Shouldn't the speed of a car, true speed and indicated speed, be dependent on the overall circumfrance of the wheel? With a person chaning out 16" rims to say, 18" rims, wouldn't the car actually be traveling faster than reported by the onboard speedometer? Or is there some sort of balencing with revs of the engine and each of the gears with maybe even some other external sources.. What are the elements of compensation?
Revelation on changing rim sizes...
- Thread starter Halz
- Start date
Of course. This is assuming you stay with a tire that has the same size sidewall as the smaller one. Most of the time your decrease the sidewall size of the tire as you increase rim size.
yes
this is why you must calibrate.
It is your responibility, too, because when you get pulled over, they don't care.
this is why you must calibrate.
It is your responibility, too, because when you get pulled over, they don't care.
<< With a person chaning out 16" rims to say, 18" rims, wouldn't the car actually be traveling faster than reported by the onboard speedometer? >>
this depends on what profile tyre is used on the larger rim. it is sometimes possible to match the rolling circumference of both tyres so that for a given rpm the speed remains the same. should the rolling circumference be increased then the speedo will under read the cars speed!
Take for example a truck then, not some super sporty mazda miata with those awesome 20" rims.. A particular minority has taken a liking to putting extremely small wheels, not just rims, wheels, on their 'trucks' or 'caddys' so shouldn't they be driving slower (by ways of the speedometer) comparably to the 670,611,629 mi/h in that of the Miata?
Edit:
And this 'recalibrating' can take place..? at a mom and pop shop? laptop to a console to the ECU? surely the dealer wouldn't condone my pimped out caddy..
Edit:
And this 'recalibrating' can take place..? at a mom and pop shop? laptop to a console to the ECU? surely the dealer wouldn't condone my pimped out caddy..
Didn't anyone tell you. You get 10 horsepower for every 5 points the ratio of rims to rubber increases. That's a 2:1 ratio, and you can't beat that with a stick...or yellow stickers as the case may be. 
um... if your rims are too big.. it can actually slow down your car...
if you want to go fast.. get the racing rims ( usually white ) 16-17 inches..
with low performance tires...
if you want to go fast.. get the racing rims ( usually white ) 16-17 inches..
with low performance tires...
I believe if you have large rims than stock, your speedo will report a speed slower than your actual speed. If you have smaller wheels, your speedo will report speeds fater than your actual speed. I think with a smaller wheel your milage will go up faster also...
-jimbo
-jimbo
The indicated speed of your car is a function of the *tire* diameter, not the rim diameter.
If you have 245/50/16 tires on your car stock, and you want 17" wheels, you would go to something like 245/45/17 tires. Note that the middle number (the sidewall %) is the one that changes. Also note that that is a rough example, not mathmatically accurate. When you change the rim, a tire shop will help you figure out what tires will come out closest to correct. You can usually live with a 2-3% margin of error so you won't have to get your speedo recalibrated. (different procedure on different cars)
For the guys that put tiny/huge tires on trucks, yes it definitely impacts their speedo and also performance. If you have 26" tires on your truck stock, and you put 31"s on, your speedometer will read way off and your acceleration will get noticably worse. You have effectively changed the gear ratio of your car. By the same token, if you put tiny wheels on your truck, you will actually get better 0-60 times, as you will have effectively lowered the gear ratio. The downside is that if you used to cruise at 70 mph / 3000 rpm, you could now be at 70 mph / 4500 rpm which isn't necessarily good for your motor. Ditto with big tires - I put 31" tires on my jeep and could no longer use 5th gear, it would try to pull 1800 rpm on the freeway at 80 mph and the engine didn't have enough power.
Changing rim sizes (and getting the correct tire) does not necessarily make your car faster/slower. The advantages/disadvantages there are strictly because of the actual weight of the rim. If your car had heavy stock rims and you put lighter, bigger ones on - it will get faster. And vice-versa.
If you have 245/50/16 tires on your car stock, and you want 17" wheels, you would go to something like 245/45/17 tires. Note that the middle number (the sidewall %) is the one that changes. Also note that that is a rough example, not mathmatically accurate. When you change the rim, a tire shop will help you figure out what tires will come out closest to correct. You can usually live with a 2-3% margin of error so you won't have to get your speedo recalibrated. (different procedure on different cars)
For the guys that put tiny/huge tires on trucks, yes it definitely impacts their speedo and also performance. If you have 26" tires on your truck stock, and you put 31"s on, your speedometer will read way off and your acceleration will get noticably worse. You have effectively changed the gear ratio of your car. By the same token, if you put tiny wheels on your truck, you will actually get better 0-60 times, as you will have effectively lowered the gear ratio. The downside is that if you used to cruise at 70 mph / 3000 rpm, you could now be at 70 mph / 4500 rpm which isn't necessarily good for your motor. Ditto with big tires - I put 31" tires on my jeep and could no longer use 5th gear, it would try to pull 1800 rpm on the freeway at 80 mph and the engine didn't have enough power.
Changing rim sizes (and getting the correct tire) does not necessarily make your car faster/slower. The advantages/disadvantages there are strictly because of the actual weight of the rim. If your car had heavy stock rims and you put lighter, bigger ones on - it will get faster. And vice-versa.
Shorter tires make the speedo read faster than actual speed.
Taller tires make the speedo reas slower than actual speed.
Taller tires make the speedo reas slower than actual speed.
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