OEM to 18/19 - change in speedometer?

[ny2ca]

The guy at the tire shop told me that since I'm going from 16/17 to 18/19 - that my speedometer will be off. If so, how much will be it be off? ie. If I'm going 80mph how fast am I really going - or the better question is how fast should I be going if I only want to go 80mph?

 

[Ojas]

ny2ca, Plug your original and current tire sizes into this calculator: http://www.miata.net/garage/tirecalc.html

 

[TitaniumVtec]

I have 225/35/18 and 275/30/19 and my speedo is exact up to 150mph; GPS verified!! I have not exceeded 150mph but I'm sure it will right on.

 

[nsxtasy]

The guy at the tire shop told me that since I'm going from 16/17 to 18/19 - that my speedometer will be off. If so, how much will be it be off? ie. If I'm going 80mph how fast am I really going - or the better question is how fast should I be going if I only want to go 80mph?

I'll assume you're getting the best matching 18"/19" sizes, which are 275/30-19 rear (and 215/35-18 front, although only the rear affects the speedometer).

275/30-19 is 3.2 percent larger in diameter than your stock rear size of 245/40-17, so the speedometer and odometer will read lower by that percentage than they do with the stock size tires.

Note that the percentage difference depends on the year NSX. 275/30-19 is 2.6 percent larger than '91-93 stock, 3.2 percent larger than '94-01 stock, and 1.9 percent larger than '02-05 stock.

 

[EIFFEL]

I would use a portable GPS unit to verify actual speed vs. speedo speed to show the difference between the two.

 

[nsxtasy]

-

I would use a portable GPS unit to verify actual speed vs. speedo speed to show the difference between the two.

There are plenty of other sources of speedometer error in addition to a difference in nominal tire size. Just to name a few, tires with the same nominal size may vary in actual size, the difference in diameter due to the difference in tread depth between new tires and tires worn to the treadwear bars and ready for replacement is 2 percent, tire pressure can affect the amount of deflection, etc. Unless you are controlling for all those other variables as well, it's quite possible that any difference you find may have other causes.

 

[CarCrazed4Life]

Re: -

Agreed. If you think about what NSXtasy is saying, a New tire with 10/32" treadwear versus a worn tire with 2/32" of treadwear is acutally 1/2" difference in total ride height.

Like Tire wear, many other calculations play into affect on speedo error. For the most part sticking to as close as possible tire height is the best way to avoid this. However, many other factors play a role as well which is why its tough to calculate them and best to verify by a GPS unit.

 

[nsxtasy]

Agreed. If you think about what NSXtasy is saying, a New tire with 10/32" treadwear versus a worn tire with 2/32" of treadwear is acutally 1/2" difference in total ride height.

Er... It's a 1/2" difference in diameter, but a 1/4" difference in ride height.

 

[ny2ca]

nsxtacy: "275/30-19 is 3.2 percent larger in diameter than your stock rear size of 245/40-17, so the speedometer and odometer will read lower by that percentage than they do with the stock size tires."

When you mean lower you mean slower? According to the Miata Tire size calculator link above, going from stock to 19's = "Speedometer reading with non-stock tire is 3.2% slow. When your speedo reads 60mpg, you are actually travelling 61.9mph." I thought that with a larger the wheel - the speedo would show a higher number - but I'll be going slower. Is this right?

 

[nsxtasy]

nsxtacy: "275/30-19 is 3.2 percent larger in diameter than your stock rear size of 245/40-17, so the speedometer and odometer will read lower by that percentage than they do with the stock size tires."

When you mean lower you mean slower? According to the Miata Tire size calculator link above, going from stock to 19's = "Speedometer reading with non-stock tire is 3.2% slow. When your speedo reads 60mpg, you are actually travelling 61.9mph."

The Miata site agrees with me.

Look at my statement: "275/30-19 is 3.2 percent larger in diameter than your stock rear size of 245/40-17, so the speedometer and odometer will read lower by that percentage than they do with the stock size tires."

Let's assume your speedometer is accurate with the stock tire sizes. Then, when you are going 61.9 mph, your speedometer says you are going 61.9 mph. When you switch to the 275/30-19 rear size, then your speedometer (and odometer) will read lower by 3.2 percent, so when you are going 61.9 mph, the speedometer reads 3.2 percent lower, or 60 mph. Just like the Miata site says. (Not 60 mpg but I assume that's just a typo. I'm sure we all wish we could get 60 mpg in our NSXs. )

I thought that with a larger the wheel - the speedo would show a higher number - but I'll be going slower. Is this right?

No. With a larger diameter tire (the wheel size is irrelevant), the car is actually travelling further than the speedometer "thinks" it is, because the conversion of revolutions from the speed sensor uses a smaller diameter in the calculation. So the speedometer reads LOWER than the distance the car is actually traveling.

 

[CarCrazed4Life]

NSXtasy is right... However Wheel size does matter. Imagine me putting 26s out back. I would then need a 295/-05/26 or something just to make it work

All jokes a side, I like that Miata tool as well as this one:
http://paspeedo.com/calculator.htm

I always set the mph to test at 100. It easy to calculate percentage in respect to speedo error.

 

[Alan C.]

tire pressure can affect the amount of deflection

How much of a change in pressure would be required to affect a noticeable change in the physical circumference of the tire?

 

[ny2ca]

thanks everyone for clearing that up - so now I'll just set my cruise control at 77.55mph which will equal 79.99582669640011mph - hopefully, no tickets now.:smile:

 

[CarCrazed4Life]

How much of a change in pressure would be required to affect a noticeable change in the physical circumference of the tire?

This part is harder to tell. Each one is dependant upon the tire size (more important is the volume of air, not pressure) to see what affect additional air has on a tire (deflection, diamater size, load rating, etc.)