Auto, Gear, Tire, and RPM Help

I currently have a 2015 GT Automatic with 3.55 gears. The car makes 790rwhp with shift points @ 7500-7600rpm. I have a set of 18x10 rims that I am going to use for a set of MT Drag Radials and there are tire diameter options from 26" all the way to 29" tall in the ET Street R and S/S line.

I've taken the car to the track with my 305/35/19 Nitto 555 G2s, which are 27.44" tall, to try and get a gauge of what RPM I'm crossing the finish line to help dial in the best tire size for the combo. Unfortunately, excessive tire spin on launch caused a short shift on the 1-2 and forced me to have to go into 5th before the finish line. So I have no data on what I'm crossing the stripe at.

I would like to go with the MT ET Street R, which only comes in 29x12.50 (P305/45R18) for the 18s. There is a new size coming out but its even wider and I don't think it will fit. The new size is 27x13.50 (P325/35R18). However, I think a 29" tall tire is way too much for what I want to accomplish. I like the 29 due to is large side wall on a 18" rim but I think it will effectively kill my overall gear ratio to a point that it slows the car down.

Using calculators out on the net it looks like a 26" tire is what I need. Assuming I will get to 135mph and cross at the optimum 7400rpm I would need a 22" tire. I do not want to swap my rear gears if possible. So that is why I am asking for assistance. These tires are not cheap and don't want to have to buy them a second time.

My other two real option for tires are the Mickey Thompson ET S/S in 26x12.50R18 (P305/35R18) and the 28x12.50R18 (P305/40R18). There is also a 27x11.50 but don't really want to give up width for diameter if don't have too.

Any feedback to guys that have gone 135+ would be extremely helpful. Thanks in advance and reading this long post.

-Cmy

My last car was an 800whp GT500 that trapped 140 in the 1/4 mile. I ran a 28 X 10 tire and it was fine.... Granted, the gearing, car weight, and several other variables are different between our two cars, but my point is don't be concerned with giving up width when selecting a tire. A 10" wide tire ( I ran 275/60/15 and 295/45/15 radials) and they hooked fine. The height and sidewall is far more important.

I agree, I think the 29" is too much tire. For reference, with a 28" tire I would cross in 4th (1:1 ratio) at about 6400 rpms running high 130mph trap speeds. The 4th gear in the 6r80 is 1.14:1, so you will make up some of the rpm difference with that, but I don't think it would be enough to get you close to redline in 4th on a 29" tire. I would stick to a 28" tire and I think it will be fine. Plus the auto is a lot more forgiving on launching, so the different compound in the S/S tire will probably do ok for traction. If your car was a manual trans, probably not.

Hope some of that is useful.

GT500Steve said:

My last car was an 800whp GT500 that trapped 140 in the 1/4 mile. I ran a 28 X 10 tire and it was fine.... Granted, the gearing, car weight, and several other variables are different between our two cars, but my point is don't be concerned with giving up width when selecting a tire. A 10" wide tire ( I ran 275/60/15 and 295/45/15 radials) and they hooked fine. The height and sidewall is far more important.

I agree, I think the 29" is too much tire. For reference, with a 28" tire I would cross in 4th (1:1 ratio) at about 6400 rpms running high 130mph trap speeds. The 4th gear in the 6r80 is 1.14:1, so you will make up some of the rpm difference with that, but I don't think it would be enough to get you close to redline in 4th on a 29" tire. I would stick to a 28" tire and I think it will be fine. Plus the auto is a lot more forgiving on launching, so the different compound in the S/S tire will probably do ok for traction. If your car was a manual trans, probably not.

Hope some of that is useful.

Click to expand...

Thanks for the feedback...and I forgot about he 1.14:1 on the autos. If this was an all out track car 4.30s and 29" tall tire would be what I would run on the car. However, this is my daily driver in the summer and I want to have a nice balance between street and strip and the 28x12.5 S/S are the ones I've been leaning towards. One, because they will handle some water on the road if I get caught in the rain. Two, for the extra side wall, over the 26", for traction at the track.

-CMY

28" tall is the ticket on my car i run 305/45/17 et street r and she dead hooks 1.3's all day long with 1.33 being the best....i have a stock base auto with the 315's and im keeping them...you must be crossing the line mid way through 4th gear....mine is just shifting into 4th about 100-150ft before the line....if i could id go to a 3.08....your 355 will be an in between gear with almost any tire size....373 gear with 28" will probably trap top of 4th...thats what i had in my 2013 auto 373 275/650/15 combo and circle d 2C she went 9.16 at 152.....i hoping to beat that in my s550 this season.

I think MT S/S 305/40/18 would be your best choice