I am looking to go from a stock 195/55/16 tire to a 17" and was wondering what issues might come from INCREASING total DIAMETER. My R56 Speedo is off somewhere about 8%...too fast. That means the odometer is registering too many miles (8%) than actually travelled?

If I go to a 215/55/17, the total diameter increase is 1.87" and the speedo will be about 7.653% SLOWER

Pro reasons for doing this:
Correct Speedo issue
Possibly fill some Wheel Gap without lowering
Have a 17" wheel with still some sidewall height to protect wheel and comfort ride.

Con reasons??:
Unsprung weight increase
Brake wear?
Handling?

What issues am I not thinking about?

Thanks.

 

Slower acceleration due to change in gearing.

Car will be higher off the ground. (Basically raising your car)

 

Thanks, didn't think of ride height...don't want to increase that really...was more worried about wheel gap and speedo...I guess I should keep size very similar to stock...rubbing, slower accel, higher ride all don't outweigh the potential benefits..

 

...not to mention going with a tire that tall with make the MINI look like a monster truck! If you want to improve speedo accuracy without messing up the aesthetics or inducing rubbing a 215/45-17 is a good option. The displayed speed on the R56 is much more optimistic than the OBDII sees. Basically the car is lying to you to keep your speed in check; very similar to most sport-bikes.

 

Too tall is risky for rubbing.

215/45-17 is your best bet. Use a 17x7 rim.

215/55-17 is too tall.

 

8% is a huge amount for your speedo to be off. When you read 60, you are only going 55 if your deviation is 8%. How did you determine this? 195/55/16 is a 24.5" dia. which is stock and should be usable as a standard to figure how far off other tires will be if you change your dia. 8% sounds like too big of a deviation unless something is wrong with the speedo.
Also, why would a company that trades on performance want to underestimate the performance? Or more importantly, what German engineer could sleep at night knowing he made a devise that was 8% off.

 

Please visit http://www.1010tires.com/tiresizecalculator.asp and run the numbers.

 

I'll bet BMW's calculator isn't 8% off.

 

I'm doing the same thing you are. I myself am looking at the 215/50-16 and the 215/45-17. Two totally different looks but they are the closest to factory.

This is a post by YYC in another thread and he provided this info from another site.

On street cars, it should have an overall diameter that is within ±3% of the factory specification. The ±3% variation is recommended for street cars in order to maintain accurate speed data going into the computer, thus keeping odometer and speedometer changes negligible. While a ±3% diameter increase or reduction in tire diameter may sound very limiting, in most cases it allows approximately a ±3/4" diameter change.

Assuming 24.00" (609.6mm) as the factory specification for overall diameter, any of the following tire sizes may be appropriate:
+-----------+-------------------+
| TIRE SIZE | DIAMETER |
+-----------+-------------------+
| 195/55-15 | 23.44in (595.5mm) |
| 205/40-17 | 23.46in (595.8mm) |
| 215/50-15 | 23.46in (596.0mm) |
| 215/45-16 | 23.62in (599.9mm) |
| 195/50-16 | 23.68in (601.4mm) |
| 185/60-15 | 23.74in (603.0mm) |
| 215/40-17 | 23.77in (603.8mm) |
| 205/55-15 | 23.88in (606.5mm) |
| 215/35-18 | 23.93in (607.7mm) |
| 175/65-15 | 23.96in (608.5mm) |
| 225/45-16 | 23.97in (608.9mm) |
| 205/50-16 | 24.07in (611.4mm) |
| 225/40-17 | 24.09in (611.8mm) |
| 225/35-18 | 24.20in (614.7mm) |
| 195/60-15 | 24.21in (615.0mm) |
| 205/45-17 | 24.26in (616.3mm) |
| 215/55-15 | 24.31in (617.5mm) |
| 195/55-16 | 24.44in (620.9mm) |
| 215/50-16 | 24.46in (621.4mm) |
| 185/65-15 | 24.47in (621.5mm) |
| 215/45-17 | 24.62in (625.3mm) |
+-----------+-------------------+

NOTES:
Stock fitments are highlighted
Fitment dependent on wheel offset and suspension setup
Tires availabilty may be limited in certain sizes

 

The only way I know I am off 8% is by a Radar Sign that posts speed as you approach. If I am doing 35, the radar says 33. Or if I am doing 50, the radar says 46....I haven't tried 100MPH yet since it is usally near school zones. and it seems like I read a thread in here about the speedo being off. The Radar may be off as well. I haven't clocked it on mile markers or GPS yet.

After looking at the many negatives of increasing diameter, I will probably go with a 215/45/17...closest/widest/tallest fit without increasing diameter more than 3/16". Thanks for the help...

 

Thanks, that is where I got my numbers...

 

There are many threads here that deal with the inaccuracy of the speedo and yes they are off by as much as 5-8%.

My 525iT wagon is off my about 6%.

and many manufacturers do this. even my 1999 Dodge fullsize pick is 5% off unfortunately it is off in what i concider the wrong direction. when the speedo says i am doing 55 i am acutally doing more like 62

Just remember one thing any - change in tire size will affect the speedometers reading. Even tires from the same manufacturer of the same size but different models can be cause as much as 1.5% to 2% difference in speed readings