Fastest Tire for a 6.5" Rim

I know this topic has been hashed out on other car enthusiast forums but I most of the answers started with "I heard that so and so. . . "  So I'm turning to a more "reputable" source.  (at least one where there are a lot of people as resourceful and cheap as I am.)

Say you are running a 15 x 6.5" wheel and to make this hypothetical scenario more common it's on a 2100 lb NA Miata.  What tire is going to be the fastest at autocross?  A 195 or 205/50 will fit the rim the best, but would it be faster than a 225 or a 245 that was squeezed on to the rim?  If this hypothetical miata were turbocharged and pushing 200+ hp would that change the answer?

In an autocross type setting, generally as much rubber as can fit will be faster. Steering precision will suffer vs a proper width tire, but that will be overcome by higher lateral force capability.

On a track with higher speeds, one could argue that the increased rolling resistance, aero drag, etc on the straights could overcome the benefit of increased cornering speed. 

In reply to bigben :

I’ve been researching STS Miata builds, which use a 7.5” rim width, and most of them are running a 205. I certainly wouldn’t try to squeeze a 225 or wider onto your 6.5” wheels, but whether a 205 will be faster that a 195 is something you’d probably have to test. 

It also comes down to competitiveness vs. budget. If the rest of the car isn’t already nearing the class limits for prep, then a 10mm difference in section width might not make much difference anyway.

Conversely, if you really want to make the car competitive you should also check the weight difference vs. tread width difference between the 195 & 205. 

I think it depends also on whether you have a power or non power steering.  A narrower tire can allow for quicker steering, as not as much rubber is grinding against the asphalt.  

Anecdote time: Was racing with some friends who have a mid-90's Sentra and lap times were faster on 195 tires than 205.  The 195's "feel" better, too, subjectively.

A 6.5" rim is 165mm wide.  Typically you want the rim to be 75% to 90% of the width of the tread cross section.  In this case, that would be 185 to 215.  I think 195 or 205 is going to be your sweet spot.  

Grm did a test probably 4-5 years ago now on an rx8. They found the skinnier tire faster (225 vs 245 I think).

Weight will be a bigger issue on smaller and less hp cars. Weight of the wheel tire package also affects suspension setup.

Short answer is it is really hard to know, but there also probably won't be a clear winner. You probably will be within a few tenths even with the "slower" tire. Compound/temperature/alignment matters a lot.

I was afraid that would be the case.

Tire rack has a great deal on some Kumho V720's right now in a 205, but I think my wife may kill me if another set of tires show up at the house. I already picked up a set of lightly used 245 hoosiers from another forum member hoping to find a set of challenge friendly 8" wheels, but 6.5" are the best I've managed to find.

Anybody have a set of 15x8 they want to trade for some silver 17x7 Kosie K1 TS?

In reply to bigben :

Is the bolt pattern the same as your hypothetical Miata?

I know that a 225 wide tire on a 15x7" wheel is hilariously squeezed, looks like a sportbike rear tire. I'd run the 195, and I wouldn't be surprised if an even narrower tire could be faster.

Running that size wheel this year on my "hypothetical" NB Miata. 15X6.5. I'm running 205/50 RE-71R. Fit with no fender rolling. That's why I chose this tire size. Was told I may have to roll the fenders with 225's. Previous years I used 14X6 with 205/55 mounted. NT01's. So far, the 2 times I've autocrossed this year it seems to have more grip but can't determine if it's the difference in tires or actual tread contact. The RE-71R's ride rougher but I'm thinking that is combination of 1" shorter tread height and possibly stiffer sidewalls. But this is all from seat of the pants dyno. 

In reply to Stampie :

Yes. They are 4x100 and 4x114.3 with a +35 offset

(And my hypothetical Miata is actually an Opel GT, but the data set for Opels is pretty small so I went with Miata for discussion purposes.)

A few years ago when you could run Rcomps in "stock" class there were a lot of people jamming 275 Hoosiers on 6" wheels.  Pinched as hell and looked silly but it was the fast way around an autocross course. 

More rubber is better.

Hell, I ran cantilevered Goodyear slicks on a Civic Sedan...more rubber is better.  

In reply to pinchvalve :

Wow! What size was that wheel / tire combo?

I’m not sure cramming a 225 on a 6.5 is worth it. I’ve beat a ton of folks running the 225s with my ES spyder in 205s.

Although I’d still like to try them at some point just to see if it feels any different. 

I always find this funny.  Back in the 80's 225/50 15's were "crammed" onto 6.5" wide rims from the factory on several "sportscars"

Spoolpigeon said:

I’m not sure cramming a 225 on a 6.5 is worth it. I’ve beat a ton of folks running the 225s with my ES spyder in 205s.

Although I’d still like to try them at some point just to see if it feels any different. 

You don't count.  This is a discussion for regular driver, not aliens.

Does it have to be a DOT-legal tire?

In reply to bigben :

Are you wanting 4x100 or 4x114?  I have some 300Z 4x114 that might even look period on an Opel GT. I'm low on 4x100 right now. 

NM these aren't wide enough. 

It all depends on whether or not you have a 3/4 race cam in it. 

In reply to LanEvo :

In this case, no they do not need to be DOT legal. 

In reply to Stampie:

The Opel is 4x100.

In reply to Spitfirebill:

It originally only had one Cam and now it has two, so it's got to be at least a 6/4 race cam.​​​​

It's probably going to depend on you, the car, and how stiff the tire is. :D

I wouldn't be shocked if a 225 was faster than a 205 on a 6.5 inch wheel in autocross, where over the years I have seem people do well with big tires jammed onto skinny wheels (although those were mostly R comps, not sure about RE71Rs). 

I wouldn't be shocked if the 205 was faster either, though. 

I think 245 is a bit much, and I WOULD be a bit shocked if they were fastest. 

Not sure if that was helpful. But hey, it's an opinion!

bigben said:

In reply to Stampie :

Yes. They are 4x100 and 4x114.3 with a +35 offset

 

(And my hypothetical Miata is actually an Opel GT, but the data set for Opels is pretty small so I went with Miata for discussion purposes.)

What a coincidence. I have an NB Miata that I autocross and an Opel GT that I do cruise-ins and shows with. My Opel is mostly stock restoration. Do have some drivability updates like Weber 32/36, pertronix ignition, European master cylinder/booster and more aggressive brake pads (EBC Green Stuff). Also put on a set of BBS Basketweaves off on old E30. They aren't 15" though, 14X6.5 with 195/50-14 BFG's on them. Not sure of the offset, had to run a spacer on the front to keep the wheel from hitting control arm. Don't autocross it, has no anti-sway bars and really isn't very fast and the engine is stock. Original seats belts probably won't pass tech either. Plenty of room to go a bit wider but not sure about wider and taller. Might fit with a slight fender roll. How stock is yours? Stock rear axle moves side to side a bit on bumps. Check some Opel forums, some are running 17" on theirs. 

In reply to wlkelley3 :

Stock? What's stock? I still have some stock parts, kind of like Darth Vader still has some Skywalker in him.

I'm currently running 17's but I think it will do better on a shorter tire. It's got Honda rotors with Volvo calipers up front and a Volvo 240 rear axle. I have a set of deep five slot mags that look awesome on the car, but at 13" they will not clear the brakes.

The engine, tranny, and driveshaft are all Nissan. The suspension is still Opel, but I've adjusted the rear geometry and added an adjustable fiberglass spring up front.

bigben said:

I was afraid that would be the case.

Tire rack has a great deal on some Kumho V720's right now in a 205, but I think my wife may kill me if another set of tires show up at the house. I already picked up a set of lightly used 245 hoosiers from another forum member hoping to find a set of challenge friendly 8" wheels, but 6.5" are the best I've managed to find.

Anybody have a set of 15x8 they want to trade for some silver 17x7 Kosie K1 TS?

be sure to check the tech specs of the 195's vs the 205's on tire rack.  On my Alfa I found a few 195 options with a larger contact patch than some of the 205's