What is best size tire to use on a 93 Z28 with the OE Brand C6 18" x 9.5" rim. Looking at P275/35/18, Close to stock dia. Or the 275/40/18 tire. App 1" larger dia? Some help needed. Thanks.

 

Which tires were on the car when it left the factory? That diameter is what the speedometer/odometer is calibrated for.

Stock tire was either a (base V8 models) 235/55-16 = 26.2"

or

(with RPO "QLC" Z-rated tires) 245/50-16 = 25.6"

Check tire size info on door jamb, or Service Parts label in glove box for RPO code.

 

Thanks Injuneer for a quick response. Sorry, I did not fully detail the Z. It came with the P245/50/16 Tires from factory. I was running the 275/40/17 tires on the SLP factory ZL-1 GM 17" 5 spoke wheels. My car now has a Stano lowering spring kit and Koni shocks. Car is app. 1.25" lower with his kit. Also have a Strange 60 rear with 3.73 Dana Spicer gear set. I have a 4L-60 built with the Speedo gear set for a 3.73 and 275/40/17 tire. My 93 is a old and rare set up by today's standard.. Has a TV cable and drive and driven speedo gears in trans rear. I have the OEM computer with removeable chip. PCM of North Carolina has done the custom chip for me. I know the 275/40/18 tires is app 1" taller , but wanted to see if anyone liked one size over the other. Other being a 275/35/18 tire. This 35 series tire should work better for my speedo gear I would guess. Thanks for any help.

 

My concern was speedometer calibration. PCMofNC indicates correction requires an interface box, and sells the Dakota Digital SGI-5. But I've always understood that the gears were the way to go. The larger diameter tires will throw the speedo/odo off by 4%, without a new gear set.

As far as larger vs. smaller on handling, I can’t help. I've always matched the original 25.6” diameter for street wheels/tires, and the car is set up for the 1/4-mile. I don’t know what effect raising the center of gravity 1/2” will have on handling. Whether the 1” added diameter causes an interference with the body will depend on the offset of the wheel. If appearance is a consideration, that’s subjective.

 

Thanks again Injunner. My Z is a pure street car. No track car. I will probably go with the 275/35/18 tires to stay close to OEM diameter for present speedo setup. Not too worried about center of gravity. I would guess that a 35 series tire will ride some what harder than a 40 series tire on the street. Just wanted some feed back on this . Also, should be safer on body clearances with lowered car. Thanks again.

 

I'm lowered with a Strano Springs and Koni 4/3 shock kit. I've been on the original 245/50/16's, as well as several different sets of 275/40/17, 275/35/18, and 275/30/19 tires over the years since I bought my Z new. My concern with the 275/40/18 tires has to do with the 1" of extra tire height with the lowering springs...you MIGHT make contact in the fenderwell if you hit a nice sized bump since you're lowered and went with an extra inch tall tire.

Just an FYI, here's my experience with the different sized tires I've run over the year. The 19's were by far the worst set I've ever run! The 30 series sidewall meant no traction whatsoever and the ride was horrendous. The 18's did improve over the 19's, but the ride was still rough and the 35 series sidewall still caused some issues with traction. However, the 18's looked the best. The 17's provided the best traction off the line (especially compared over the 245's of the stock 16's) and the ride difference between the 16's and 17's was hardly noticeable. My rim/tire selection over the years have varied! Original 16's, 3 different sets of 17" rims, 2 different sets of 18" rims, and one set of 19" rims. I'm currently back down to 17's for the best combination of ride and handling.

 

Thank you so much for your information. This is exactly what I hoped to hear about. Actual experience with some of your combo's on rim and tire sizes on a lowered Z. Thanks.

 

Thank you so much for your information. This is exactly what I hoped to hear about. Actual experience with some of your combo's on rim and tire sizes on a lowered Z. Thanks.