how does wheel size affect rear gears?

EXTRPR50 · Jul 20, 2005

I have 4.10's in my 95 Gt. I also have an AODE. I currently have 16 inch stockers, but if I put on ford 17 inch Bullitts, doesnt that affect my 4.10's?

wyominghotwheel · Jul 20, 2005

just think of it like this, the larger the tire and rim, the longer it takes for the tire to rotate in a forward direction..........so it takes a 17" tire longer to go around in a complete circler verses a 14" tire and rim..........

You will probably be off a few miles an hour, but just learn what the difference is and keep your cruising speed there.......if you are going 50 and you know 47 mph is 50 in your car after the 17's, slow her down and go 47......

hotwheels of insanemustangs.com

18mustangs · Jul 20, 2005

wyominghotwheel said:
You will probably be off a few miles an hour, but just learn what the difference is and keep your cruising speed there.......if you are going 50 and you know 47 mph is 50 in your car after the 17's, slow her down and go 47......

I doubt he'll be off a few MPH by simply upgrading to 17" from 16" tires. GTs came with either 16s or 17s depending on the options one ordered and the dealer is not likely to modify computers or speedos for wheel size. He'd most likely notice a difference going from 16" to 18".

srothfuss · Jul 20, 2005

If he adjusts the size of the rubber correctly then there should not be an issue with the speedometer. Performance wise you will loose a small amout, but you won't notice it. If you switch, You want the same overall diameter of rim/tire.

Now, when you move up in tire size correctly, that means more metal and less rubber. Which translates into more weight.This requires more energy to rotate the tires and that is where you will lose some power. (probably not really noticable) As a side note... Bigger tires take longer to stop because of the increased inertia so make sure your brakes are up to the challange also.

If you search google for "tire size calculators" they can be a great help to make sure that you don't throw off your speedometer or effect your gearing.

For me going from a 225/60/R15 to a 225/55/R16 or 245/45/R17 would give the same overal diameter of wheel so my speedometer would read correctly. But then my car would ride stiffer and stop worse so I am not ready for that move yet

powertrax91 · Jul 20, 2005

srothfuss said:
If he adjusts the size of the rubber correctly then there should not be an issue with the speedometer. Performance wise you will loose a small amout, but you won't notice it. If you switch, You want the same overall diameter of rim/tire.

Now, when you move up in tire size correctly, that means more metal and less rubber. Which translates into more weight.This requires more energy to rotate the tires and that is where you will lose some power. (probably not really noticable) As a side note... Bigger tires take longer to stop because of the increased inertia so make sure your brakes are up to the challange also.

If you search google for "tire size calculators" they can be a great help to make sure that you don't throw off your speedometer or effect your gearing.

For me going from a 225/60/R15 to a 225/55/R16 or 245/45/R17 would give the same overal diameter of wheel so my speedometer would read correctly. But then my car would ride stiffer and stop worse so I am not ready for that move yet

the loss in acceleration is quite notiable... I lost .5 sec in ET when i went from 16x7 ponys to my 17x9 cobra Rs... the weight diff. is huge

srothfuss · Jul 20, 2005

powertrax91 said:
the loss in acceleration is quite notiable... I lost .5 sec in ET when i went from 16x7 ponys to my 17x9 cobra Rs... the weight diff. is huge

I ment on the street... Not on the track when the clock is running. I have to remember to clarify my thoughts when I post.

12sec67 · Jul 20, 2005

check out this tire calculator.. it also shows the ---- %---- that your speedo will be off, and a comparison of tire size hieght and width so you know what size to get.

hope this helps,

Rich

http://www.eunos.com/tirecalc/tirecalcold.html

tjm73 · Jul 20, 2005

powertrax91 said:
the loss in acceleration is quite notiable... I lost .5 sec in ET when i went from 16x7 ponys to my 17x9 cobra Rs... the weight diff. is huge

Totally true. I went form easy 13.8's to 14.4's when I switched to my new wheels from my old draglites. Nothing else changed.

On the street things feel very similar almost can't tell a difference. Liek soem one else said the wheel size doesn't effect the speedometer reading, it's the tire height.

vristang · Jul 20, 2005

I created an Excel spreadsheet for calculating what the speed and rpm differences would be.
Only a Nerd would think this was as cool as I think it is.

Send me an email address and I can shoot it to you.

maverick0716 · Jul 20, 2005

tjm73 said:
Totally true. I went form easy 13.8's to 14.4's when I switched to my new wheels from my old draglites. Nothing else changed.

On the street things feel very similar almost can't tell a difference. Liek soem one else said the wheel size doesn't effect the speedometer reading, it's the tire height.

A loss that big was due to either A) major spinning B) taller tire diameter. That is quite the drop in ET. The wheels would have to be at least 20 lbs. (each wheel) heavier than the stock wheels, which is not likely.

grrrrrrr · Jul 20, 2005

I think the loss in ET is a result of more tire diameter. Thats why drag racers with huge tires run gears in the 5.30's and up because their tires are so big. I'm sure theres a rule of thumb somewhere but I know that as you increase tire diameter (bigger rims, more sidewall, that sort of thing) it's effectively the same as decreasing your rear gear ratio. So as someone mentioned, they lost .5sec in ET it was probably because they effectively went from 3.73's to 3.27's. Prolly not that big a jump actually, but you get the point.

90bronconate · Jul 20, 2005

powertrax91 said:
the loss in acceleration is quite notiable... I lost .5 sec in ET when i went from 16x7 ponys to my 17x9 cobra Rs... the weight diff. is huge

i wouldn't say it's totally the weight if the only thing changed was the rim size. if you didn't change the tire diameter than you lost some sidewall and .5" all the way around is a significant amount. how i understand it is, the more sidewall you have the easier it is to hook up cuz the energy has more material to spread throughout. so i'm guessing that you're 60 foot was worse.

GRGT1994 · Jul 20, 2005

vristang said:
I created an Excel spreadsheet for calculating what the speed and rpm differences would be.
Only a Nerd would think this was as cool as I think it is.

You also could just use Google to compute tire height. It does computations directly through the search.

For example, enter "255 millimeters multiplied by 45 percent multiplied by 2 plus 17 inches in inches"

When you hit search, it will tell you that equals 26.0354331 inches.

Kinda neat.

super302 · Jul 21, 2005

powertrax91 said:
the loss in acceleration is quite notiable... I lost .5 sec in ET when i went from 16x7 ponys to my 17x9 cobra Rs... the weight diff. is huge

are your cobra rims made out of lead, lol

My cobra rims didn't feel any heavier than my shtty turbines
There is prob a 5 pound per wheel dif, and extra 20 pounds isn't going to slow you down, its has to deal with traction and tons of other factors

vristang · Jul 21, 2005

GRGT1994 said:
You also could just use Google to compute tire height. It does computations directly through the search.

For example, enter "255 millimeters multiplied by 45 percent multiplied by 2 plus 17 inches in inches"

When you hit search, it will tell you that equals 26.0354331 inches.

Kinda neat.

Yes, Google kicks @$$.
See attached Bart Simpson pic.

The calculator I created in Excel will use rpm, trans gear, rear gear, and tire diameter, to calculate vehicle speed. I set this up because I wanted to see how changing fifth gear may affect my fuel economy and power levels.

As an aside, if anyone thinks they can proof a CR calculator that tries to compensate for piston to side wall clearance as well as all of the easy stuff, I would like to have someone else look my CR tables over.

It is best to measure all things that can be measured DIRECTLY. Any inaccuracy in the above calculation will carry out through the rest of your calculations, compounding each time you multiply or divide.

It is best to break out a tape measure, and check this for your self.

For example I ran the following with Google.
((275 millimeters) multiplied by (45 percent) multiplied by 2) plus (17 inches) = 26.7440945 inches

and

((275 millimeters) multiplied by (35 percent) multiplied by 2) plus (17 inches) = 24.5787402 inches

I ran both sidewalls because I can't remember what size tire I had to go with to get things to fit. The actual diameter that I measured is 25.25"

Maybe there is a conversion step missing for the mm width, and solution being in inches?

If you can measure it, it is usually best to measure it.

GRGT1994 · Jul 21, 2005

I think the descrepancy you mention could be caused by a number of things. Tread wear is surely one. But probably more importantly is how tires dimensions are computed to begin with.

I would imagine the 255/45/17 dimension does not contemplate the distance from the ground to the top of a tire mounted on a wheel, at pressure, supporting a car. There are too many variables there within the range of potential applications for any given tire. So the advertised measurements are probably off the wheel. And then all those variables mentioned above (plus vehicle weight and wheel width) will add to the variance you mentioned.

So any calculation off advertised dimensions will, of course, only give you some hypothetical information to work from. But you still should be able to make some useful comparisons among tires based on a strict, cold calculation.

vristang · Jul 22, 2005

GRGT1994 said:
I think the descrepancy you mention could be caused by a number of things. Tread wear is surely one. But probably more importantly is how tires dimensions are computed to begin with.

I would imagine the 255/45/17 dimension does not contemplate the distance from the ground to the top of a tire mounted on a wheel, at pressure, supporting a car. There are too many variables there within the range of potential applications for any given tire. So the advertised measurements are probably off the wheel. And then all those variables mentioned above (plus vehicle weight and wheel width) will add to the variance you mentioned.

So any calculation off advertised dimensions will, of course, only give you some hypothetical information to work from. But you still should be able to make some useful comparisons among tires based on a strict, cold calculation.

You bring up alot of good points. Tires are very difficult to model or even just accurately predict any given behavior.

I would be interested to see how much a tires height changes between sitting on the shelf, and mounted on the vehicle at full pressure.

Cool subject if you ask me.

powertrax91 · Jul 25, 2005

super302 said:
are your cobra rims made out of lead, lol

My cobra rims didn't feel any heavier than my shtty turbines
There is prob a 5 pound per wheel dif, and extra 20 pounds isn't going to slow you down, its has to deal with traction and tons of other factors

when you factor in that it is all unsprung weight, it makes a huge difference...

I've heard that 1lbs of unsprung weight is the equivalent to 10 lbs of sprung chassis weight...

so adding that 20 lbs unsprung weight would be like adding 200 lbs to your car...

I think that would slow anyone down.

how does wheel size affect rear gears?

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