So I went and put a new set of tires on the GMG Daytona. I could not find anything in a 235/55-R18 that I liked. I purchased some 235/50-R18 instead.

Here is my question:
According to the tire calculator
size diameter revs per mile
Tire1 235/55R18 28.18 738.11
Tire2 235/50R18 27.25 763.30


My old tire, Tire 1 is .93" taller than my new tire, Tire 2. When I went to adjust the tire size with my trusty SuperChips tuner, it was already done! Superchips reported that my tire size is 27.25".

Can anyone else with a Superchips tell me what tire size it says you have? Doesthe carfigure it out on its own?

Is it possible that I had the wrong tire size before? It should have been 28.18".

 

The superchips doesn't report the tire size. It tells you what it's going to set it to... 27.25 must be it's default. You need to change that to match your tire size.

The Predator is no different. I have to change mine to 28.5 which is closest to the size of the 20" tires I have on the car.

 

After doing some testing, I noticed an improvement in 0 to 60 times.

Over the past year I have been hovering in the 5.7 sec 0to60 range pretty consistently. I made several passes today and now I am running 5.4 sec. The hundredths digit varies but I consistently ran 3/10 sec quicker. Now I can't wait to try the 1/4 mile, to see if there is any change there....

I think the smaller diameter tires may work well with the rest of my setup. Has anyone else noticed changes after changing tires/wheels?

 

I haven't gone smaller yet but your right a smaller diameter tire will improve the gear ratio and low end times.

 

how wide can we go on the stock rims? I'd like to be able to put more rubber on the ground to help with the future mods.

 

I believe you can go up to 9" wide on the stock 18" rims.

 

You can go wider than 9", I am at 9.6" currently and they arejust fineand the tire manufacturer has our stock rim size in the recommended range formy tire size. Best bet is to go with what the tire manufacturer recommends for a rim width range.

 

There you go... byebyeblue's recomendation "go with what the tire manufacturer recommends" is perfect.

 

The 27.25 is what DC determined to be the "rolling diameter" of the 235/50x18 tire at recommended pressure. The programmer/tuner reads this from the controller and allows you to modify it to match the tires and wheels on the car. The rolling diameter is always smaller than the tire diameter because the flex of the radial sidewalls reduces the distance between the axle and the ground. If you measure the distance between the center point of the axle and the ground (rolling radius) and double it (rolling diameter) you should be pretty close to 27.25 for the 235/50x18 stock tires that come on the car (assuming that the air pressure is 30 psi). If you set this to the diameter of the tire (28.18) your car will actually be going slower than indicated on your speedometer. This will also affect your odometer and mpg readings with the EVIC.

Make sure that your new tires are at proper pressure and measure from the center point of the wheel to the ground and double that measurement to get a good starting setting. (At the risk of stating the obvious, you have to have the car's weight on the tires for this to work). If you have access to a measured mile or 5 mile strip of road, check your ET through it at a steady 60 (cruise control helps here). If your setting is right, you should do a measured mile in exactly 60 seconds and the 5 mile in exactly 5 minutes. If you're slow or fast, tweak the tire size up or down a notch and recheck your speedo until it's as accurate as you want it to be. If you're slow, decrease the tire size, if you're fast, increase the tire size.

I have the stock Michelins set at 32 psi cold and with my tire size set to 27.5 and it hits the those measured miles right on. For grins, I also checked it against my wife's Garmin GPS that has a speedometer function and both speeds matched. I just measured my tire with a level and a tape measure and it came real close to 13.75 inches so this works.

Have fun and Happy Holidays!!

Redbeard

 

Midnight Blue, thanks for the info!

My question really is, the original tires were 55 series, and the new tires are 50 series. The original tires are .9" taller than the replacements. When I checked the computer, using a SC tuner, it reported the correct diameter of 27.25, however it should have reported 28.18, and then I would have changed it to 27.25.

How did it know? Can anyone with a SC Tuner report on the programmed tire size, in the stock tire size configuration?

Thanks!