Tire size fail

[Phil Z]

Before I got my z running again, it sat more than 25 years in my garage. So I took the wheels off about 4 years ago and had new tires mounted so I could make it a roller. Once I was driving it I noticed my speedometer was off by 10 mph. 65 = 55. Figured out the tires weren’t the same diameter as my still original spare tire. A long way to go to get to my question. What should my RPM be, cruising at 65 mph on the correct size tire? I am assuming the RPM runs higher on smaller tires, but I defer to anyone who has this knowledge. 4 speed Manuel transmission all original drive train. Thanks.

[SteveJ]

What size tires did you mount, and what is the size of the spare? There are tire size calculators that can help you figure the engine speed.

[7tooZ]

As memory serves me a stock tire and an early 4 speed in an original Z would be about 3000-3200 RPM


72 body and block, everything else 71, Tokico springs, Illumina, R180 CLSD, 83 close ratio, 3.90 gears, Ztherapy SUs, BRE 15X7 Libre wheels and BRE front spoiler.

[7tooZ]

I believe an early car had 175srX14. That tire has a 24.96 dia.


72 body and block, everything else 71, Tokico springs, Illumina, R180 CLSD, 83 close ratio, 3.90 gears, Ztherapy SUs, BRE 15X7 Libre wheels and BRE front spoiler.

[AlbatrossCafe]

My '78 280z stock tires are 195/70/R14.

The speedo is off about as much as yours even with stock tires. It's most imprecise of any vehicle I've owned lol. Even worse than my truck which has tires 3" larger than stock.

Edited August 7, 2018 by AlbatrossCafe

[Phil Z]

As memory serves me a stock tire and an early 4 speed in an original Z would be about 3000-3200 RPM


72 body and block, everything else 71, Tokico springs, Illumina, R180 CLSD, 83 close ratio, 3.90 gears, Ztherapy SUs, BRE 15X7 Libre wheels and BRE front spoiler.

At 65 mph checked by radar and GPS I am turning 3800 rpm. I felt that was higher than my memory recalls. I want to upgrade to 15 inch wheels and the correct tires to achieve a proper reading on the speedometer. Thanks for the the replies.


Phil Smith

[Phil Z]

What size tires did you mount, and what is the size of the spare? There are tire size calculators that can help you figure the engine speed.

I will measure my tires and spare ASAP and let you know. Thanks


Phil Smith

[SteveJ]

You don't have to measure your tires per se. You just have to get the sizes off the sidewalls. (Note, that it has been pointed out in a different thread on this board that the wheel diameter can vary even with the "same" tire size.)

Assuming that we're starting with a stock 175 R14 tire, I used 175/78R14 as the size. (Google is your friend.)

You want a 15 inch wheel (There are not many good choices at Tire Rack, but if you don't mind the road noise, Advans  are available.).

This gives you available tire size (at Tire Rack) for 15 inch wheels: https://www.tirerack.com/tires/sizes/results.jsp?diameter=15

[7tooZ]

You don't have to measure your tires per se. You just have to get the sizes off the sidewalls. (Note, that it has been pointed out in a different thread on this board that the wheel diameter can vary even with the "same" tire size.)
Assuming that we're starting with a stock 175 R14 tire, I used 175/78R14 as the size. (Google is your friend.)
You want a 15 inch wheel (There are not many good choices at Tire Rack, but if you don't mind the road noise, Advans  are available.).

This gives you available tire size (at Tire Rack) for 15 inch wheels: https://www.tirerack.com/tires/sizes/results.jsp?diameter=15

Steve
I was under the impression that the early Z 175R was 82 sidewall.
Don


72 body and block, everything else 71, Tokico springs, Illumina, R180 CLSD, 83 close ratio, 3.90 gears, Ztherapy SUs, BRE 15X7 Libre wheels and BRE front spoiler.

[SteveJ]

36 minutes ago, 7tooZ said:

Steve
I was under the impression that the early Z 175R was 82 sidewall.
Don


72 body and block, everything else 71, Tokico springs, Illumina, R180 CLSD, 83 close ratio, 3.90 gears, Ztherapy SUs, BRE 15X7 Libre wheels and BRE front spoiler.

It could be. I was going off a post from one of the more respected members at Zcar.com. 

A little more digging revealed this size chart at Tire Rack: https://www.tirerack.com/tires/tiretech/45_conversionchart.jsp

If the OP wants 15 inch wheels, the 205/60R15 should give a comparable tire size and not much speedometer error with the actual speed a little lower than the indicated speed.

[7tooZ]

2 hours ago, SteveJ said:

It could be. I was going off a post from one of the more respected members at Zcar.com. 

A little more digging revealed this size chart at Tire Rack: https://www.tirerack.com/tires/tiretech/45_conversionchart.jsp

If the OP wants 15 inch wheels, the 205/60R15 should give a comparable tire size and not much speedometer error with the actual speed a little lower than the indicated speed.

Most tech sites agree that the original tire dia was in the 24.5 - 24.9 range which makes it easy to chose a tire and wheel size using the tire conversion charts. I am running 205/60X15 and find the accuracy of the speedo really close. But then again I have and 83 trans and a 3.90 diff so in my case who knows???

[siteunseen]

I have 205/60/15s on my 240 and riding next to a friend talking to each other on the phone, my speedo was very close to his at 70mph.  Maybe 1 or 2 mphs either way.  Less than 5 for sure.  They are Hankooks from discounttiredirect.com, $65 each.  I really like them.