No, a snap-on scanner cannot. The only way I am aware of to re-calibrate the wheels to the rest of the data (like MPG and speedo) is to have the PCM reprogrammed/tuned. That requires a tool like and HP Tuner (a device that can read AND write to the PCM).

 

i have 3.8 200 hp NA.....which is without supercharger

 

Are you doing city driving mostly? Otherwise, L67's can get almost 30mpg pretty easily on the highway. If you're talking about highway driving then there's something wrong with your car. I get about 27mpg on the highway (17mpg city/hard driving) with my Monte.

 

On the highway i'm getting anywhere from 25-28 on 20's with a L36 ! might have to try a 245/45R18

 

Not sure what "L" number my engine is but it's a 3.5 LOL. Thanks for all the input folks, I guess I'd better get my GPS out and figure out how far off my speedo is so I can avoid getting a "performance award" from the local law enforcement.

 

the rims dont have to be heavier just because theyre bigger. i dont think the op stated if hois factory wheel was steel or aluminum. if he switched from steel to aluminum the 18s should be lighter. the rim diameter shouldnt matter as long as he went +2/-2. if he went up 2 inches in diameter he should have gone down 2 sizes in sidewall height. if he had a 26" diameter tire on there with the factory whell he should try and get as close to 26" with the new tires. if the wheel tire combo is shorter than the factory setup it most probably did hurt his mileage, but not as much as the computer says. plus is that shorter wheel tire gave you a little extra gearing!

 

i agre with chibi and the monte man. i did it with my monte went with 18 inch from steel rims to aluminum 245/40/18. right know i am waiting to get new rubber for them got new rubber for the steels because didnt like not having a already emergince set with bald set of rubber. the 18 have decent little rubber but not in my comphert zone. it threw off my speedo by 5 right know i am looking at that speed shop with that dyno to see if they can set it up for 16 and 18 read out on the computer if it is possible. if it is going to be one or the other it would be the 18 inch and use the 16 for spares like this time around.

 

that size i said is a rough guestimit because i am not fully awake to go back out side this morning to look in my shed sorry

 

If his spedo is off than so is his actual mileage readout off too. If you overall tire(rims and tires) are smaller you will actually put more mileage on the car than you would normally, if you have overall bigger tires than you will put less mileage on your car. So at that point their is no way to calculate actual MPG. But a 235 40 18 should be almost dead on if not a tad smaller. I used to have 225 40 18 and it was almost .75" smaller than normal size.. it made it appear if my MPG had gone up by almost 3 or 4 as well as added more miles faster. Don't use the digital read out its always off so is how many gallons you have used.. by my fill up it was always 1.5 gallons behind.

 

Checked the speedo against my GPS and it's only off about 2 mph at 60 mph...oh well, the price of looking good and better handling I guess. LOL