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?

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.

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

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.

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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

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ForkenSwift cheap electric car conversion- Geo Metro fuel economy mpg modifications

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

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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.

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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.

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ForkenSwift cheap electric car conversion- Geo Metro fuel economy mpg modifications

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

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Distance traveled by bicycle in 2007= 1,830ish miles
Average commute speed=25mph (yes, that's in a car)

I'd never put narrower tires on my car. My car is 3000lbs so I need all the contact patch I can get.

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Yes, we should. I went from 175/70R13 to 185/70R13 because the 185s came on the rims when I bought them. I sort of wanted to get rims with tires. Now I kind of wish I'd just found some rims and swapped. Whatever.

I guess the good thing about this is that I have another set of rubber in my garage when my current tires go out. Considering how often I go through tires that is a good thing.

It's easy to correct odometer and mileage when you change tire size. If anyone wants to email me sludgy@aol.com, I'll send an Excel spreadsheet that calculates an odometer correction.

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By putting wider tires on your car, you have spread out the 3000 pounds over a greater surface area. Your rain, snow, and ice performance will probably be negatively affected by the change. What problem were you trying to solve, or did you just want tires that looked wider? You said your gas mileage didn't go up or down with the new tires. Why did you change sizes?

'

The spreading out part is true. But negatively affected is not true. By the magic of physics, we know that 3000 pounds on 1 square inch of tire will experience the same frictional force in total as 3000 pounds on 100 square inches.

As I mentioned in a previous post, these tires came with the rims I bought. I purchased these rims because they were lighter. The tires came with them so I saw no need in taking them off.

Unfortunately this is not true.

Friction is thought by teachers as not being dependant on the area for educational purposes but in fact it is affected.

Also there are 2 types of friction:
1) Sliding Friction
2) Rolling Friction.
Rolling friction is always greater then sliding friction.

For best MPG, narrover and smaller tyres should be used in order to decrease contact area, and wider and bigger tyres have more contact area therefore increases grip at dry surfaces and decreases braking distance (with or without ABS).

Also: Losing grip at snow/rain is highly dependant on the geometry and shape of grooves on outer surface of the tyre.

There are indeed two types of friction, static and kinetic. But if you look at the friction equation you will get Friction = uForceNormal where u is the coefficient of friction and Force Normal is the force that the ground exists on the tire (in this case). The force normal equation is FN = mass * gravity. So therefore, you get your frictional forces without any consideration off size, only mass.

I know.
I was referring to this formula that being thought on priliminary physics.

Unfortunately "u" (thecoefficient of friction) increases as the area increases. In fact you should never compare different areas of same materials under same load.

And also why do you think wider tyres decrease braking distance (with or without ABS) as compared to narrover tyres ?

Aha, I see now that cylindrical surfaces have friction proportional to length, but I dunno if the tire is considered a cylinder or a square/rectangular surface, as I don't think cylindrical implies give in the substance. I dunno.

the other critical factor to consider is the shape of the contact patch on rain & snow performance.

all else being equal, a narrower tire resists hydroplaning better, and anyone who drives regularly in snow will choose a narrower tire for winter where possible for similar reasons.

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ForkenSwift cheap electric car conversion- Geo Metro fuel economy mpg modifications

You guys are making me want to switch back to my old 175s