Little car needs a new pair of shoes...
September 27, 2010 6:21 AM Subscribe
I'm looking to change my car tyre size, and I've got some questions (more maths related than automotive related, I'd guess).
I've just bought myself a car to use as a weekend toy. It's currently got 15" wheels with high profile economy tyres. Since the manufacturer offered a 16" lower profile option, I'm planning on changing the wheels over and putting on something a bit grippier.
The thing is, that this means changing the tyre diameter slightly.
So... since the speedo on my car (like most cars) is reading 77mph when the GPS is claiming 70mph, I was wondering about killing two birds with one stone.
Question time:
Am I correct in saying that if I increase the tyre circumference, I will be moving the speedo reading closer to accurate not further from it? What increase in circumference would result in the speedo reading accurately? (I've a feeling it's somewhere about 10%, which sounds like a hell of a lot.)
Additionally (and this is for interest not the basis of any decisions to change tyres):
If I increase the tyre diameter slightly, am I likely to (fractionally) increase or decrease fuel efficiency? What about torque?
I'm assuming that the car will become (fractionally) faster but lose a tiny bit of torque. Is that correct? (and I appreciate that we're probably talking a difference of 1 or 2mph at the top end, whichever way it goes. I'm just making sure my maths is right...)
I've just bought myself a car to use as a weekend toy. It's currently got 15" wheels with high profile economy tyres. Since the manufacturer offered a 16" lower profile option, I'm planning on changing the wheels over and putting on something a bit grippier.
The thing is, that this means changing the tyre diameter slightly.
So... since the speedo on my car (like most cars) is reading 77mph when the GPS is claiming 70mph, I was wondering about killing two birds with one stone.
Question time:
Am I correct in saying that if I increase the tyre circumference, I will be moving the speedo reading closer to accurate not further from it? What increase in circumference would result in the speedo reading accurately? (I've a feeling it's somewhere about 10%, which sounds like a hell of a lot.)
Additionally (and this is for interest not the basis of any decisions to change tyres):
If I increase the tyre diameter slightly, am I likely to (fractionally) increase or decrease fuel efficiency? What about torque?
I'm assuming that the car will become (fractionally) faster but lose a tiny bit of torque. Is that correct? (and I appreciate that we're probably talking a difference of 1 or 2mph at the top end, whichever way it goes. I'm just making sure my maths is right...)
If you put on larger diameter tires your car will be traveling farther with each revolution, and therefore faster at the same speedometer reading. So yes, the speedo reading will be more accurate. You may lose a bit of torque, but you have a gearbox to compensate for that. In other words, if you stay in the lower gears just a bit longer, you won't notice the loss. As for fuel efficiency, you may not even notice, since the extra fuel spent staying slightly longer in the lower gears is balanced by the greater distance covered by the larger tires.
posted by weapons-grade pandemonium at 6:55 AM on September 27, 2010
posted by weapons-grade pandemonium at 6:55 AM on September 27, 2010
Your new 16" tires will make the speedometer read approx. 72 mph when you're going 70mph.
If you want the speedometer to read 70mph when you're going 70, you'll need 16.5" tires.
IANAmechanic.
posted by Salvor Hardin at 6:55 AM on September 27, 2010
If you want the speedometer to read 70mph when you're going 70, you'll need 16.5" tires.
IANAmechanic.
posted by Salvor Hardin at 6:55 AM on September 27, 2010
Response by poster: That's true (and I'd found a similar site before), but... the speedo is already reading 10% fast, and flexibility about which tyre size I use means I can get a tyre that is sensibly priced, easy to find and works well (as well as addresses speedo over reading) rather than blindly going for the same circumference and accidentally trapping myself in a tyre size that may be expensive or hard to find.
Which is why I asked my question... how much flexibility do I have without the speedo reading slow, and what other effects will the changes have?
posted by sodium lights the horizon at 6:57 AM on September 27, 2010
Which is why I asked my question... how much flexibility do I have without the speedo reading slow, and what other effects will the changes have?
posted by sodium lights the horizon at 6:57 AM on September 27, 2010
Response by poster: Sorry... that was to Thorzdad, who was the only person who had answered when I started typing...
posted by sodium lights the horizon at 6:58 AM on September 27, 2010
posted by sodium lights the horizon at 6:58 AM on September 27, 2010
Best answer: If I increase the tyre diameter slightly, am I likely to (fractionally) increase or decrease fuel efficiency?
Not even in the slightest, no. Unless, of course, you're using your odometer to calculate the 'm' in mpg, in which case the numbers will change bot you won't be creating any measurable difference in actual mpg.
What about torque?
You will reduce torque by increasing wheel size, but the amount you are looking at is teeny tiny. The car will accelerate faster at the same (teeny tiny) increase. You will notice neither.
For reference, you are talking about (using a 70 vs 77 mph difference) 9% difference in tyre circumference not diameter, so it's really not that big a change. After all that's something like 2-2.5% diameter change, which ain't so much. Work out the circumference for you tyre as follows:
165/75 15 (to pick a random size) is 15" diameter wheel = 75%*165mm (sidewall in relation to tread width).
Diameter = 15*25.4 + 123.75 = 381 + 123.75 =504.75mm
Circ = Pi * D = 1585.71
For 9% reduction in speedo reading, D=1585*1.09 = 1728.4
Now, by law, speedo's over-read (yours is actually within legal tolerance), so maybe don't go the full 9% - I'd work on either the same rolling circumference or up to 5% bigger and work from there. The sums are pretty easy.
So to reduce your speed reading by 0.9% you
posted by Brockles at 7:03 AM on September 27, 2010
Not even in the slightest, no. Unless, of course, you're using your odometer to calculate the 'm' in mpg, in which case the numbers will change bot you won't be creating any measurable difference in actual mpg.
What about torque?
You will reduce torque by increasing wheel size, but the amount you are looking at is teeny tiny. The car will accelerate faster at the same (teeny tiny) increase. You will notice neither.
For reference, you are talking about (using a 70 vs 77 mph difference) 9% difference in tyre circumference not diameter, so it's really not that big a change. After all that's something like 2-2.5% diameter change, which ain't so much. Work out the circumference for you tyre as follows:
165/75 15 (to pick a random size) is 15" diameter wheel = 75%*165mm (sidewall in relation to tread width).
Diameter = 15*25.4 + 123.75 = 381 + 123.75 =504.75mm
Circ = Pi * D = 1585.71
For 9% reduction in speedo reading, D=1585*1.09 = 1728.4
Now, by law, speedo's over-read (yours is actually within legal tolerance), so maybe don't go the full 9% - I'd work on either the same rolling circumference or up to 5% bigger and work from there. The sums are pretty easy.
So to reduce your speed reading by 0.9% you
posted by Brockles at 7:03 AM on September 27, 2010
Your new 16" tires will make the speedometer read approx. 72 mph when you're going 70mph.
If you want the speedometer to read 70mph when you're going 70, you'll need 16.5" tires.
This advice is utterly wrong - feel free to ignore it.
posted by Brockles at 7:03 AM on September 27, 2010
If you want the speedometer to read 70mph when you're going 70, you'll need 16.5" tires.
This advice is utterly wrong - feel free to ignore it.
posted by Brockles at 7:03 AM on September 27, 2010
Your new 16" tires will make the speedometer read approx. 72 mph when you're going 70mph.
If you want the speedometer to read 70mph when you're going 70, you'll need 16.5" tires.
Salvador Hardin is assuming that nominal tire diameters refer to the outside diameter of the tire. They do not.
posted by jon1270 at 7:04 AM on September 27, 2010
Well, ok then, disregard that.
posted by Salvor Hardin at 7:08 AM on September 27, 2010
posted by Salvor Hardin at 7:08 AM on September 27, 2010
Best answer: how much flexibility do I have without the speedo reading slow, and what other effects will the changes have?
To chime in again, your flexibility is 0-8% (to be close but safe) of a larger tyre circumference than you have now. 0-5% is conservative and my personal preference.
Other effects - none detrimental to any significant degree providing you stick to the max dimensions (wheel width and tyre width) that your manufacturer suggests for your car. If you choose a fatter tyre than the max one offered on the fastest of your specific models (ie BMW 540 if it's a 5 series, not an M5 which is significantly different, or whatever the appropriate is for your car) then you will need to check for tyre clearance on suspension compression and full lock. If use the tyre size range (from the options the Manufacturer offers) suggested as a guide then you're being much more sensible and have more freedom for a particular size that is cheap to get hold of.
If you choose a much reduced sidewall of tyre or one much wider, you may notice a reduction in ride quality and road noise (especially over bumps and harshness on rough surfaces) but that's the price you pay for more rubber on the road. If your car came with a 15" wheel, I'd not go more than one size bigger without considering changing suspension a little as the loss of sidewall flex could be detrimental to driving enjoyment - basically you make the wheel/tyre combination much stiffer than it was and this may not be within the range of your damper/shock valving to control accurately and this would be unpleasant.
posted by Brockles at 7:11 AM on September 27, 2010
To chime in again, your flexibility is 0-8% (to be close but safe) of a larger tyre circumference than you have now. 0-5% is conservative and my personal preference.
Other effects - none detrimental to any significant degree providing you stick to the max dimensions (wheel width and tyre width) that your manufacturer suggests for your car. If you choose a fatter tyre than the max one offered on the fastest of your specific models (ie BMW 540 if it's a 5 series, not an M5 which is significantly different, or whatever the appropriate is for your car) then you will need to check for tyre clearance on suspension compression and full lock. If use the tyre size range (from the options the Manufacturer offers) suggested as a guide then you're being much more sensible and have more freedom for a particular size that is cheap to get hold of.
If you choose a much reduced sidewall of tyre or one much wider, you may notice a reduction in ride quality and road noise (especially over bumps and harshness on rough surfaces) but that's the price you pay for more rubber on the road. If your car came with a 15" wheel, I'd not go more than one size bigger without considering changing suspension a little as the loss of sidewall flex could be detrimental to driving enjoyment - basically you make the wheel/tyre combination much stiffer than it was and this may not be within the range of your damper/shock valving to control accurately and this would be unpleasant.
posted by Brockles at 7:11 AM on September 27, 2010
Response by poster: Thank you very much boys and girls.
The plan is only to shift from a 15" to 16" rim. The tyre width will either stay the same or stay within the model specs.
But I'm happy that my "intelligent" guesses about profile and width were roughly right and I'll aim to shift the recorded speed down by about 5%.
posted by sodium lights the horizon at 7:22 AM on September 27, 2010
The plan is only to shift from a 15" to 16" rim. The tyre width will either stay the same or stay within the model specs.
But I'm happy that my "intelligent" guesses about profile and width were roughly right and I'll aim to shift the recorded speed down by about 5%.
posted by sodium lights the horizon at 7:22 AM on September 27, 2010
For reference, you are talking about (using a 70 vs 77 mph difference) 9% difference in tyre circumference not diameter, so it's really not that big a change. After all that's something like 2-2.5% diameter change, which ain't so much.
Not to argue with Brockles on automotive matters, but are you sure about this part? I think a 9% difference in circumference would also be a 9% difference in diameter, since circumference is just diameter x pi. Or maybe I am misunderstanding what you meant.
posted by FishBike at 8:19 AM on September 27, 2010
Not to argue with Brockles on automotive matters, but are you sure about this part? I think a 9% difference in circumference would also be a 9% difference in diameter, since circumference is just diameter x pi. Or maybe I am misunderstanding what you meant.
posted by FishBike at 8:19 AM on September 27, 2010
Well, on the second cup of coffee, then OF COURSE it is. On the first one, not so much. Not sure where that hump of complete horse-arse came from. Note, also, the trailing sentence in the first post. At least the sums are right, even if there are some odd = signs.
Christ, I actually have to fly to Georgia and do some proper work tomorrow. Maybe I should get a caffeine IV...
posted by Brockles at 8:30 AM on September 27, 2010
Christ, I actually have to fly to Georgia and do some proper work tomorrow. Maybe I should get a caffeine IV...
posted by Brockles at 8:30 AM on September 27, 2010
Something most people overlook when changing tire/rim sizes (and mostly they are changing for looks first so it doesn't really matter) is weight and offset.
If you put on a heavier tire/wheel than what the suspension was designed for it will not work as designed. It is likely to wear out faster, not give as good of a ride and will affect acceleration and braking. You are changing what is called the unsprung weight and small changes in this can have huge effects. Now a small change (a couple of pounds) will not produce a change and for what you are talking about you will be in this case, but putting 20" rims on car designed for 15" rims can have some bad consequences. Also buy changing the offset (the distance the rim extends out from the suspension) changes the moment arm of the unsprung weight and this multiplies any change to the unsprung weight causing even more disruption.
I have found the best way to accomplish what you are discussing is buying the manufacturers rims in the size you desire if possible. You say the model car you are buying came with 15" rims also so look on ebay or craigslist for those rims, also junkyards can be a great source for this kind of thing. Even rims for a different model of car can often fit but make sure you match lug nut patterns before you spend any money. If you can't do this or the manufacturers rims are too ugly or something just try to match the OEM wieght as closely as possible. Good luck and changing tires/rims can have a bigger impact than almost any other bolt on modification you can do to car and far, far cheaper than most. Also don't forget to check weight of the new tires also as the total assembly weight of the tire/rim is what matters.
posted by bartonlong at 10:14 AM on September 27, 2010
If you put on a heavier tire/wheel than what the suspension was designed for it will not work as designed. It is likely to wear out faster, not give as good of a ride and will affect acceleration and braking. You are changing what is called the unsprung weight and small changes in this can have huge effects. Now a small change (a couple of pounds) will not produce a change and for what you are talking about you will be in this case, but putting 20" rims on car designed for 15" rims can have some bad consequences. Also buy changing the offset (the distance the rim extends out from the suspension) changes the moment arm of the unsprung weight and this multiplies any change to the unsprung weight causing even more disruption.
I have found the best way to accomplish what you are discussing is buying the manufacturers rims in the size you desire if possible. You say the model car you are buying came with 15" rims also so look on ebay or craigslist for those rims, also junkyards can be a great source for this kind of thing. Even rims for a different model of car can often fit but make sure you match lug nut patterns before you spend any money. If you can't do this or the manufacturers rims are too ugly or something just try to match the OEM wieght as closely as possible. Good luck and changing tires/rims can have a bigger impact than almost any other bolt on modification you can do to car and far, far cheaper than most. Also don't forget to check weight of the new tires also as the total assembly weight of the tire/rim is what matters.
posted by bartonlong at 10:14 AM on September 27, 2010
I'm assuming that the car will become (fractionally) faster but lose a tiny bit of torque. Is that correct?
Just to address the fractionally faster part, assuming you mean the car's top speed, then it depends on what is limiting its top speed now.
If, as is common with many modern cars, a rev limiter or speed limiter cuts the power to prevent you from going any faster, then yes, you'll gain some actual top speed if you install a wheel and tire combination with a bigger rolling radius. The limiter will still cut in at what the computer thinks is the same speed, but you'll be going at a higher true speed when that happens.
Whereas if your top speed is presently limited by where the engine runs out of power to make it go any faster, then it's more complicated. The engine makes the most power at a specific RPM, so the best possible top speed is achieved when the gearing and tire size is such that the engine can exactly reach that peak power RPM at top speed.
If, with your current tire size, the car's speed tops out with the engine RPM already above the peak power RPM, then slightly larger tires would push the RPM down a little at that speed, closer to the peak power, and you might get a little extra speed out of it. Conversely, if the car runs out of steam now below the peak power RPM, larger tires will make that even worse and you'll lose a bit of speed. It's very little different either way, though, for the percentage change in tire size that you can reasonably achieve.
posted by FishBike at 10:41 AM on September 27, 2010
Just to address the fractionally faster part, assuming you mean the car's top speed, then it depends on what is limiting its top speed now.
If, as is common with many modern cars, a rev limiter or speed limiter cuts the power to prevent you from going any faster, then yes, you'll gain some actual top speed if you install a wheel and tire combination with a bigger rolling radius. The limiter will still cut in at what the computer thinks is the same speed, but you'll be going at a higher true speed when that happens.
Whereas if your top speed is presently limited by where the engine runs out of power to make it go any faster, then it's more complicated. The engine makes the most power at a specific RPM, so the best possible top speed is achieved when the gearing and tire size is such that the engine can exactly reach that peak power RPM at top speed.
If, with your current tire size, the car's speed tops out with the engine RPM already above the peak power RPM, then slightly larger tires would push the RPM down a little at that speed, closer to the peak power, and you might get a little extra speed out of it. Conversely, if the car runs out of steam now below the peak power RPM, larger tires will make that even worse and you'll lose a bit of speed. It's very little different either way, though, for the percentage change in tire size that you can reasonably achieve.
posted by FishBike at 10:41 AM on September 27, 2010
This thread is closed to new comments.
Here is a handy tire size calculator. Simply plug-in your current tire size, and then adjust the sizing parameters for the larger wheel until you hit on a new size that is similar in overall dimension to your current tire/wheel package. It also calculates the marginal difference in your speedometer reading. Requires java.
posted by Thorzdad at 6:29 AM on September 27, 2010