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Discussion Starter #1
I currently have 225/70/R15 and I am getting a set of 225/50/R16 for the Bird. Should I worry about the speedo being wrong after the change.

ps. I will eventually go with 235 (at least) for the car but the wheels are off a 93 and the tires a pretty much new so I will have to wait for the new tires.
 

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try this:

tire size calculator

looks like your speedometer will read 9.3% too fast, or when your speedometer says 60 mph, your actual speed will be 54.4 mph.

At least you will be going slower than it says, not the other way around and risking a ticket.

One other thing to consider, if you still have a speed limiter (stock I think 106 mph), your new top speed will be only 96 mph, although your speedometer will still read 106 mph.

edit: isn't the stock size of tire either 205 or 215/70R15? the above numbers were assuming you speedo was right with the existing tires, compared to stock tires, the speedo would be a little more accurate.
 

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WAY too short. Your going to have huge fenderwell gap, like 4". The tires will wear out fast due to being overloaded, and won't handle as well. IMHO, pull off the tires, sell them to an import buddy, and get some tires big enough for your car.
 

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I'm running 225/60/16's, and they are pretty close, about 3mph too fast on the speedo, but Ford's speedos aren't exactly known to be right on the money to begin with. Another choice, if you want something wider would be a 255/50/16.
 

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GreenBird said:
WAY too short. Your going to have huge fenderwell gap, like 4". The tires will wear out fast due to being overloaded, and won't handle as well. IMHO, pull off the tires, sell them to an import buddy, and get some tires big enough for your car.
I don't know if you saw my other request, but you didn't answer it. So I'll ask the question again;

How will running a SHORTER tire cause the tire to be over loaded and wear out quicker?
 

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Eric Z. said:
How will running a SHORTER tire cause the tire to be over loaded and wear out quicker?
The load rating of the tire is lower (generally), and while it's probably not unsafely overloaded, it is beyond optimum. Greater load on the tire means it has less abilty to corner. Think of it this way....could you run through an obstacle course faster with or without a 25lb weight on your back? Also, since the tire is shorter, it will rotate faster for the same vehicle speed. The faster the tire spins, the less grip it will have.

Not saying there will be a huge difference, but it would handle better with a properly sized tire of the same characteristics.
 

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If your set on the tires, you should plan on droping it a little. I tried them on before the drop and it looked like sh!t. That is the exact size I have on my SC's with Sprint Tech springs. I wish they were just a little bit taller, 60's would fill the wheel well perfict. You can see them click here for my cardomain
 

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GreenBird said:


The load rating of the tire is lower (generally), and while it's probably not unsafely overloaded, it is beyond optimum. Greater load on the tire means it has less abilty to corner. Think of it this way....could you run through an obstacle course faster with or without a 25lb weight on your back? Also, since the tire is shorter, it will rotate faster for the same vehicle speed. The faster the tire spins, the less grip it will have.

Not saying there will be a huge difference, but it would handle better with a properly sized tire of the same characteristics.
200 lbs difference in tire load rating is not going to effect tire wear when the lesser rated; smaller tire's load limits far surpass the requirements of the car.

Example:

Dunlop Qualifier GT's:
225/50TR15 1301lbs. x4 = a max vehicle weight of 5204lbs
225/70TR15 1753lbs. x4 = a max vehicle weight of 7012lbs

My bird with me in it, a half tank of gas and in full street trim only weighs 3960 lbs. No where near the weight limit of 225/50's. It's not going to be a factor.

I think you a couple things backwards. You have tried to draw a comparison of having a smaller tire to with running with more weight on your back. :2huh: First off, a smaller tire is typically going to weigh less and we all know that less rotating weight = faster times. More rotating weight is going to decrease acceleration, braking and slalom times. Taller tires are also going give your car a higher gear ratio. Take my car for example, it has 3.73's. When I switch from the 27" street tires to 26" drag tires it makes my final drive equivalent to switching a 3.89 gear. The same theory should also apply to breaking.

In the turns the shorter tire will also perform better than and last longer because of the reduced amount of sidewall flex. While a shorter tire will rotate more times in a measured mile, I don't thing it will be so drastic that it should be any thing to prevent someone from buying the shorter version. The 2 tires I show above both have 440 tread wear ratings and 10/32's of tread so apparently the manufacturer doesn’t thing it will make much difference either.

As for this part of your post:
The faster the tire spins, the less grip it will have.


I'm not sure what line of logic you’re using here so I will let you explain that to me a little more if you like. It just doesn’t make any sense at all.

I personally like to have my fender wells filled out. IMHO a 225/50/15 would look extra tiny and out of place on 'Bird that was not lowered 2+ inches. On a stock ride height 'Bird a 27" tire looks perfect. I'm running 235/60/16's right now myself. When/if the GP springs come in I'll switch to a 26" tire size. Probably a 255/45/17.

Eric
 

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Well it seems we aren't in disagreement, you just think the effects would be less than I do...

I was only saying that a tire that can handle a greater load would perform better because less of it's stress will be from load, so there could more stress due to cornering.

As for the tire speed and grip...

ever driven on glare ice? if you go REALLY slow you can get across without a problem. however, if you go even 10mph, you will lose control and go sliding.

I will admit I didn't think about the mechanical advantage of a lighter tire. However I wasn't counting on a shorter sidewall, as I said "with the same characteristics".

Thing to do would be to compare to tires of the same design, one a 235/60-15 and one a 235/60-16 (as an example).
 

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Discussion Starter #10
Oh, I guess I forgot to note that the wheels and tires are already together on a 93 Bird. I am getting the whole car for $200 and selling the 5.0 thats in it for $100 if not more. The tires are brand new, they were put on the car just a few weeks before it was destroyed in an accedent.

ps. I did check, all the wheels and tires are fine, the accident did nothing to them.
 

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GreenBird said:

As for the tire speed and grip...

ever driven on glare ice? if you go REALLY slow you can get across without a problem. however, if you go even 10mph, you will lose control and go sliding.

And WTF does that have to do with a shorter tire height causing you to lose traction on pavement? No matter the conditions, if you drive to fast for them, you’re going to lose control, regardless of the tire height. Sorry GB, that dog just isn’t going to hunt.
 
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