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hi (noobie here ) i have a 96 328i (complete stock, and automatic, not my first choice but at the time im dying for a bimmer :p hehe) ... well the thing is i got new 18" and the car seems to accelerate slower.. :( getting lower mpg's use to be around 20-22mpg now only 17-18.5... also everytime i accelerate and reach around 20-25mph it feels like im switching gear.... i was wondering if anyone else who changed their rims had this happen to them... is it normal?? bad for the engine or what???? friend said have it recalibrated... but i wanna know from ya'll bimmer owners ;) anyways any idea what's up... will it damage my engine...??? :help well thanks in advance!! -Ron
 

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welcome :)

first off, to plus-size rims and tires, you have to get the right size. its mainly the tires that does it. a 235/40/18 tire will be taller than a 235/30/18 tire. different cars require different sizes; you should check your owners manual to see what size tires you need. my E46 takes 235/40/18 for 18" rims. Now why is this important...well if your tire's overall diameter is wider OR narrower than stock, you will have wrong readings of speed and that will yield to wrong mpg numbers. Try measuring the diameter of both the stock tire and the new tire (overall tire diameter) to see if they equal up. it should be +/- an inch.

another is the fact that your rims are wider; wider the rim, more force is needed to turn that rim. this could slow you down. (the general rule is, small diameter wheels accelerate faster, but has low top speed : larger diameter wheels accelerate slower, but has a higher top speed)

and finally, the weight of the rim greatly decreases speed and accelation. there are reasons why rims can be cheap or expensive; style is one factor, but one major factor is weight. alot of aftermarket rims are heavier than stock; creating more weight for the car thus using more power from the engine to turn the rim. this also could slow you down as well.

hope that helped a bit
 

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Here is a simple calculator to figure where you ended up in comparison with where you started. For example the 19s I got on my M3 actually lowered the gear ratio slightly. Thousands of a percent. Very slightly. The engineers at BMW did their home work and went to a shorter sidewall. By doing that they did not need to run a different rear gear. IE keep manufacturing costs down. You pick up some handling do to less side wall flex and you get a larger contact patch in my case. If you are way off and performance is a big issue a gear change is not a huge deal. An experienced garage can do it in a couple of hours.


http://www.car-videos.com/info/tirespecs.asp
 

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Discussion Starter #6
ah no its not some replica :p , i got adr cypher.... so can it be fixed having it recalibrated...???
 

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Discussion Starter #7
thank you :D .... ohhh i see... but is it bad for the engine though? since it needs more power and such to turn the wheels??? thnx :)
 

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Anytime you plus size your wheels, you run the chance of getting something heavier and possibly bigger in diameter than what is stock..I have 19's on my car and it did seem a little slower but damn they look good! I would check and see if you can get it recalibrated...Alot of times aftermarket wheels are heavier than stock ones, that will slow a car down...
 

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to keep going with scarface said, plus sizing wheels will usually always add weight thus making the weight to power ratio different (you need more power to turn heavier wheels) now your mpgs may be off as well as your speedo because you may or may not have the correct overall diameter rims and rubber on your car causing all the calculations to be thrown off.
 

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Discussion Starter #10
ohhhh alright i get it ;) thanks guys :cheers guess i need to up the power of the car :devil , i did alot of the exterior stuff already but none to boost power yet =\ heres a pic, what do u guys think? oh and for the rear is it knoticable that i didnt use an m3 bumper.. just a stock 328i and attatched a diffuser??? :p
 

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Discussion Starter #12
ya soon as i get some money im thinkin of lowering it... still thinkin if i should change the rear bumper to real one, it doesnt seem to be much difference... oh well if i get the money i may hehe ;) thanks
 

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Originally posted by woobiee@Dec 29 2003, 05:26 AM
welcome :)

first off, to plus-size rims and tires, you have to get the right size. its mainly the tires that does it. a 235/40/18 tire will be taller than a 235/30/18 tire. different cars require different sizes; you should check your owners manual to see what size tires you need. my E46 takes 235/40/18 for 18" rims. Now why is this important...well if your tire's overall diameter is wider OR narrower than stock, you will have wrong readings of speed and that will yield to wrong mpg numbers. Try measuring the diameter of both the stock tire and the new tire (overall tire diameter) to see if they equal up. it should be +/- an inch.

another is the fact that your rims are wider; wider the rim, more force is needed to turn that rim. this could slow you down. (the general rule is, small diameter wheels accelerate faster, but has low top speed : larger diameter wheels accelerate slower, but has a higher top speed)

and finally, the weight of the rim greatly decreases speed and accelation. there are reasons why rims can be cheap or expensive; style is one factor, but one major factor is weight. alot of aftermarket rims are heavier than stock; creating more weight for the car thus using more power from the engine to turn the rim. this also could slow you down as well.

hope that helped a bit
wow pro write up :cheers
 

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Discussion Starter #15
18x8.5, and no i dont think it rubs against it :lol:
 
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