are not the same, to match you need the following

F 235/40 zr 18
R 265/35 zr 18

I have no problem (money well spent)
My Question is M parallels wheels came standard on the newer M5 with 245 size, did BMW change anything for the Speedo?
Or am I just wasting you time?

Thanks

Graham

IvanDias · 3rd September 2002, 14:28

I agree with your calculations.

Taking just the back wheels, as this is where the speed sensor is on the rear diff, then going from 235/45/17 (25.33") to 265/40/18 (26.35") is a 5.4% difference, leading to a speedo reading 100mph and you going 94.7mph.

The 265/35/18 you mention gives (25.30"), a difference from 265/40/18 (25.03") of only 1.1%.

I would suspect that it would be childs play for BMW to reprogram the OBC to adjust the translation from the pulses-per-second of the diff sensor to whatever signal it sends to the speedo.
No, let me rephrase that.... It would not have been hard for the engineers at the BMW GMBH factory to alter the programming for the cars when being manufactured in the production run for 1994/95.

It would probably be less hassle to change the rear tyres once your current ones wear out. Until then, just remember not to go too fast over the speed limit when passing Gatso cameras. LOL.

I don`t think you`ll have much luck with walking into a main dealer, giving them a big beaming smile and saying "I`ve put M-Parallel wheels on a early E34 M5, resulting in a 5.3% error in the speedometer. Please can you advise me of the software modifications to the OBC on the 1994/95 model years and the possibility of flashing the software on my car."

g boyd · 3rd September 2002, 15:56

Ivan

Thanks for your input, I will sleep well tonight,

No problem for speeding in Saudi.(got caught at 230kph between two check points only a 150 sr or 30 pounds fine )
I only worry when I am in the UK

Regards

Graham

ps Have a pint for me tonight

kevin034 · 3rd September 2002, 16:04

For the US folks, this would make a GREAT SAT question!

Mmm-Five · 3rd September 2002, 17:54

This is probably the reason behind the official BMW max. HP figures being at 7000rpm and the actual figures being at 6500rpm.

They probably left the 17"s on the test vehicle.

mywong23 · 5th September 2002, 10:20

tire size calculator:

http://www.miata.net/garage/tirecalc.html

I like the 245/40x18 front & 275/35x18 rear

g boyd · 5th September 2002, 11:19

mywong23
275/35 zr18 on the rear
Do you have spacer's fitted?

I am asking because I will order new rubber soon.
265 or 275?

Thanks

Graham

mywong23 · 5th September 2002, 18:52

I've been told that a 275/35x18 tire mounted on 9" m parallel wheel does not need a spacer. I have seen some mounted on 18x10 wheels that required the fenders be 'rolled'.

Double check the clearances when you install, as some rear suspensions have sagged or have been modified over the years.

The tires look great - very high on my wish list.

Captain Kirk · 19th September 2002, 20:28

Luckily, BMW chose to engineer error into the speedometer, making it optimistic. This helps reduce traffic tickets as you are not going quite as fast as you think. It also gets you out of warrantee earlier...

I have new 245/40R18's as well. Using milemarkers over a hundred mile stretch I calculated the speedo to be accurate to 0.1% (It read a 99.9 miles afer going 100 miles).

dronfieldm · 20th September 2002, 12:23

Just a note regarding the accuracy of BMW speedo's, 3 years ago in my 735i, I set the cruise control just below 100mph and wafted along. (silly I know). The result - done for speeding at 98.64 mph. Pretty accurate I thought !!!!

All the best
Mark