Restoration Tire Size?

[st0878]

Can anyone tell me the tire size for a 75 280z with the stock aluminum aloy rims? The tires on there now seem to be wider than they should and I think the raidius is smaller, resulting in incorrect spedometer readings.

[dga]

For a 77 280z, it was 195 70H14

[DoctorMuffn]

Is that the same for a '73 240Z?

The manual tells me 175 HR-14 (Tubeless)... What does that mean? Where's the aspect ratio?

Please help,

The Doctor

[MikeW]

The factory tires had an aspect ratio of about 75.

[DoctorMuffn]

I need an idea on width too. An aspect ratio means nothing without width. Will I be looking for 175/75 R14? Is that going to make my speedo read properly?

My rolling circumference is 69" yielding a rolling diameter of about 21.9"; this is with 185/60s which look way small in my wheel wells and read almost 90mph at common freeway traffic speed of about 75-80.

Thanks for your help to this point though, MikeW.

The Doctor

Oh yeah, I've got the 4 speed tranny right now.

[MikeW]

I'm surprised that 185/60's cause you to read 90 when you're really going 75. Maybe that other traffic is moving faster than you think. I'd try to hold it steady on 80mph (straight up on the speedometer) and time a measured mile on the highway.

175 is the width whether the aspect ratio is 75 or 60.

Here's a place that might help you figure out the numbers better:

http://www.miata.net/garage/tirecalc.html

Plugging in your numbers 185/60 instead of 175/75 you should only be going 56mph when your speedometer reads 60mph. Perhaps a previous owner swapped the transmission/differential without changing the speedometer gear?

BTW, I'm currently running 205/60 on 7" rims. According to the calculator (using 175/75 as the old size) that means I'm going 58.4mph instead of the indicated 60mph. My needle bounces around enough that a small difference like that doesn't bother me.

[DoctorMuffn]

Wow excellent find, MikeW. Thank you very much for all of your help with regards to my plight. Thanks especially for taking the time to share your wealth of knowledge.

Furthermore, I read something on this forum that recommended a 24.75" rolling diameter to measure accurate speed, and either 195/70s or 175/75s will yield very close measurements.

The Doctor

[lordbiotree]

http://members.cox.net/nissan4wheelers/Calculators.htm

Includes

Speedo converter

Find new Gear Ratio with new tire size

Crawl Ratio( 4 wheel drive vehicles only)

MPH Calculation

RPM Calculation

P-Metric tire to inches

All these calculators can be found at the above link.

[DoctorMuffn]

Thanks, lordbiotree, that's an excellent resource.

[MAX]

Greetings MikeW,

I'm looking to upgrade my tires and wheels from 185/75-14 to 205/60-16. However, I don't care for the really low profile tires and was wondering how this combination would look. Do you have a picture of your wheels and tires that you could post.

Thanks

[MikeW]

Greetings MikeW,

I'm looking to upgrade my tires and wheels from 185/75-14 to 205/60-16. However, I don't care for the really low profile tires and was wondering how this combination would look. Do you have a picture of your wheels and tires that you could post.

Thanks

Here ya go ...

[sblake01]

MAX, Mike is running 205/60-14s and you're considering 205/60-16s.