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Old 12-17-2008, 06:56 PM
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tire size vs top speed?

What would give you a higher top speed, a tire with a larger diameter or one with a smaller diameter. For expample would you get a higher top speed from a 35" diameter tire or a 25" diameter tire. I am guessing that the larger in diameter the tire the higher top speed you will achieve but slower the acceleration would be. This makes sense to me because a larger diameter tire would have a larger circumference so if every turn the ring gear makes the tire makes, the larger tire would travel a further distance?? Please correct my if I am wrong.

Thanks,
Keith

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Old 12-17-2008, 08:19 PM
How fast is fast enough?
 
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well if you were limited by your rev limiter it would be the larger tire for a higher top end speed. But most cars aren't limited by their rev limiter but by their power- or lack there of. fitting a taller tire means you're putting less power to the ground for a given distance and it usually means your car's height increases which increases drag.

Welcome to the world of automobiles- where there is rarely a black and white answer.
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Old 12-17-2008, 08:21 PM
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The bigger tire should give you a better top. The smaller tire would accelerate. Its messing with the gear ratios though.. I have heard the off road guys with tall tires really suffer with MPG. The tall tires give the motor a hard time getting the truck moving.. People I know with short tires have a bad time on interstate.

Seems that you should stay close to your factory wheels and tires for the best performance.
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Old 12-17-2008, 08:26 PM
How fast is fast enough?
 
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Quote:
Originally Posted by weirdbeard
The bigger tire should give you a better top. The smaller tire would accelerate. Its messing with the gear ratios though.. I have heard the off road guys with tall tires really suffer with MPG. The tall tires give the motor a hard time getting the truck moving.. People I know with short tires have a bad time on interstate.

Seems that you should stay close to your factory wheels and tires for the best performance.

common misconception i once held aswell. shorter tires will actually give you more top speed if you gear apropriately. Get that sucker 1/2" off the salt (use whatever tire will get you there) and gear to make up the rest.

When you switch over to top speed records 2.73 gears all of a sudden look steep.
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Old 12-17-2008, 08:28 PM
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Taller tire = higher gear ratio= higher MPH at the same RPM
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Old 12-17-2008, 08:38 PM
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Quote:
Originally Posted by weirdbeard
The bigger tire should give you a better top. The smaller tire would accelerate. Its messing with the gear ratios though.. I have heard the off road guys with tall tires really suffer with MPG. The tall tires give the motor a hard time getting the truck moving.. People I know with short tires have a bad time on interstate.

Seems that you should stay close to your factory wheels and tires for the best performance.
When you add the BIG tires it adds alot of weight and most of the BIG tires are pretty aggressive tread pattern that adds alot of drag,both factors in the bad MPG, most of the trucks I do with the BIG tires like 40's or 42's I have to gear them to 513 or 538 ratio to get some performance.
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Old 12-17-2008, 09:15 PM
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Yeah, too many factors to say definitively.

Your top speed will happen at the HP peak. It should be geared so that wind resistance equals the HP.

For instance, if your car makes 300 peak HP at 5000 rpm and you're hitting your top speed at 5000 RPM, you know you're geared right. If you decrease tire size from that point, the RPMs will increase slightly. Now at the same speed you are making the same wind resistance, but making less power since you are past the peak. The net result will be less top speed. The same is true for increasing tire size.

Think of it this way. Let's say you're travelling at max speed and you're at 5000 rpms. Now downshift (hypothetically). Now you're spinning to 6000 rpms which makes less power. The result is the car will slow down. Reducing tire size in that case would have the same effect.

So for us to say definitively which way yours is would be impossible. Adding tire diameter might get you closer to the idea, or it could be taking you further away from ideal.

Most stock production vehicles have a pretty tall gearing with OD, so most of the time you will reach top speed (limited by wind resistance) long before you reach the HP peak. In that case, increasing tire size will almost always reduce top speed AND reduce acceleration. If your vehicle is limited by an electronic speed limiter, increasing tire size without telling the computer will fool the car into going a couple mph faster. The speedo will still indicate the same max speed, but you'll actually be travelling a few mph faster.
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Old 12-17-2008, 09:48 PM
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i know i have a buddy i helped put a slug 305 in an s-10 and we had put two of the spare donut tires on the back and drove around doing burnouts all day. when we actually had to take the truck serious we put the 14x7 or whatever it was on and it wouldnt do burnouts so good anymore...the point is a smaller tire spins way faster around than a larger tire therefore a larger tire, once it gets rolling will cover a greater distance faster
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Old 12-18-2008, 06:09 AM
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Quote:
Originally Posted by ap72
common misconception i once held aswell. shorter tires will actually give you more top speed if you gear apropriately. Get that sucker 1/2" off the salt (use whatever tire will get you there) and gear to make up the rest.

When you switch over to top speed records 2.73 gears all of a sudden look steep.
I am talking just tires and stock gears.
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Old 12-18-2008, 07:10 AM
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Quote:
Originally Posted by weirdbeard
I am talking just tires and stock gears.

Well if you're talking stock gear and tires then get the smallest stock tire and the shallowest stock gear- like a 2.73 and a 26" tire. It doesn't matter if the gear is from the factory or not it all works the same. You need to get the viehicle as low and as slick as you can get it, use whatever tires can get you close to the groudn and use "factory gears" to compensate. BTW factory gears range from about 2.41-4.56 on most common cars/trucks. Keep in mind your tire should be rated for the speed you're hoping to achieve too, or more if you want to be a little safe.

And you need to minimize drag (ie get it low and slick). As your speed doubles your hp loss to drag goes up by 4. custom air dams, spoilers, no mirrors, custom wheels, all sorts of good tricks... and some people use duct tape. Seriously. Look it up.
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Old 12-18-2008, 08:24 AM
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If the question assumes EVERYTHING ELSE on the vehicle (gear ratio, drag, etc) remains the same, then a larger diameter tire effectively raises the final drive ratio (equivalent to a lower numerical), so assuming the vehicle has enough HP to overcome the drag, then the taller tire provides a higher top speed. As everyone else has noted, there are really too many variables. Power curve, actual gearing, drag, etc all play in this. Some cars are so underpowered that they actually go faster in fourth than fifth.
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Old 12-18-2008, 01:55 PM
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Quote:
Originally Posted by sexypizzaman
What would give you a higher top speed, a tire with a larger diameter or one with a smaller diameter. For expample would you get a higher top speed from a 35" diameter tire or a 25" diameter tire. I am guessing that the larger in diameter the tire the higher top speed you will achieve but slower the acceleration would be. This makes sense to me because a larger diameter tire would have a larger circumference so if every turn the ring gear makes the tire makes, the larger tire would travel a further distance?? Please correct my if I am wrong.

Thanks,
Keith
Were it this simple. But first, pi (3.1416... on forever) times the diameter of the tire gives its circumference which is the distance the vehicle attached to that tire will travel for each revolution of that tire, assuming it's not slipping.

The rub for top speed gets into how fast the engine can turn against the load imposed on it. Sometimes, a stiffer gearing such as seen with the smaller dia. tire actually allows enough leverage against the load to allow the engine to spin up enough more that the top speed is increased compared to a higher gear ratio which is represented by a larger diameter tire. So while higher gearing or larger diameter tire theoretically would net a higher vehicle speed, the engine looses leverage against the loading as the gear ratio becomes less (higher ratio in the jargon of axles). This is where Car and Driver aways gets into trouble, they like to calculate a vehicle's top speed based on the engine's red line. Well Bunkie, there's damn few cars out there whose engines can achieve redline RPMs with the transmission in overdrive. There aren't many that can even do that with a 1:1 final drive in the tranny.

So the answer to your question is a definite "maybe". The answer lies somewhere in the engine's power development curve versus the chassis' power consumption curve.

Bogie
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