Tire/Tyre Size Calculator - Rolls-Royce and Bentley Forums
Reply
 
LinkBack Thread Tools Display Modes
post #1 of 11 (permalink) Old 03-08-2013, 10:08 AM Thread Starter
Senior Member
 
guyslp's Avatar
 
Join Date: Aug 2007
Location: Staunton, VA - USA
Posts: 2,919
Tire/Tyre Size Calculator

As anyone who owns one of our cars that's not in current production knows, it's incredibly common for the tire sizes originally fitted to go out of production. Again and again the question comes up, "What tires can I get for [insert your car year and model here]?"

An online tool that's incredibly handy for figuring out the answer to this question is the Tire Size Calculator at miata.net. You simply plug in the size of the tire originally fitted on the left then can enter the sizes currently available on the right and see how close a match they are.

One common example that has been causing much consternation is what tire to put on a Bentley Turbo R that was originally fitted with Avon 255/65R15 tires. If you plug this in on the left side and start playing with sizes on the right (and you need to have some idea of roughly where to start) you quickly find that 235/70R15 or 225/75R15 tires are negligibly different in size from the originals and have almost no effect of significance on your speedometer reading.

One word of warning, though, is that the calculator uses nominal standard sizes for its calculations. There is a +/- percentage allowed between a nominal size and the actual size of a given tire manufacturer's version of that tire. Always be sure to check out what he manufacturer of the tire you're considering gives as its actual size if you think you're anywhere close to a "tight fit" with the original tires.

Brian

The opposite of a correct statement is a false statement. But the opposite of a profound truth may well be another profound truth.
~ Niels Bohr


guyslp is online now  
Sponsored Links
Advertisement
 
post #2 of 11 (permalink) Old 03-13-2013, 12:21 PM
Member
 
Join Date: Feb 2012
Location: United Kingdom
Posts: 28
Send a message via AIM to dougal
Can i just sugest that sticking with the correct size tyre is better.

You didn't buy a Rolls Royce because it was cheap. you bought it because it is a superb car. part of the reason it is a superb car is because it it made of top quality components. fitting cheap tyres will spoil the car.

Regards

Dougal
www.longstonetyres.co.uk
dougal is offline  
post #3 of 11 (permalink) Old 03-13-2013, 12:32 PM Thread Starter
Senior Member
 
guyslp's Avatar
 
Join Date: Aug 2007
Location: Staunton, VA - USA
Posts: 2,919
Might I suggest that if you don't have anything useful to say that you refrain from being insulting?

The fact of the matter is that many of the OEM specified sizes at the time of production are no longer available for many of our cars. It is quite helpful to know what you can reasonably and safely fit as alternatives.

Let's not even get in to how awful the OEM Avon tires were and are from a wear perspective.

Each owner can and will make their own decisions based upon actual circumstances. Some of us think that's perfectly appropriate.

Brian

The opposite of a correct statement is a false statement. But the opposite of a profound truth may well be another profound truth.
~ Niels Bohr


guyslp is online now  
 
post #4 of 11 (permalink) Old 07-09-2013, 09:51 AM
Senior Member
 
Join Date: Feb 2013
Location: Pembrokeshire UK
Posts: 318
I remember in my youth fitting wide wheels to cortinas and the likes we would go from 155's to 175/70 to keep a similar rolling radius but get more rubber on the road, I don't think I would want to go narrower and taller on a Turbo R to maintain the rolling radius, wider and lower perhaps, but for now I'll stick with the Avons.
Also the link to longstone wasn't that helpful as their price can be bettered by a substantial margin if you do a bit of research, I recently had 2 Avons delivered to my door for less than £300 each.
I had to replace the Avons due to old age, lots of tread left but they were cracking up and didn't relish the thought of a blowout on such a heavy car.
Best regards
Graham
Graham is offline  
post #5 of 11 (permalink) Old 07-12-2013, 09:35 PM
Senior Member
 
Join Date: Feb 2013
Location: Pembrokeshire UK
Posts: 318
By way of an update to the Turbo R tyre story, just bought 2 via tyre shopper for less than £300 each including fitting at a local tyre depot(National Tyres)
Mytyres where I got the last pair have gone up to £404 each!!! then you have to get them fitted as well, other suppliers are out of stock so could be stocks are getting low so might be a good idea to buy now while you can if you think you'll need some soon?
Regards
Graham
Graham is offline  
post #6 of 11 (permalink) Old 08-27-2013, 06:28 PM
Member
 
Join Date: Aug 2013
Location: North East England
Posts: 34
the Miata calculator is actually quite incorrect.

the main problem is the maths that they've used.

I believe that they've just done the width x aspect ratio + wheel diameter thing, which can be up to 15% wrong.

Thyres slip as they work, the slip being the thing that creates the heat which enables grip to happen. The rolling radius calculation doesn't take account of slip, which is unfortually a quality that can only be measured ( which is what tyre manufacturers do ). In addition, the larger the aspect ratio, the more the difference.....

The point of all of this is that when comparing rolling radii, only tyre manufacturer's measured data is valid.

From Avon, the original 235/70 carries a 319mm static laden radius (SLR) and rolls at 743 wheel revs per mile (WRPM).

If attempting to find an alternative, which incidentally I could condone under certain circumstances, the manufacturer's wrpm and SLR must be matched.
2woody is offline  
post #7 of 11 (permalink) Old 08-28-2013, 05:35 AM
Member
 
Join Date: Aug 2013
Location: Eastern England
Posts: 36
Tyres and alternatives

With regard to tyre sizes becomeing obsolete I have found with many years experience that if you Deal with Rolls /Bentley Specialists you can always source Tyres of the correct size as per Original fitment.Only when changing from Cross plies to Radials does it become more complex however good Tyre retailers can usually help finding a suitable replacement.

Bentleyboy
Bentleyboy63 is offline  
post #8 of 11 (permalink) Old 08-28-2013, 05:37 AM
Member
 
Join Date: Aug 2013
Location: Eastern England
Posts: 36
Tyres and alternatives

With regard to tyre sizes becomeing obsolete I have found with many years experience that if you Deal with Rolls /Bentley Specialists you can always source Tyres of the correct size as per Original fitment.Only when changing from Cross plies to Radials does it become more complex however good Tyre retailers can usually help finding a suitable replacement.

Bentleyboy
Bentleyboy63 is offline  
post #9 of 11 (permalink) Old 09-11-2014, 07:40 PM
Senior Member
 
Join Date: Nov 2013
Location: Spartanburg, SC
Posts: 54
Speed and Load Rating

I have been able to find the exact tire size the owner's manual calls for on my 86 Spur. However, I am wondering about the speed rating and the load rating. I can get a tire rated at 118mph (OEM was 130mph). Though I won't be getting close to either speed, will the lower rating be an issue?

As for the load rating, it is harder for me to get my head around it. Should I combine the load rating of all four tires to ensure that it exceeds the GVWR plus passengers, luggage and fluids?

Or, am I making too much of this?

Thanks for your replies...and letting me resurrect this topic.

Duane
NAG14318

Duane Ridenour
South Carolina
86 Silver Spur
kg4ibj is offline  
post #10 of 11 (permalink) Old 09-11-2014, 08:25 PM Thread Starter
Senior Member
 
guyslp's Avatar
 
Join Date: Aug 2007
Location: Staunton, VA - USA
Posts: 2,919
Pulled directly from the RR & Bentley Parts, Repair, Restoration & Other Resources Compilation that I maintain, in a discussion of tires for Silver Shadows, but this part applies to SY or SZ series cars:

What is most critical is that you pay attention to the load rating of the tire you select once you know which size youll be choosing. It must be above 100 due to the weight of these cars. Most tires in the above sizes meet this criterion, but some do not. The vast majority will have load ratings between 102 and 105, with some available in XL (extra load) rating of 108. Speed ratings are generally not at all important, particularly if you live in a country with speed limits and dont routinely flagrantly violate them for extended periods of time. For instance, an S speed rating means that a tire can carry its specified load at 112 MPH/180 kPH sustained driving speed. Most tires in the sizes above have a minimum speed rating of S. Few of these cars ever see that speed for short periods of time, let alone for sustained periods of driving. At one time, before load ratings were routinely indicated, speed rating was the closest indicator you had to roughly gauge load capacity, so speed ratings of H and above were the OEM spec. Now that load ratings are on the tire these should be your main guide.


Yes, you combine the weight that can be loaded on to each tire to arrive at the total that all four tires can carry. I have yet to see a tire in the sizes that can be fitted to these cars that doesn't have a load rating above 100. The page on How to Read Speed Rating, Load Index & Service Descriptions at tirerack.com covers all of this quite well and in an understandable way. A tire with a load rating of 100 can handle a load of 1764 lbs/800 kg, so a set will carry 7056 lbs/2400 kg. You are not going to come close to that with a fully laden SZ car, and most of the tires you're going to find will have a load rating higher than 100.

Brian

The opposite of a correct statement is a false statement. But the opposite of a profound truth may well be another profound truth.
~ Niels Bohr


guyslp is online now  
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Rolls-Royce and Bentley Forums forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in









Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.



Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page
Display Modes
Linear Mode Linear Mode



Posting Rules  
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome
 

Garage Plus vBulletin Plugins by Drive Thru Online, Inc.

SEO by vBSEO 3.6.1