So, I finally did it! Took the 15 inch wheels off and put the 17's on. (Pictures will be coming soon, as I just got a new digital camera.)

Sorry for the long post, but here goes...

I already had two good Pirellis (P7000 SuperSport) so kept them. Then put on two new Falken ZIEX 512's (chosen after months of research). The 15's were stock size, P205/65/15. Absolutely every tire chart I looked at said that the correct 17" tire would be P225/45/17 - which is what I have.

First thing I noticed was that my speedometer is now fast - meaning that it shows higher than what I'm actually traveling. I've checked this with a couple of different traffic radar monitoring units they set up around here. I figure the difference is about 10%.

Second thing I noticed is a whine coming from the tires. Can't quite determine if it is the fronts (Pirellis) or the back (Falkans) but think it is the front.

Third thing I noticed is that I used to get about 30 mpg on the road. Now I'm only getting about 25 mph which is a double wammy - since the speedo is now measuring more miles for the same distance I would've expected my mpg to go up, but the fact that it is now lower means I've significantly lost fuel economy.

The tire dealer (Discount tires) told me to run 35 psi in these 17's. That seemed low to me and, since the mpg went down so drastically, I thought it might be from low pressure. So I bumped them up to 38 psi but haven't seen much of a difference.

So far, at 38 psi, the ride seems to be pretty good - not too harsh and very responsive!

I got an alignment so I know everything should be about as good as I can expect it to be in making this setup work right - short of altering the suspension.

How high of pressure can/should I go to achieve the best fuel economy without making these tires run hard as rocks?

Also, anyone else experience the speedo difference? Is there anything in the ECU that can be set to correct this?

 

if you could get a 245 45 17 on there you'd be as close as you can get to the first tire size.

your mileage went down because you have increased your rolling resistance and unsprung weight.

 

Rolling resistance...okay I'll you give that. Wider tires, bigger footprint - that's all a good reason to make the switch.

Unsprung weight...not sure what you mean - weight of what is "under the springs" maybe? I put both the old and new wheel/tire combinations onto a scale before swapping them out and maybe 2 lbs difference? So, maybe 8-10 lbs for the frame weight of the vehicle? - say even I'm wrong by a factor of 2 and it's 20 lbs, negligible I would think!

Tire whine and lower fuel economy are both possible indicators of underinflation...and I didn't mention in the previous post but now the tach runs around 26-2800 at what is 75 mph instead of where it used to run - around 22-2400. But could these factors contribute to what must be about a 25% decrease in gas mileage? (I figure the 25 mpg as calculated is now overstated by 10%, thus is really 22.5 mph. So 7.5 (decrease in mpg) / 30 (orig mpg) = 25%)

So what I'm really asking here is what ya'll would recommend for inflation on 17" tires, given the weight of our 3rd gens? Should I be running 40-45 lbs instead of 35-38?

 

10% sounds high. Find a tire calculator online and it should tell you the (theoretical) difference in speedometer reading %-wise. I don't think it'd be as high as 10%. If you're revving a bit higher at the same speeds as before now, that takes a bite out of fuel economy too.

 

You're right. According to the site http://www.miata.net/garage/tirecalc.html I should be seeing a 2% difference...

But that doesn't fit with what I thought I found, guess I'll have to run by another traffic radar setup again...wish I could get by with running one at 60 mph instead of 30. Then I'd have a much better idea.

 

By the sound of it, I'm probably lucky I didn't have a state trooper explaining my radar reading to me this last week. I made several trips across state (probably 1000 miles total) running the speedo at 81 or so on a 65 mph highway.

...you see officer, I figure my speedo was about 10% off, so I thought I was only going 73 which I figured you'd give me...

 

Hopefully, matt will chime in and explain unsprung weight. I do know that unsprung weight is very sensitive to changes

 

so when you get bigger rims you lose on fuel economy?

 

That's what I'm trying to determine, I guess. Then I'll have to decide if that is a long-term price worth paying...

As I said, I had weighed them side by side with the old rim/tire combination, because I had already read here that larger rims were typcially heavier, and decided there wasn't really that much difference...

Either that or hoping that someone can tell me whether the air pressure needs to be higher. I'm driving a couple of hundred miles again tomorrow. I think I'll bump the pressure up a little more and see what happens...

Anyone have any input into the pressure question? What kind of pressure do you run in your 17" tires?

 

goign that high on pressure, you're going to kill the inside tread band on your tires, and you're really not going to see much more increase in gas mileage.
it will also ruin the ride quality.

the P7000SS are noisy tires. I used to have some on my 16s and noticed they were very noisy. go with another pair of 512s when you get new tires and you should be happy.

Your gas mileage is more indicative of a change in driving style than in tires.. have you changed your driving style now that you've got these responsive tires on there? usually the Ouija board says yes- even though you don't think you do.

unsprung weight is exactly how it sounds- it's the part of the car that is "outside" the springs. This includes the control arm to some extent, the bottom half of the strut assembly, the spindle/steering knuckle/wheel hub assembly, brake rotor and caliper, and the wheels and tires.
the handling of a car is very sensitive to changes in unspring weight, but it's even more sensitive to rotating unspring weight- which will be your wheels, tires, and brake rotors.

Gas mileage is somewhat sensitive to the rotating weight- which you have changed some, but it shouldn't be enough to cause a drop in gas mileage that much..
even though you only changed the weight by a couple of pounds, think about where the weight is.... the wheels you used to have had a large portion of the weight at 7.5" from the center ( 1/2 of a 15" wheel). you've now moved a large portion of that weight another inch farther from the center. think about trying to spin yourself around in circles with a bowling ball held at your chest. now hold it at the end of your arms' length and try to spin around. much more difficult now!
same thing, only much more exaggerated.

I hope that makes some sense to you.
you *might* see a small drop in gas mileage, but you shouldn't see something that big.

Also keep in mind that a 225/45/17 tire in a Pirelli may not be the exact same size as a 225/45/17 in a Falken.. that can cause another bit of small error in the speedo/numbers. I have a friend with some 265/45/18 michelin pilot sports that just got some 255/45/17 Dunlop SP9000 for his spare wheels. the 255 Dunlops are almost an inch wider than the 265 Michelins!

Anyway, make sure you always keep them on the same axle- front or rear. if you run different size tires on the front end, you can really heat up the differential in the tranny and possibly do some damage if you drive too hard and too long on mismatched tires.
If you keep the same tires on the same axle, you won't have a problem with it. might freak out the ABS system under hard braking, but so will different compound tires, as they'll grip differently under hard braking anyway...

 

Thanks, Matt, for your informative reply...answers a few questions, especially the description of "rotating weight".

I wasn't sure about the Pirellis...just happened to already have them and thought I'd save a few $$ by only buying two tires until I knew for sure I really wanted to run with 17" wheels. I think you're right about the 512's. They seem to be a good tire and I think I'll like them. I also think they'll be a better all-season tire here in Colorado once winter sets in...

I've made one switch in my "driving style" - I now keep the A/T switch on the "performance" setting instead of in the middle where I used to...and love the difference it makes around town!

Keep in mind, though, that the gas mileage I'm measuring is highway mileage. This is with the cruise control on and over the same roads and distances that I've travelled for years. There really isn't much that can change in that scenario.

Thanks again for the input. I guess I'll keep the pressure where it's at - 38 lbs. It seems to give me about the "stiffest" ride I really think I want and going higher isn't ideal if it'll make the ride qualify suffer any.

And thanks to everyone else who responded with your information and input as well!

 

yeah, definitely stay away from the P7000SS if you have to drive in the snow or slush. they're okay in the rain, but suck ***** in the snow...

I'd even be wary of the 512 in the snow... but you'll find out on that one. may want to look at some other all-season tires for the winter.