rnalecz

I was running Kumho Espt on the rear of my Nismo with the size of 275/30R19 but they have become almost bold. So today i replaced them with used 285/35R19 Goodyear F1s. When i placed the tires next to each other the 285's where a lot taller then the 275. My question is would this make the car slower? is it better to have bigger but wider? Matter of fact, I always had this little road section i would run from marker to marker starting at 40 and with the 275's i always hit 109-110, but i just ran the 285's and i got 103-104 Is this really slower due to the engine having to push a bigger and heavier tire? Or is my speedo messed up from the bigger size, even though i was told the speedo is only on the front tires? Fronts are still the same.

DavesZ#3

iTrader: (26)

The more rubber you put on the road, the more rolling resistance you'll experience. from 275 to 285 the difference is small, but should be measurable.

Your new tires are taller than the old ones, so that means for each revolution of the driveshaft, the car will actually cover more ground than before. Even though your speedometer was reading 100 mph, your actual speed will be greater. As a result of that greater speed, the wind resistance against the car will be greater, requiring more throttle to get to that speed.

So basically, your speedometer reads 100mph, you have more rolling resistance to overcome so it takes a little more throttle to get there and your tires are bigger diameter so the speed of the car is actually greater than 100 mph so the drag is greater, again requiring more throttle. So it's possible that you think you're giving it more gas to go the same apparent speed (based on the speedometer), when in reality, you are probably going faster.

BTW - in addition to all that, inertia from the larger tires means that they take longer to "spin up". That's why guy who drag race usually try to use 17" wheels and low profile, wide tires.

SniperHunter

iTrader: (9)

Well, assuming you had 18s to start with, the OEM tire size is 245/45/18. Using 275/30/19 was the wrong size/too small of a size and probably made your car a bit quicker, at the expense of some serious speedo error (4.5%). Where as the 285s you're running now are precisely the size you should've had in the first place, and are a recommended +1 tire upgrade for larger wheels, with little-to-no error.

So yes, the tires made you slower. But at the same time, they are the correct tires. The other ones were too small and incorrect.

hartsickdiscipl

Taller tire= fewer revolutions per mile= slower acceleration when traction is the same.

Even a 285/30/19 is slightly taller than a 275/30/19... And a 285/35/19 is even taller. Yes, you have slightly decreased your car's acceleration, but I suspect you've improved the traction off the line.

My car has a stock rear tire size of 245/45/18, and I moved to a 255/45/18. My new tires are a hair taller, so theoretically, I hurt my acceleration a bit. However, since I do alot of bracket racing (drag), the benefits outweighed the tiny loss of acceleration for me. By going with the wider and slightly taller tire, I can now be SURE that I don't have to shift out of 3rd gear before the end of the 1/4 mile.

Check out the tire size calculator below:

http://www.miata.net/garage/tirecalc.html

hartsickdiscipl

For drag racing the best setup is really a 15-inch wheel/drag radial or slick combo that gives you more sidewall (flex is good for traction) but a smaller overall diameter than stock. That is, instead you want to make sure that you don't need to shift out of 3rd gear in an HR using the stock rims.. in which case I found that going up a size or to to 255 or 265/45 is the smart way. Taller tire gives you a taller effective gear.

rnalecz

Ok so your saying now on the speedo the 103 is the same thing as the old 109? So what tires is the speedo located at? I was told its only on the front and therefore to that it should have been the same speed. So having bigger tires there will be no lose to same preformance correct? Just the speedo gets messed up? And arnold the Nismo comes 265/35R19 stock

Ok so hartsick your saying that it will be slower? I mean fewer revolutions per mile would make the speedo slower but would that make the acceleration actually slower? I don't understand why would bigger slower acceleration? I do understand more traction tho

I mean its so confusing to me that now the tire does not have to spin as much to go as fast as the smaller one, but wouldn't the engine be able to spin both the same speed therefore making the taller one even faster?

FATPUBUS

iTrader: (30)

The first answer was spot on. I drive a semi and tire sizes, compounds, and tread design all affect fuel mileage and speed. Just new tires alone are taller which means more rolling resistance and less fuel mileage...as was already stated. Is it truely measurable what the losses will be in a car, idunno. But in a big truck it is literally thousands of dollars a year you could lose in fuel mileage from the worst tire to the best. Michelin has a rolling resistance calculator for the truck tires...now i wonder if they have one for passenger car tires. Doubt it, but it would be cool. Guess i kinda got off topic there lol.

terrasmak

iTrader: (8)

Well you have a Nismo, your stock rears are 265/35-19. The 275/30 is actually shorter than stock and will read a higher MPH than what you are actually going. 109 indicated MPH was probably closer to 105 or 106. Now you running tires that are taller than stock and indicating 103 , while in reality you actually going 105 to 105. Its actually really close, you would have to do actual time passes on the drag strip to figure out what tire would actually be better to make it faster.

davidv

iTrader: (2)

Rear axle RPM is the same.
Rear tire RPM is the same.
Rear tire revolutions/mile varies with tire circumference.

See: http://www.miata.net/garage/tirecalc.html

hartsickdiscipl

Yes, when you have the same engine and the same gears, going with a wheel/tire combo that has a larger overall diameter WILL result in slower acceleration. I can't say how big of a real-world effective difference you will see in your case.

That's why drag racers say that they are "gearing up" when they put a taller slick on their car, or "gearing down" when they put a shorter wheel/tire combo on the car. Changing the wheel/tire combo in order to change to overall diameter is a common practice for drag racing, because it's essentially a cheaper way of changing your car's effective gearing.. like changing your rear end from a 3.54 to a 3.90 gear, or vice-versa. Having a shorter wheel/tire combo allows your engine to push through each gear faster, because the wheel is achieving more revolutions per mile. This also has the effect of keeping your RPMs higher with each shift, keeping you in the powerband more consistently.

As a rule- Taller wheel/tire combo= higher top speed
Shorter wheel/tire combo= quicker acceleration (it's like putting a shorter gear in your car)

davidv

iTrader: (2)

Correct. The quarter mile drag racer is set up to take advantage of horsepower and torque for 1/4 mile ET. That's it. Faster and slower is relative. Only thing that matters is 1,320 feet.

hartsickdiscipl

Although it's true that most dedicated drag-racing cars (pro and super pro, and some really serious street cars) are set up purely for max acceleration over 1/4 mile (to the extent that they literally might hit 120mph through the traps and that would also be the car's top speed due to having short gearing), the same rule applies before and after 1,320 feet with street cars.

If you have 2 350z's both running 3.54 gears, but one has a 26-inch rear wheel/tire diameter, and the other has as 28-inch combo, the one with the 26-inch combo is going to accelerate faster all the way to top speed. The car with the larger wheel/tire combo will have a higher top speed, but it will be behind the car with the shorter effective gearing all the way until that one tops out. One car might top out at 150mph, while the other would make it all the way to 165 (assuming the limited was removed). But it wouldn't get to 150 as fast. Changing the wheel/tire diameter has the exact same effect as changing gears.

terrasmak

iTrader: (8)

you also have to take shifting into consideration, if the 26 inch tire car has 5 shift and the 28 inch tire car has 4 shifts, the times will be closer do to the time it takes to shift.

rnalecz

Thanks guys so much for clearly explaining the differences, all my questions have been answered plus more besides the question where is the speedo located lol, davidv showed the wheel calculator but on that one it does not specify if its on the front or rear. Lets say i went bigger in rear but smaller in the front lol what would happen then, i would have no idea what speed i can travel for the cops to not bother lol

terrasmak

iTrader: (8)

It's off the rear tires.

hartsickdiscipl

If you're running all the way up to 140+, the extra shift won't make much of a difference. The car running 26's will be far enough ahead that it won't lose much ground with 1 more shift. 2 inches of tire/wheel is a BIG difference in effective gearing.

Over a 1/4 mile it's a different story, because you're only hitting 100-105ish mph (depending on DE or HR). That's why I actually went with an ever so slightly taller rear tire on my 07 for bracket racing. With the stock 245/45/18, I had to shift into 4th or at least let off the gas just before the stripe.. But with 255/45/18, the tire not only gained width, but also just a hair of height. I don't have to let off or shift out of 3rd gear now. This really helps my consistency because in bracket racing, you really don't want to have to shift up right before the 1/4 mile.

davidv

iTrader: (2)

Cool. Show us how that works for you here: https://my350z.com/forum/drag/233840...ons-stock.html

J. Dub

iTrader: (18)

Wait.... What?

I thought our speedo automatically calculates changes in tire size to show the correct speed we're going basically no matter what size tire we have.

Correct me if I'm wrong please.

Cause if I run a 295/35/19 rear tire and I am going the same speed as John Smith on the highway whom has stock tire sizes we will both show the same speed on our speedo... Correct?

TheFarmer

iTrader: (19)

No it does not.

youngh

iTrader: (4)

lol I knew I kind of felt weird when I went from 245/45/18s to 245/40/18s.. same for fronts i also went 5mm smaller height and felt like I accelerated faster..