"dont get bigger tires... it will throw off your speedo", i beg to differ Options

[x_itchy_b_x]

its not a myth. its fact, larger tires = more radius = the ecu doesnt know any better and its going to affect the speedo because the sensor is getting the speed reading from the tranny and not the tires radius. it affects you less at lower speeds because its more of a ratio and you more the faster you go.
it wont affect it like a truck going from 33" to 38" tires, my old truck was over 10mph off on the highway.

[GTS13]

The MPH difference isn't what would bother me, it's the fact that you engine was designed to rotate a certain circumference and increasing that circumference could possibly decrease your acceleration. You may not notice the small difference with you GT but I know for a fact that I would most certainly notice with my ST until I do some engine mods to increase the numbers I put out, but even then depending on the wheel and tire size and the engine mods I could just end up running like stock and where's the fun in that?

Edit!
While I won't remove what I stated earlier because it's good stuff. I forgot that I have 215/45/17s and uhh...it's pretty close to stock. The 225s are going to be a little taller though.

This post has been edited by GTS13: May 25, 2007 - 12:48 PM

[Defgeph]

Got my new tires today. They are MEAN boy, they look like monster truck tires compared to my 205's. I can't wait to slap these suckers on. Fender gap..... end yo self foo.

[spolski07]

car companys actually set there speedo 2 to 3 mph higher then what your going, thats y cops get u for 5mph+....ever see a ticket for 3mph over? I learned with my gps that on stock wheels i can go 50mph on the speedo, but im actually doing 47mph on the gps....gg

there is a neighborhood in San antonio (windcrest) that pulls people over for 21 in a 20
no lie... ive seen it happen (one of those cop things and the cop hiding near it.)

[topshed]

Do what I've done and move the speedo needle to read a couple of mph faster.

keith

This post has been edited by topshed: May 25, 2007 - 4:18 PM

[jason]

a good benefit to bigger itres is also fuel economy, not as many rpms needed to go 70 or so on longer trips i imagine would help out alot with fuel economy, as far as power, i dont see it have THAT big of an effect, may like a loss of .1 second on a 0-60 run and some issues with accelorating fast while on the highway, but hey these cars arent that fast anywho unless you drop some $$ into them

[lagos]

not to mention that your ride quality is a bazzilion times better with larger tires.

[topshed]

not to mention that your ride quality is a bazzilion times better with larger tires.

seconded on that. doesn't make much difference on roadholding anyway unless your on track days

This post has been edited by topshed: May 26, 2007 - 12:32 AM

[mnwolftrack]

so, for years ive always had people say to stay with low profile tires, because anything bigger will throw off your speedo. im sure we've all seen the online calculators and countless threads. ive even had tirerack call me back and try to convince me to change my order from 225-45-17s to 205-40-17s because of this..lol ...

well anyway, i just wanted to say that all that, is mostly a myth. i recently got a gps for my car. my speedo is dead on with the mph reported by the gps. i was a bit shocked because i expected it to be off by at least 1-3mph. so go ahead, get bigger tires and enjoy the ride quality..your speedo will be just fine

I come from a world of Toyota 4x4's, mainly 1979 to 1995 pickups and 4Runners and widely varying tire sizes. There is DEFINITELY a difference in speedo and odometer when changing tire size. However, I don't think the inch or two in tire size that most of you would be dealint with would be very significant. My '90 4Runner was geared for 225/75/R15's (28" tall) and I put 35x12.5R15's on it (actual size is closer to 34"), and the speedo/odometer were off by almost 20%. With this much of a change, there is a HUGE affect on performance as well, and I had to regear the axles from 4.10 to 5.29 just to make it drivable. Without the gear change, I was way off the power band, and 5th gear was unusable. On my '85 pickup, I went from 225/75R15 to 33x12.5R15 and still had to re-gear (to 4.88) because 5th gear was also unusuable. In my pickups/4Runners, I prefer the 22RE or 22R carb'd engines. While highly reliable, these engines are quite weak (so my transition to a Celica is easy!).

Not only was 5th gear unusable and I was WAY off in the power band, fuel economy suffers greatly (!). The 22R and 22RE 4 cylinders have sweet spots they like to cruise in, and if you are above or below it, you are not producing efficient power and fuel economy takes a huge dump. While lower RPM's theoretically should mean better mpg, it normally does not. If the engine is turning slower than it's happy turning at, it has to work harder. If it's turning too slow (lugging), then it's working very poorly and can do bad things such as burn valves.

I'm sure you guys are considering this, but if you change tire sizes larger or smaller, you have to figure out your percentage off so that you can use it to recalculate your mpg. If you are going to a 3% larger tire, you have to add another 3% speed and 3% to the odometer/trip meter.

If you're only talking about an inch or two difference from stock of the overall tire height (and diameter), you'll likely be talking 5% or less of an affect on speedo/odometer. 5% at 60mph is 3mph off.

That being said, tire sizes are not an exact science. Even when new, the sidewall size vs. actual size might be different, and you have to check the manufacturer specs to see what the actual size is. As I said earlier, my 35x12.50R15 mudders claim to be 35" tall, but they are closer to 34" (new). They started with like 21/32" of tread, but they are half worn, and they are probably UNDER 34" by now. Tire wear, new vs. near bald, will also affect things a bit since the diameter and height of a tire decreases as it wears.

While I'm not trying to say you're wrong or anything, I'm just explaining what I see in more drastic tire size changes in my more familiar world of Toyota 4x4's.

This post has been edited by mnwolftrack: May 26, 2007 - 9:01 AM