I just changed my tires from 17 front/18 rear to 18 front/19 rear. Did that affect my HP rating or my speedometer? If so, how?

Also, ever since I changed my tires the ABS and Active Handling warning lights come on everytime I turn on the car. What gives?

 

Tire and wheel size will not really effect your HP at all, except for some very small nit-picky and nearly insignificant details below.**

Speedometer: it could, slightly. Its the change in rear tire size that will affect the speedometer. The way to tell is to look up the rolling diameter of the old tires, versus that for the new tires. You can find tire diameter on any of several internet tire sites like tirerack.com, etc (look up a particular tire size, ask the site for all tires that size, then find your brand and type, and look under "specs" for that tire -- it's on the right).

Hopefully, old and new are the same or nearly same diameter: usually when you upgrade wheel size and tires you should keep the diameter the same, just change the wheel size as you did, and the width of the tire, perhaps, for more traction. Any good shop, or on line store (TireRack) will advise you so that your upgrade leaves you with the correct rolling diameter if you are in doubt.

As an example, I replaced the original rear 295/35-18 Goodyear F1 supercar tires on my vette (diameter 26.1) inches, with BFGoodrich 345/30-18 G-Force drag radials (26.2 inches). In my case I was not upgrading the wheel diameter, just wheel width and going to a larger (wider) tire - 14 inches treadwidth instead of 12, and stickier rubber, too. The important point for the speedo is that the diameters are very close. This slight difference though, four tenths of a percent (26.2/26.1=100.4%) will introduce .4% speedometer error - insignificant given speedometers are seldom within even 2%. However, if the new tire, were, say, 28 inches in diamter, the error would be 7%, so when the speedo said I was doing 65, the car would actually be doing 70.

It is important to stick with the same rolling diameter, front and rear, most so you do not upset the ride height and handling.

*** Details. Technically, changing tires can affect HP slightly and fuel economy noticeably. It will not effect the HP your engine is producing, but some tires are more sticky and flexible than others, and so absorb -- soak up -- a bit more HP rather than transmit it to the road (or the test dyno). The difference is very small. However, it is noticeable in some cases. This is why electric and some hybrid cars have narrow, stiff sidewall tires made of very hard compound, a l to limit rolling resistance -- the tire soaks up less energy so the car has a tiny bit more power to go farther on a batter charge, etc. The worst offenders here are drag radials: some parts of rolling resistnace mean friction, and of course drag radials are made to create gobs of friction (traction). They have a soft, sticky compound, a wide tread to create as much traction as possible, and very flexible sidewalls to help the launch: all also contribute to rolling resistance and probably mean they absorb a few HP at full throttle. But its too small an amount to measure accurately on the dyno (I'm guessing about 5 HP at full throttle) and my case I don't care

 

Wow, Lee, thanks for the information.......so, based on what you tell me chances are that my ABS/Active Handling warning lights may be related to the fact that I went from 18 inch to 19 inch wheels. In fact, according to your calculations I went from a rolling diameter of 22.32 inches (P275/40ZR18) to a rolling diameter of 22.92 inches (P285/35ZR19).

So, (22.92/22.32=102.7%) which would introduce 2.7% speedometer error. Maybe my computer chip is having difficulty with this change. Your thoughts?

 

I doubt that something like only a 3% change would make a difference. It might be that if the ratio of front to rear wheel diameter changed, that might do it, but frankly I'd start looking elsewhere. If the probably started immediately after the new wheels were put on, and it were me, I would look to whether the wheel sensors or something like that were knocked about or loosened when the new wheels were put on. I don't have any experience with repairs to this, -- I would think maybe mech 259 would know more about this.

 

most likely when your car was lifted to change the wheels, it triggered the sensor. Mine does it half the time when I switch wheels before heading to the track. Disconnect the battery and you should be safe once you plug it back in...sometimes it will take a couple of stop and goes for it to reset itself.

As far as hp increase or decrease, the only thing you may notice is unsprung weight...it takes more effort to move a heavier wheel/tire...so sometimes you "might" feel a difference if you increase the size per corner say 5-10 lbs...you must have to have an excellent butt dyno...the average person probably won't be able to tell unless you are doing track events then you will notice it more.

 

thanks Lee and Tailwag,

I disconnected the battery for about an hour....but I still get the error code flashing on my display.....guess I'll just have to take it in to see about the tire sensor thing Lee is referring to. Hopefully it's nothing serious....thanks guys!

 

With an increase in wheel diameter, the effective rear axel ratio is lowered. Also, since the wheels are heavier and most of that mass is further from the axis of rotation, it is more difficult to get them moving than a wheel with the mass closer to the center. However, I think this is a pretty slight change since the tire sidewall is also shorter. All things being equal, the car will probably accelerate a tiny bit slower and I think the only place it could be measured is at the drag strip since the change is so small. Even then, it would be hard to do since there are so many other variables getting in the way of an accurate measurement. You could also do a before and after dyno run. But that also doesn't yield 100% consistent results from one run to the next.

 

thanks reactor2....welcome to the forum!

 

Thanks! I just purchased a 2004 commemorative edition A4