Effects of larger wheel

[A_Whelan]

Hello,

I'm putting this thread in General Discussion because more people will view it than in the tire section and I feel it's not just about tire/wheel size but a few different aspects of the car. If you feel it's in the wrong section please move it.

I'm planning and ordering parts for by '89 325i and I'm down to the tires and suspension. After looking at a lot of pictures I think a 16 inch wheel will fill the wheel well more than a 15 inch and look better. However, I'm wondering how much a 16 inch tire will affect things like the odo and speedometer?

I understand that a larger tire will travel a further distance in one revolution, it will be harder to move the car from a dead start and my speed will not be what is displayed. How much of a change am I looking at. Also for a lot of city driving should I look into swapping the gears for better driving performance (this is a dd, so I mean fuel economy/wear and tear not performance).

Cheers,
Aaron

[devon.818]

i've been told the e30's speedometer is off, by like 4-5mph if it says 50mph. a guy told me he put larger tires on the rear and it helped a little. idk how much truth there is to this. someone else will chime in

[bloodshot-e30]

size of rims doesnt matter as long as the rear tire diameter matches a stock wheel diameter...

like a 225 45 16 is equal to a 195 60 14

theres a calculator here somewhere but i cant remember the name of the thread!

[unrlmth]

i've been told the e30's speedometer is off, by like 4-5mph if it says 50mph. a guy told me he put larger tires on the rear and it helped a little. idk how much truth there is to this. someone else will chime in

The early models have a smaller stock tire size than the late models, so this may be true for them. In my late model with the stock tire size, the speedo is pretty much spot on.

Also as long as you get the same diameter tire, you shouldn't have any issues with the speedo or odo.

[Mike325]

Try this:

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

[Adrian_Visser]

Use this tire size calculator to determine how the speedo will react:

http://www.miata.net/garage/tirecalc.html (taken from the wheel and tire fitment thread)

Edit: Beat me to the punch

[devon.818]

even with 225/45/16 you are still 3% off according to that.

[bmwstephen]

You alsi have to factor in that going up in size would equate to additional unsprung weigt and rotational mass. Plus tires will be slightly more expensive compared to its smaller counterpart

[A_Whelan]

Thanks for that link, I'll use that when ordering. I guess thinking about different gears is too foolish for such a small change.

Is there a way to recalibrate the speedometer?

[Marcos]

Width too!

Also consider that a "+1,+2,+3" increase (that's the accepted way of referring to up-sizing wheels) in wheel diameter is usually associated with a wider wheel as well.

This most often leads to a greater amount of holding power and less sidewall flexion but is has some drawbacks as well. As with most things, there is a compromise.

A lower profile tire is often prone to "masking" it's breaking point, ie when the tire breaks loose from lateral forces. So basically you will not have as much warning about when the car will slide. This has been somewhat addressed with more advanced tire compounds.

There will tend to be a slightly harsher ride and the car will most certainly follow grooves in the pavement more than before.

Most older guys run 15" wheels on an e-30 , young guys often larger. Sure the bigger wheels look slicker. Also, don't forget about curb rash. My first set of 19"s (Scion xb) got totally violated by an x-girlfriends parking job. I was sad...

My vote is 15" with a nice fat tire to protect the rim.



Marcos

[unrlmth]

even with 225/45/16 you are still 3% off according to that.

3% is very close in size. You can have around a 2% difference between a brand new tire and a completely worn out one and most of the cars I've driven (except my E30) have speedos that read 3-5% off.

[Adrian_Visser]

16's do look good on an e30 but the selection of tires isn't as good as 15 or 17 so keep that in mind as well.

[jeffnhiscars]

You can check tire diameters on Tirerack and use the recommended calculators to see how big a difference it makes. I went from stock to 205\50\16 and the 3% figure is about right since you go to a lower profile when you increase wheel size. It is also true that the lower profile tires are less forgiving as far as harshness and road hazards. I lowered my pressures to 28R\25F which helps and may yet go lower, but you have to watch the load rating of your specific tires as they are spec'ed for minimum pressures to hold your car up. You may also need spacers if the offset changes (mine are 10mm)

As far as weight, my rims are 13lbs each and the 50 profile tires do weigh less than stock, so I have a net reduction in unsprung weight.

An older guy on 16's :-)

[Marcos]

Indeed

And a very nice set of wheels at 13 lbs each. Unsprung wheel weight is my biggest concern personally when increasing diameter.

[bimmerteck]

My 16's with tires(205/50r16) were within 2oz of my 14" weaves with tires. So it is possible to upgrade without throwing the weight too far off.





(Posted by yet another older guy running 16" wheels)