B1N

Im kind of a noob to wheels and just wanted to get my facts straight.

Are offsets always the same for a specific car/model? will a car always have a certain offset based on its dimensions?...can rims affect offset? what do spacers do? do they increase or decrease offset? say my current rims have an offset of 20 do i need to always get rims with that offset or can i change it?

how does tire size effect the cars measuring devises like speedometer or mileage calculator? say i wanted a 275 as opposed to a 265, how would that affect measurements or offset?...

another tire size question is the sidewall width, my current one is 40 front and 35 back but people recommend me 35 front and 30 back for my next set of wheels. They are the same size rims (19") so since the sidewalls are smaller wont the wheel be smaller thus affecting computers measurements eg speedometer?...do i need the same sidewall measurement 40 front 35 back for my new tires?

thanks

Heloha

Check out: Tire Size calculator
It will help determine the sizes you can change to while keeping the same overall height..etc.

jsal

Diddo on the wheel calculator. You can plug in numbers to determine overall tire size. The 30/35/40 number is a percentage of the width of the tire that determines the side wall height (I believe). So, if you keep the same width of the tire, but change from a 18" rim to a 19" rim, you will have to lower the sidewall to keep the same overall diameter (i.e. a 245/40/18 is the same overall diameter as a 245/35/19). The tire calculator will help you with comparing tires and rim sizes.

The offset will tell you how much of the tire is outside of the hub versus inside of the hub. Every rim has an offset number. Cars are setup to accept different rim offsets. The e60's are the same, but the M5 will take a different offset in the front due to the fender being larger. The all-wheel drives take a different offset tire.

So you have to balance the offset of the rim, with the car and the tire size for look and function. There are a lot of posted rim designs with offsets and tire sizes on the forum that you should be able to see something you like. Take advantage of all of the work the people have done in choosing a tire/rim combination to get you close.

Spacers are used in between the rim and the hub to push the tire out for better look/fit. If your rim sticks out farther than you want it, there is not much you can do.

Good Luck.

B1N

thanks for the post I appreciate it!...so i think my stock tire sidewalls are 40 and 35 front back respectively on 19 inch rims. Im thinking of buying a new set of wheels also 19 inch but vendors are recommending 35 and 30 sidewalls front and back respectively and said its ok and it will 'fit'. Wont this change the diameter of the wheels from stock? isnt that bad?

thanks again

monacobmw

You also have to look out because changing the tire diameter will play games with the speedometer, and thus, the odometer. As I recall, a bigger diameter means your speedometer reads lower numbers than your actual speed (correct me if I am wrong here).
If you are leasing your car, the changes can affect your mileage.
As we understand, BMWs are already set a few miles per hour higher than actual speed, so the offset can bring you back to even (if you go with larger diameters). If you are going smaller, the opposite effect will occur.

Best regards,

B1N

thanks for the reply, so being that my car is leased and I have a set mileage i can only use per year, wouldnt it be in my best interest to get a wheel with a larger diameter?....I just dont understand how vendors are telling me that the optimal tire size would be 245/35/19 and 275/30/29 front back respectively compared to my stock 245/40/19 and 275/35/19, would the decrease in sidewall effect speedometer and odometer significantly?

also how would changing the rear to 285 from 275 affect those measurements?...i want a wider rear tire than stock

monacobmw

The vendors are correct. The optimal size is the one they recommend. The size that you suggest is a bit larger and will give you more rubber (more cushion). Personally I like the look of the size that you are looking at. It looks more like the 7-series wheels (fuller). These recommended tire sizes make a 19" wheel look like a 17" or 18" wheel on other cars.

I would not make a choice based on odometors or speedometers. I just pointed this out as a side note. The changes are also not that great. There is a website or two that will show you how the tire size changes will change your speedometer. Just google the issue.

Alot of people recommend 285 rather than 275. I do not see any changes to the speedometer when you change wideness. In all of this, you have to remember that your tires may start to rub the fender if you go too big with the tires. It is not just the car sitting still, but think about what happens when you hit a bump and the shocks compress.

I am not sure that 285 will work with 275/35/19 (unless you have a nonsport package).

Good luck.

olli_535d

The numbers for the sidewall height are called aspect ratio and are a percentage of the tyre width, rather than a fixed figure.

If you go up one size in rim dia, you loose 5% of the aspect ratio of the tyre size to keep the rolling dia and circumference roughly the same. Little mathematical millimeter differences can be neglected.

example, your front now is 8x18 with a 245/40/18. The correct conversion going 19" would result in the correct tyre being a 245/35/19
Another rule of thumb is that if you go wider by appr. 20mm (sometimes a little less or more), you loose another 5% of aspect ratio.

example, you have 8x18 rears, also with a 245/40/18. Instead of going 245/35/19, you would now go 275/30/19, as you go larger in the rim and also wider.

Oversizing on 19'2 is not really necessary (usually done with 20's, though)