"SR" Aspect Ratio Tires

[JHZR2]

Hello,

Hoping someone will have a definite answer. My 81 MB 240D was OEM set up with 175SR14 tires. It currently has 195/70R14 tires (which were used extensively on the 5-cyl variants). The speedo reads fast compared to my GPS reading.

Ive read that "SR" means an "80", "85" aspect, but then have also seen that it means "82". Trying to grasp for certain what SR really implies.

I have a set of 185/65R14 tires which Id be interested in using if not too ridiculously off. Using the miata.net tire size calculator, Im showing 6.5-8.7% too fast using 185/65r-14 in place of 175/80 or /85r-14. Since Im probably already off by 5ish MPH at 60, I dont want it to get out of hand...

Thanks!

 

[Audi Junkie]

They are too small, won't support the weight of the weight of the vehicle well. Double-check the load rating, but even if adequate, my opinion is, no way.

 

[eljefino]

I think the S is the speed rating and the R is the default height/width ratio, 80 or 82. It's the lack of numbers that mean 80-82, not the S. Wiki says: Prior to 1991, tire speed ratings were shown inside the tire size, before the "R" character. The available codes were SR (112 mph, 180 km/h)...

Despite being 10mm wider your proposed 185/65/14s carry less weight. I bet the original tire size was barely adequate in the pursuit of MPG.

 

[Trav]

According to the tire book i have here SR prior to 1991 was a speed rating 112 MPH post 91 is aspect ratio 80. I think the closest you will get is 185/75/14.

175/SR (80) would be a 140 mm sidewall.
185/74 would be 138.75mm the 1.25mm difference should be negligible speedometer wise maybe 2MPH.

Right now with the 195/70 you have 136.5mm
with the 185/65 it is only 120.25mm i think your speedo will be way off,its a full 20mm difference from original.

I think the calculations are correct but double check the numbers.

 

[JHZR2]

Originally Posted By: Audi Junkie
They are too small, won't support the weight of the weight of the vehicle well. Double-check the load rating, but even if adequate, my opinion is, no way.


Good point, a 240d is about 400-500# heavier than an integra.

 

[Astro14]

I owned the same car - same tires...

The SR in this case is not an aspect ratio, nor is it a speed rating. Tires that were sold back then were understood to be an aspect ratio of 78 if it weren't specified. That was a holdover from the days of bias-ply tires that were all a 78 aspect ratio. Your MB came with 175R14s, most likely Michelin X radials...they had no speed rating...

You simply can't find those tires in that size at a regular retailer anymore...leaving two choices:

1. got to a specialty old car tire supplier, e.g. http://www.cokertire.com/ - they've got Vredesteins in the 175R14 for about $96 each.

2. continue to buy 195/70R14s and mount them. (my recommended option). they are the same diameter as the 175R14s...

The speedometer difference is a red herring...interesting but not related to the tire size. 195/70R15s will be just fine, they have the same sidewall height and the same rolling diameter as the OE tires.

Great car, slow, but I wish I still had mine...the most reliable thing I've ever owned...

 

[CapriRacer]

Amateurs! Sheesh!

As has been pointed out, the "SR" is not an aspect ratio. It is the speed rating and indicates a radial tire.

Just so everyone understands, the fact that there is no aspect ratio in the size means the aspect ratio is imbedded in the way the size is expressed. In this case, a 175SR14 means that this is an 82 aspect ratio.

If the tire size was 7.50-14, that's a bias ply tire with an 88 aspect ratio. You can read more about this here:

http://www.barrystiretech.com/tiresizing.html

But what about the rest of this? Well, first, converting to modern tire sizing has its problems - and the first one is the rim size.

According to Tire Guides a 1981 MB 240D came on 175R14's (verifying the OP), with an inflation pressure of 28 psi front / 32 psi rear mounted on 5½" wheels. My experience is that it is the narrow wheels that seem to cause the most problems. So let's see if that is the case here.

ETRTO (European Tyre and Rim Technical Organization) lists a 175R14 as being a LI of 88 in a Standard Load with a diameter of 634mm.

A 195/70R14 had a SL Load Index of 91 (OK there!) with a diameter of 630mm (OK there, too!) with an allowable rim width range of 5" to 7" (OK there as well!) Overall, that seems a good fit!

What it sounds like is that the car has a built in error!

What about the 185/65R14? - LI 86 (Too Low!), diameter 570mm (a 10% loss!), and an allowable rim width range of 5" to 6½" (That's OK). Overall not a good choice.

 

[Astro14]

OK- you got me...I am an amateur...and remembered the Euro-metric aspect ratio wrong (must have been thinking of the G78 bias-ply tires on an old muscle car I once had...)...but I've owned the exact same car as the OP and had the same challenge in equipping the car with tires...

However, we (amateur and professional) reached the same recommendation on size and on his speedometer error....means that the 195/70R15 must be the right answer...and that's the answer I reached years ago when I had the same problem (no more 175SR14s sold anywhere).

Here is an even better resource for fitting modern tires to older cars: http://www.tirerack.com/tires/tiretech/45_conversionchart.html

Still doesn't help me find an equivalent for the 6.50x19 on the '32 Packard though...

Cheers,

 

[JHZR2]

OK, .82 it is, and wont be using the 185 tires. Thanks!

 

[Trav]

Thanks for the link.
I have a 2000 ADAC book here with that tire.They show the S as 112 MPH and the R as Radial with no aspect ratio given for pre 1991 and aspect ratio of 80 for this size on newer tires.

Is there a difference in U.S. and Euro?

 

[CapriRacer]

Originally Posted By: Trav
......Is there a difference in U.S. and Euro?


Not enough to worry about. I cover that here:

http://www.barrystiretech.com/tirestandardizingorgs.html