I went from the stock 18s to 19" wheels with 245/35/19 on the front and 275/35/19 on the rear. It feels like my speedometer has me going faster than I actually am. I was rolling down the highway at 80mph and it seemed like everybody was going 80. How much of difference does the increase in wheel size have? It is marginal, correct? Thanks

 

this should help http://www.miata.net/garage/tirecalc.html
according to that site, you should actually be going faster that you think you are. actually a positive, less miles on the OD

 

Your speedo is fine. IIRC the speedo runs off a sensor that works in conjuction with your brakes, its not affected by changing tire size, gearing etc...other cars can go 80mph too you know

 

yeah, thats how I felt when I had my scion xb r.s. 1.0, I had 15" hub steel wheels(w/hub caps)(came with the car) and I put on 19's on it with 225/35/19 toyo proxe4's. I had the biggest wheel and tire combo on a scion xb(before warranty was void) and it felt like my speedo was way off. But I went up 4 wheel sizes and lascala26 went up 1. I would think one inch would mess up the speedo calibration. ?

 

What matters is overall tire diameter not wheel size.

 

It doesnt matter on the Z. If you stock diameter is 26" and you do a 27" or a 28" diameter, the speedo is not affected like in other cars.

 

How can that be? All a speedometer sensor can do is count revolutions of the tire over a fixed amount of time. Since with each revolution of a 26” diameter the car travels 93% of the distance of a 28” tire the speedometer will be off by 7%. Am I thinking incorrectly?

I find it hard to believe that a company like Nissan who takes two years to determine how to properly align the Zs suspension could somehow figure a way around the laws of physics.

 

Wow! A lot of BS being posted here!

Our speedo reads off the rear AXEL. So, tire diameter DOES affect your speedometer readings. The math isn't too complicated, but I'm lazy.

An easy check requires a stopwatch & a cruise on a road with mile markers. Find a stretch of road that is clear of traffic, and measure the time it takes to cover one mile, while maintaining a constant speed. Divide that number into 3600, and like magic, you will have your REAL speed! For better accuracy time a 10 mile stretch, and divide that into 36,000.

 

I believe it's pi(d) for circumference. I think we all finished the sixth grade

 

yea

 

It isn't off by that much, is it? I'm not too worried. I just didn't want to be doing 65 when I'm actually going 57....something like that. The diameter of my tire/wheel shouldn't haved changed much.

 

275 35 19 is a tad bigger then the stock size.If anything your going slower.If memory serves me right i think there is a 2 mph bust.Nothing to worry about.How the hiros look?

 

Yeah...if I'm the speedometer says I'm going 60, I'm actually going 58.8mph. No big deal. The Hiros look killer. Love them. Thanks!

 

Sorry guys, after doing some research I have to admit I was wrong. The speedo reding comes from the tranny, essentially the back tires, so if you do change overall diamter it will affect your speedo readings. I got confused somehow with this same question when it was brought up in regards to swappping the rear end gear-350 evo final drive, that is when the speedo is not affected. Again, sorry for posting misinformation