Does tire size really affect speedometer?

[Matt Timion]

Okay, I've heard time and time again that putting the non-stock tire size on your car will alter the speedometer. This would alter your mpgs as well if you didn't adjust for the difference.

but i'm not sure if it's true or not.

I recently went from 175/70R13 tires to 185/70R13 tires. While the difference isn't too great, there should be at least a few percentage points difference in the speedometer.

Not a single change. When I cruise at 3000rpms in 5th gear it is still 65 mph. When I drove past one of those digital speed displays near a school the other day at 39mph it also said 39mph.

So does the ECU compensate for tire size? Any theories?

[SVOboy]

It does indeed affect it. That's why larry always does those things with compensating when he does super mileage runs.

[Matt Timion]

Originally Posted by SVOboy

It does indeed affect it. That's why larry always does those things with compensating when he does super mileage runs.

that's all fine and dandy except for the fact that I just told you that mine didn't change with the bigger tires.

[SVOboy]

Indeed, but you have to figure that digital thingy has error too.

[diamondlarry]

So does the ECU compensate for tire size? Any theories?

It's possible that some cars may adjust for tires but mine doesn't. I did the same thing(185 70 14's versus 175 70 14's)and my mileage is now "short by about 5%. This is why some of my mileage figures are carried out to three decimal points sometimes because I have to add the difference into my miles driven. I figured out that it was off by driving from one county road to another and watching my odometer. All of our roads are 1 mile apart around here and after 10 miles I would be .5 miles off. I checked the speed by timing myself at 60 mph on the speedo and it showed that it took me significantly less than 60 seconds to cover the mile.

[MetroMPG]

how can a car "know" it's got different sized tires on? the indicated speed (speedometer) doesn't change relative to rpm regardless tire or rim size.

175/70/13 to 185/70/13 results in a 2.4% change at 65mph, or 1.6 mph.

that's probably small enough to be in the margin of error (human, radar & mechanical) to be able to detect reliably (or at least easily).

http://www.paspeedo.com/calculator.htm

[rh77]

I just realized something by this thread. Within the last few months I've noticed that my test vehicle is traveling at an actual 54 mph when it displays 60 mph, for example -- it keeps me out of trouble, BUT

I'm travelling fewer miles since the rotation of the tire is increased (higher RPMs of the wheel translating to a higher speed reading). My only guess is that the tires are worn down. The speed has been verified using 2 separate "your actual speed is..." radar trailers and by my wife following me and telling me, "why were you driving so slow?" -- when I set the cruise for 10-over the limit. Calibrated it versus the other car, which is pretty much spot-on from the radar trailer.

So, I guess my gaslog is inherently flawed and I've travelled fewer miles per gallon. The changes in mileage between fill-ups still can be accurate since the speed problem is a constant.

Bummmer.

RH77

[Compaq888]

I've changed from 195/65/15 to 205/60/15. The rpm is the same at all speeds and my mpg didn't change at all. It only changed when I inflated the tires more.

[MetroMPG]

shouldn't you guys be going to narrower tires for mpg? less frontal area & aero resistance.

unless of course you're using the improved grip to corner at higher speeds, thereby braking less and conserving more momentum.

[kickflipjr]

^
I'm shure you heard about the one guy running trailer tires on his metro. that is crazy dangerous!