i believe i read somewhere that the speed rating required the tire to sustain the given speed for at least 1 hour. i forget where i read that... maybe tirerack.com? regardless, it's not a magical barrier that will cause the tire to instantaneously self destruct.

i ran the BFGs (235/60-15) for a year or so. highway cruising for me at first was 75-80mph, with occasional jaunts toward triple digits, with no untoward effects. i dropped down to ~70mph for cruising simply because mpgs were about 13 vs 18 respectively.

i switched them over to a primewell(?) tire, which i'd never heard of before, because of wearing tread. i dont even remember but they were probably an S rating. they ran fine highway cruising at 70mph for about 3,000 miles. wasn't concerned because i was planning to upsize.

which gets to my final point... if youre planning on testing the high-end speeds of the car frequently, perhaps you should be on a larger wheel.

i always felt a bit of "squishy-ness" when cruising at highway speeds. i thought was the tires (not suspension) and their 5.5+ inches of sidewall.
i went up to an 18 inch wheel with an H rated tire. the car feels extremely planted at speed now. 18 is pretty large for these cars, but definitely improved my confidence at higher speeds.

 

This goes back to what Camcor mentioned about sidewall construction. Even with the 15" wheels you can get the same feeling, or close, if you can find the right tire, as the higher load rating and higher speed ratings tire generally have a stiffer casing and sidewall. Of course, finding 15" tires like that isn't easy these days. Yoou can still get the 215/70x15 Mchelin XWX Ferrari-spec tire....at $400 each !

Those BFG 235/60s are just a basic tire, not a performance tire. There's nothing *wrong* with them but they're really an ordinary tire but in a rather fatso 15" size . I've used them myself. And they really need at least a 7" wide wheel...8" would be better....to properly support the (already softish) sidewall. Most XJSs had just a 6.5" wide wheel until '93 or so....or even 6" wide before '81-82.

BFG offers a wide variety of tires but, like other brands, very little in the 15" sizes.

Not familiar with the other brand you mentoned.

What size tires are you running on your 18" wheels? I ask because, IMHO, there are some real world disadvantages and risks to the almost-no-sidewall-at-all tires that we often see today.

Cheers
DD

 

totally agree... and i wanted to go with the largest sidewall possible, that would still fit and not rub. i decided 40 was probably too short, and wanted to try and squeeze a 45 sidewall in.

i went with a michelin winter tire (for now) at 235/45-18.
(the wheels are 18x8 asteroids from an '01 XJR.)

the diameter of the tire is listed at 26.3 inches compared to the 26.1 inches on the BFG 235/60-15.

they're tight, but they fit and dont rub with no rolling of fender lips and a 1/2 inch spacer up front.

 

I found some V rated tire information that I had collected and it is as follows.
Kumho ECSTA LX Platinum 225/60 VR tires for about $100.00 ea. may want to consider 235's to stay closer to stock wheel height. Vredestein makes a 215/70 VR15 which sell for about $500.00 ea. I wanted to go with the Kumho's but the tire shops wouldn't put them on because they weren't factory specified replacement size.

 

I like Falken ZX912s and the cheaper pirellis. If going with a 1000$ pricepoint rather than 400-500 I'd get P6000 or maybe some nittos.

The speed rating largely affects the sidewall thickness. they aren't necessarily less safe unless you intend to use them at high speed. If you never drove over 80 and had tires rated for 85 and tires rated for 185 the 185s are going to giving a firm solid feel, but good tires won't feel hard or uncomfortable.
Its similar to the tradeoffs of aspect ratio but there is less negative affects of getting the better build high speed tires. If you aren't going to go 150 then thats a lot of money ot pay though.
My 85$ Falkens were purchased because they were the best I could find for the money and mine are 130mph rated...which I have used