We’re looking at different vehicles and note that the Toyota Rav4 offers 3 different tire sizes; 16", 17", and 18"
The handy tire calculator at http://www.1010tires.com/tiresizecalculator.asp indicates about a .65" difference in outside diameter of the specific 16" and 17" models. (The 18" falls between them). That’s about a 2.5% difference in circumfrence.
With different tire sizes each rotation of the wheel means traveling a (slightly) different distance. So I am wondering;
Does this have any effect on the transmission, odometer, or anything else? Is the car’s fuel mileage affected?
I am assuming Toyota doesn’t tweak the transmissions and/or odometer for each tire style; or maybe they do?
No, it’s going to make a significant difference*. First, the odometer and trip computer will under-read a bit since the wheels won’t be making as many rotations, and thus your perceived fuel economy will drop. Actually, though, your fuel economy will improve slightly (and your acceleration will drop slightly) as bigger wheels/tyres will act like a higher-ratio gear.
This is assuming you do it yourself. I’m sure there’s a method for the dealer to recalibrate the odometer and speedometer since they probably offer more than one wheel size on the vehicle anyway.
*Significant as in noticeable, not necessarily as in meaningful.
If you are ordering the vehicle new it will be geared and calibrated to the tire size I would bet.
When you get picked up for going 59mph in a 55 zone you will know the reason why.
My speedo is off by 4 mph I do use my GPS to verify and then i mistakenly go to fast.
I even wrote to Garmin and asked if a feature like an alarm could be set to sound if i were to set it to alarm at 58mph, They say no, no interest.
There are 101 trip computer features in my unit that are mostly useless. Something like an alarm would(could) pay for the device several times over if it were used.
I will venture an educated guess that the tire size on each diameter wheel will be different, but will have the same overall diameter. This will equalize revolutions per mile and keep odometer/speedometer settings the same.
The tire size on the 15" rim might be something like 195/70 x 15. On the 17" rim it might be 195/55 x 17. A lower aspect ratio will provive better traction, especially in turns, because there is less sidewall to flex. Because there is less sidewall, however, the ride will be a bit stiffer. Conversely, a 70 series tire won’t handle as well, but the ride will be smoother.
The lower aspect ratio tire, because it provides better traction (friction), will wear out a bit faster. Nothing dramatic, though.
Generally speaking, the taller rim with a lower aspect ratio tire is considered preferable.
The PCM or power control module (car’s computer), or BCM (body control module) will be calibrated for the tire size that comes on the vehicle. So the speedo will read acurately.
To answer Uncle Fred, Yes Toyota does tweak every car so the speedometer reads correctly. But it has nothing to do with changing anything with the transmition or odometer, only the computer program that reads the sensors. In a much older car you might have a gear driven off the transmition, that turns a cable, that is connected to the speedometer. Now days it is all done by sensor readings and the program in the computer. They just change a value in the program and the computer adjusts for the sensor readings.
If you want to put larger or smaller after-market wheels and tires on any car with a PCM you can have the computer recalibrated for the new size with little trouble. That is unless it is a fairly old car with a carburator and no computer anything.
Most modern cars can also be reprogramed to change the shift points, remove top speed limiters, etc. Called a performance ‘tune’. The way your car behaves and performs is all in the computer now.
Good guess, but wrong. You are thinking in mechanical terms about issues that are all electronic now.
There are many sensors that the computer now reads to do things that you might think are being done mechanically. Push your foot down on the gas pedal and a cable is allowing more gas into the engine? Nope, the Throttle Position sensor tells the PCM that you want to go faster. The Mass Air Flow sensor gives a reading about how much air is coming in. The Knock sensor reads predetonation and changes the firing timing. The Oxygen sensor reads the gasses leaving the engine. And the PCM adjusts accordingly.
There are also wheel sensors that check for spin or loss of traction and work with the brakes, and if you have active handling or traction control, with the trottle too.
It’s all active programing reading sensors and making the car go zooom.
I think you’re misunderstanding his contention. I had to read it twice to follow it. He’s suggesting that even with different wheel sizes, the outer circumference of the tires could be the same. Obviously the hole in the tire (bead circumference) will be smaller for a 15" wheel than for a 17" wheel, but if the 17" tire has a smaller aspect ratio, it’s OD could be identical to that of a 15" wheel with a larger aspect ratio. If this is the case, one could change wheel sizes without having to recalibrate the speedo.
Sorry if I mis-read his meaning, you could get the result that way, but it seems like too much trouble. It is fairly common to just reprogram the computer. The dealer can do it or even a home enthusiast with a hand held programmer.
Pickup trucks often come with some basic wheels and tires under the assumption that the buyer will be replacing them to suit their needs. Where I live it’s common to jack the truck up by installing a lift kit in the frame and put on big ass wheels. But enough about the girls in the NW.
After market wheels are a big deal with those that lower their cars too (You!, in the Honda with a Fart Pipe and itty bitty wheels, I’m talking to you.)
Other guys (mostly) that take their cars to the drag strip sometimes replace the gears in the rear end with a different ratio and that can be adjusted for too.
Actually, that IS the most common way to adjust tire sizes… The whole reason for going with a larger wheel size is for low-profile tires. It’s a combination of esthetics and performance, essentially.
…mostly esthetics. Unless you’re spending the big dollars on forged wheels, larger wheels/lower-profile tires can add a significant amount of weight. That’s bad on a couple of fronts. The additional unsprung mass requires your suspension to work much harder, leaving you with a harsher ride and possibly grip issues in bumpy turns. Also, the larger rotational mass will eat up more power to accelerate and to brake.
The major reason that performance cars originally went to the larger wheel sizes (besides looks) was to fit larger brake discs inside the wheels. Shorter sidewalls give better feel and quicker direction changes, but that can be achieved these days with taller, stiffer sidewalls.
This is exactly what the OP was saying and he provided the link to the tire size calculator, but not the actual tire sizes, which are: P215/70R16
P225/65R17
P235/55R18
With the 16" tire as the base, the 17" will make the speedo read 2.4% slow, and the 18" will read 1.2% slow.
Note that a 2.4% variation means that when the speedo reads 60, you’re actually going 61.4 mph, not something likely to get you stopped by the cops. The 4 mpg variation Gbro mentioned is 7%. And if the factory setting were for the largest circumference tire, then the others would cause the speedo to read too high, not too low.
Do you have a cite that 1) the manufacturer adjusts the computer to adjust the speedo for the wheel/tire selection, and that 2) the computer can be similarly reprogrammed to adjust the speedo by a mechanic or a “home enthusiast” with a code reader?
Doesn’t every state have laws that prohibit tampering with a car’s odometer? If changing the speedo settings was as easy as you suggest, the indicated mileage on any used car sold would be meaningless.
I suspect that all RAV 4s are shipped with the same settings on their computers and that they are not reset by anyone, and probably **can’t **be reset outside the factory for legal reasons. The 2.4% difference the OP noted is simply considered to be within an acceptable range of variance.
I replaced my stock 15 inch tyres with 17 inchers. I also went from a 195 / 65 rubber to a 215 / 55. (on a Toyota Wish, which is a small people carrier)
Convential wisdom is that the larger tyre width / circumference gives better grip (because of a larger footprint) which leads to better stopping and handling in addition to the previously mentioned benefits of a lower sidewall having less flex.
The trade-off is of course fuel economy and acceleration. Others that have done the same trade I did note a 5-10% drop in mileage.
I would question the idea that larger tyre = faster wear. This is more often a question of the tyre you buy than the size. Similiar tyres in different sizes will have the same wear pattern / life.
If you go for a replacement tyre, be careful to get one with the same, or better, speed rating as the factory tyre. (ask the tyre shop, they can advise)
I can’t speak for Toyotas, I don’t work on them. On a Volvo inside the Central Electrical Module is a file called the car configuration file. This file lists somewhere between 60-100 different parameters for this individual car. Inside this file is all kinds of interesting facts. Among them are things like:
[ul]
[li]VIN[/li][li]Production plant[/li][li]Factory order number (beats me as to why, but it is there)[/li][li]Exterior color[/li][li]Interior color[/li][li]Engine type[/li][li]Alarm system (Y/N)[/li][li]Type of horn on alarm system[/li][li]type of climate control[/li][li]transmission type (stick, auto 5 speed/6 speed AWD)[/li][li]Various other options on the car[/li][li]Tire circumference[/li][/ul]
The circumference is listed in mm. To the best of my knowledge it is either not adjustable, or if it is adjustable it is only adjustable between a few preselected parameters. I honestly don’t know if it is adjustable, as I have never needed to or tried to adjust one. If it is adjustable, the options would correspond to what size tires were available on the car as OE equipment.
So can a shade tree mechanic adjust the CEM on a Volvo? I doubt it as I have never seen a really good (as in can match what my factory tool can do) scan tool/r- programmer for a Volvo.
With other brands YMMV.
You aren’t tampering with the odometer, you are changing the calculation used to determine that X number of rotations equals Y speed and miles traveled.
I will find the cites for you. I cannot believe that this is not common knowledge.
Here is one link to a hand held tuner that does just what I said you can do. Adjust for tire/wheel sizes and gear ratio changes. For the shade tree mechanic. It’s all perfectly legal because your odometer continues to read the mileage acurately, which is something that is not happening if you don’t readjust the settings. Or you can just take it to the dealer and have them do it.
Dealers or other shops that have a Tech 2 tool can change the settings too. And many vehicles have a series of programing steps specific to the vehicle that the dealer uses.
On my 2001 Jeep, which may not be the most advanced vehicle but is still fully computerized, it’s as easy as switching out the speedometer gear in the back of the transfer case with one with a different number of teeth. They’re small, cheap, take about 5 minutes to switch out, and are available from the dealer for nearly any gear ratio and tire size you can think of.
You’re tampering with the odo in the sense that you can set it to undercount actual miles traveled. If I bought a car and the first thing I did was reset it so it thought my tires were 10% smaller in circumference than they really were, a few years later I could sell it with a reading of, say, 50,000 miles when it had actually done 55,000. Maybe not a huge difference in value, but probably worth something. (Yes, my speedo would have read wrong the whole time.)
I knew there were tuning tools like these that adjusted engine parameters, but I assumed that adjusting the speedo, and hence, indirectly, the odometer, would have been off limits. I guess not. Thanks for fighting my ignorance!
Still, although it is apparently possible, do we know for sure that the factory or dealerships actually go to the trouble of adjusting the speedo for the wheels installed? Or do they just consider a 2-3% variation as acceptable? Are wheel/tire sets factory options, or something that may be installed/changed at the dealership? Does anyone care enough to bother with this?