245/45/17 vs 255/40/17

[Toni]

I am currently running 245/45/17s. My best with the 15gs was an 11.67 @ 121.56. I have gotten 122s. I basically max out third gear at the line to 7300. Now I am rolling with dr650s and 40 more horse.

That being said, I need new tires and am looking at the two sizes mentioned. The 255/40s will give me a smaller diameter of .65". This means I hit a higher rpm in third to net the same MPH. I imagine I don't want to shift into 4th unless I really need to.

Is this really a big deal off paper? What will happen to 1st and 2nd and my times/shifting in general?

Thanks

[j2k4]

Stock gearing and tire size leaves you basically at redline/top of third in the lights if you were trapping 121-122, Toni.

With the DR650s your traps are gonna be a bit higher, so I sure wouldn't go to to a shorter tire if you're trying to avoid shifting to 4th.

[HellBringer]

I went 124.7MPH on 265/35/17's back in the day tapping the rev-limiter right at the traps. They're slightly smaller than stock diameter. Are you logging your RPM or just watching the tach?

Shifting to 4th will definitely hurt unless you're trapping closer to 130. You're now at that MPH range where the 5sp is a hindrance due to where 3rd gear ends. Either raise the rev-limiter or run a taller tire. :sigh:

[stealthee]

If you went with 255/45 they would be .35" taller. Not a huge difference, but at least not shorter.

[R/T93]

26" slick time?

From the phone.

[B-Man]

If you went with 255/45 they would be .35" taller. Not a huge difference, but at least not shorter.

Strut clearance becomes a concern with factory struts. I have no idea what Toni is running.

[stealthee]

That's definitely true.

[JasonY]

I'd find a 26" tall tire and run that, street tires for the street and track tires for the track.

You could get some AMR wheels for cheap and run 245/40 MT ET street 2's and still retain the ability to drive to/from the track on them. Wont hook nice like a 16" wheel with a slick on them, but better than street tires or a hard side-walled high performance but sticky tire.

Jason

[Toni]

Turns out they don't make many variations of the 17" tire so I was limited to what I could really get in the tires I wanted. I ended up going with the 245/45/17. Stock struts so I don't want to get stuck with taller tires that don't fit.


I went 124.7MPH on 265/35/17's back in the day tapping the rev-limiter right at the traps. They're slightly smaller than stock diameter. Are you logging your RPM or just watching the tach?

Shifting to 4th will definitely hurt unless you're trapping closer to 130. You're now at that MPH range where the 5sp is a hindrance due to where 3rd gear ends. Either raise the rev-limiter or run a taller tire. :sigh:

I was already hitting the stock limiter and I already rose it up it up a bit. Not sure how high I can safely raise the stock pump/valve train. I figured 4th will hurt me here but nice to get an idea of what mph I'd have to hit to be worth it.

Thanks

[JasonY]

Stock valve train if new is good to 8k or so. I ran the old school gforce 8300 rpm ECU in my old 93 for years just to stretch 1st gear out and not hit the rev limiter. Normally shifted around 7800-8k in 1st and rode 3rd out as far as needed.

Traps were 122-125mph. But had plenty of RPM to go before 8k. Power was fading fast but was better than short shifting into 4th. I dont think i ever broke 120 using 4th gear.

Jason

[RealMcCoy]

Strut clearance becomes a concern with factory struts. I have no idea what Toni is running.

Many people run 255/45's on stock suspension and wheels. My sig pic is one example.

[Toni]

That tire size seems to be hard to find in a max performance tire. A few regular street tires and all seasons but the pickings are slim.

[drew0u812]

This tire has a relative soft sidewall and decent traction.
http://www.tirerack.com/tires/tires.jsp?tireMake=Continental&tireModel=ExtremeContact+DW&partnum=545WR7ECDW&vehicleSearch=false&fromCompare1=yes