Big Wheels vs Computer????????

[roberto]

Thanks Great information and makes perfect sense

285/35 R22 both front and rear --- the tyre measures from top to road 29" (740mm) is this typical???

[roberto]

Not a bad looking truck for a 2001 !!!!

[mrkbbd]

on 20's you run a 35 in the rear and a 40 in the front, so that is definitely not right

[roberto]

so forgetting the original Question are you saying every SAV has to have bigger sorry wider back tyres ????????

[mrkbbd]

not wider, but thinner sidewalls

[roberto]

so what do I gain by making the front tyres lower??? ie 45's on the back and 35's on the front???

[FSETH]

Originally Posted by roberto

so what do I gain by making the front tyres lower??? ie 45's on the back and 35's on the front???

2 bent rims and steering wheel shake!

I don't think you really gain anything except making your X look like a dragster. I am not sure there would be any advantage to that visually or with regards to performance. You would probably bend your rims easier if you hit a pot hole.

[TurnAround]

MRKBBD: "on 20's you run a 35 in the rear and a 40 in the front, so that is definitely not right". That would be for 20" wheels that are staggered (rear wheels wider than the front). From what Roberto reported, it sounds like his front and rear wheels are the same width, so a single aspect ratio percentage would be correct.

Roberto: "so forgetting the original Question are you saying every SAV has to have bigger sorry wider back tyres ????????" No. It depends on which X5 model you bought, or, which after market wheels a buyer goes for. When the rear wheels are wider this is referred to as "staggered". It is generally considered a cooler look by most. The idea is that it adds traction. Ironically, wider rear wheels make any BMW understeer even more than they have a tendency to do (handle worse, not better). Dinan and others have noted that.. interestingly.. on a BMW, you'd want wider FRONT tires to make the vehicle perform better. But hey.. that is all irrelevant. BMW's and Porsches look much cooler with wider rear wheels. I'm going with non-staggered (same width front and rear) because my 3.0i engine doesn't have as much torque and I would feel the negative weight impact of spinning up wider heavier rear wheels. Bummer.. but look at it this way.. I get to rotate my tires for maximum wear life. :-P

Roberto: so what do I gain by making the front tyres lower??? ie 45's on the back and 35's on the front???. Other way around bro. The lower aspect ratio percentage number would be for the rear... if.. you had wider rear wheels (staggered), which you apparently don't. Why is the rear lower? Because.. if it was the same, when you do the math you'd wind up with a taller side wall on the rear, because the formula is the aspect ratio percentage X the width of the tire. Literally.. do a couple of calcs, and you'll see the sidewall picture. If you have a wider rear wheel, you have to use a smaller percentage. Usually, it's 5% lower.

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You're tire size should be ok. Here's the math and the stats:

- You said you have 285/35 R22 Front and Rear. This likely means you have the same rim width front and rear. Check 'em.

- According to the tire calculator at TireRack.com you have the correct tire size for a 22" rim.. in order to make your speedometer happy and more or less correct.

- Comparing tire diameters for several different brands within the 22" wheel size, and the other wheel sizes, the range of actual tire diameters is 28 1/4" to 30 1/8". You reported 29" which is somewhat in the middle. BMW speedometers register 3.5 to 5 mph slower than they should (actual speed). The larger the tire diameter, the faster the car actually travels for a given speedo reading. So in other words, your X5 is probably very close to being accurate on the speedometer.

Borrow someone's GPS and take it with you to see if the car's speedo is actually reporting the correct speed.... then let us know if you would.


On a side note, if the outside diameter of your tires was truly different from the front to the rear by more than a half inch, this would cause the DSC computer to freak out and turn the DSC on when it shouldn't.

[roberto]

that last comment is not happening therefore we can presume that if you stick to the Tyrecalc philosophy you are driving a safe car!!!!

this also puts to bed the philosophy that if you have bigger wheels you go faster hence your BHP is increased --- The overall diameter of the wheel/tyre combo is the issue here

Thanks for an interesting forum -- I would like to know however, that the original BMW Wheel/Tyre combo is the same as my combo ie around 29"

thanks Robt