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I recently purchased a '03 VW Jetta TDI for commuting to work. While setting up my scangauge I noticed the speedometer is indicating about 3-4 mph slower than the speed shown on my GPS.

This made me wonder if I were to go to a taller tire it should get the MPH more closely matched to the actual groundspeed. Correct?

Also, by doing so, the car should cover the same distance with fewer engine revolutions and thus better fuel economy. Am I correct on this assumption or is my thinking here completely backwards?

I believe car manufacturers typically set up the speedometer to show show slightly higher than your actual speed. The logic being to reduce the incidence of speeding, ie: they tell you you've reached the speed limit slightly sooner. They are walking a fine line though: push that adjustment too far and they get accused of trying to accelerate customers through the warranty period.

Anyway, any time you put on a wheel/tire combination that has larger diameter than stock, you effectively change the gear ratios. You'll improve your fuel economy. You'll also reduce the speed at which miles accumulate on the odometer, making calcuations somewhat confusing.

There is only so much clearance from the the wheel well, brakes and supsension components though, enough change and you might start hitting something.

A taller tire will slow your speedometer even more (less revolutions per mile), but the larger tires will most likely change the gearing enough to improve FE. You'll need to figure the odomenter correction for the size tires you are running vs. the original size in order to get an accurate MPG reading.

To increase the speed your speedometer reads you'd need to go to a smaller tire (more revolutions per mile).

If you do mostly highway miles, a taller tire will help. But if your miles are mostly stop and go, you may not gain, and might actually lose.

Keith

Many up-sized wheels are built with the conventional performance enthusiast in mind, and are wider for a larger contact patch on dry surfaces. That is the opposite of what you want, and width will hurt your fuel economy by way of aero, rolling resistance, and inertia.

http://www.barrystiretech.com/rrandfe2.html

In a nutshell, no.
http://www.caranddriver.com/features/10q1/effects_of_upsized_wheels_and_tires_tested-tech_dept

Most modern manufacturers set their spedometer/odometer using one specific tire size, then set their computers and manufacturing process to all use the same across the entire line. Then as the official manufacturing roll out happens and options change, usually they end up releasing that vehicle with multiple tire size options. The lower priced basic models tend to have cheaper tires (not always smaller though), and the high end premium models get the premium wheels and usually the same tire size as originally programmed.

Perfect example is my current SUV (2004 Dodge Durango). The spedometer and everything was programed to use the 265/65R17 tires. Yet mine came with 245/70R17. This means that as the speed increases, the spedometer difference grows faster than my actual speed. At 20 mph, it shows 1 mph fast (spedometer shows 21), 40 mph shows 2 up, 60 = +3, 80 = +4 (84).

So for the original poster, I would keep an eye on the entire speed spectrum (20 to 80, may need to go fast once if possible just to test this). If it is a constant 3-4mph difference whether it is 20 or 60 mph, then it may need to be reset/reprogrammed by the dealer or superchips type of program tool. If it is like mine where it is off a little to start but the difference grows at higher speeds, then you have a slightly off tire size, and in your case, going down one size will help resolve this.

Looking at the VW information online, it appears they actually used a wide range of sizes for these cars:
195/60/r15, 205/55/r15, 205/50/r16, 225/45/r17, most level models used 195 65 R15 which is likely what their original spedometer program set it to use. With the 205/225 tires, your speedo will show slower than actual speed. When it comes to mpg, you would actually be getting 42 mpg and not 40 mpg (as an example).
In my case, it would be the opposite effect. If I figure (as an example) 350 miles and used 22 gallons, that is 15.9 mpg (about average for this vehicle for city), BUT figure in the 5% mileage discrepancy, that 350 is actually 332.5 miles. 332.5 / 22 gal = 15.1 mpg which comes to more money spent. If you figure in the number of people across this country and possibly world that think they are getting better mileage than they actually are, it makes up a big chunk of change they believe they are not really spending. Some may actually cheat the system the exact opposite way like your VW, program it for smaller tires and use larger ones, so it appears you get decent mileage and the EPA reports it as such, when you actually get slightly better than expected and save a little money, and the EPA and their testing models are none the wiser.

Now if you already have the 195 tires then there may be a sensor problem, recall or programming error with your car that may need to be fixed.


As a side note, the real problem comes to actual miles. Since mines shows about a 5% difference, that means at its current 107K miles, the actual mileage SHOULD be around 102K. That 5K miles can make a big difference when it comes time to sell or trade in the vehicle. With the original poster, it would actually work in his benefit, at 100K miles on the odometer, it would actually be closer to 105K.

Regarding the above post: The last 4 VW's and 2 Audi's that I have owned have all had a speedo error of 3-7 mph depending on vehicle speed. If the vehicle is equipped with climatronic (auto climate control), this can be used to access a diagnostic function where actual vehicle speed is displayed. Also ross tech's vcds or vag-com software will allow you to accurately determine vehicle speed and anything else you could ever want to know about your vw. with that said, i have used these functions to determine my speedo error, and to compare the odometer read out over a measured distance. while the speedometer is reading an optimistic speed of 3-7 mph faster than actual vehicle speed, the odometer is in fact recording accurate distances. This is the case for the last 6 VW/Audi's that I have owned, I cannot speak for every VW product or any other manufacturers of course.

Regarding the original question: When it came time to replace tires on my wife's 2004 Jetta, I increased the aspect ratio and went with a taller tire, thinking the very same thing regarding revolutions per mile and helping fuel economy. The mistake I made was forgetting that 95% of my wife's driving is in city traffic. The taller tire effectively inceased the gear ratio and rotational inertia and has hurt her fuel economy in city traffic.

3-4 mph slower than the speed shown on my GPS

As said before, "taller" tires will slow your speedo even more.

A "taller" tire will in effect gear the car higher, lower rpms ,, you must consider the terrain and if you're already geared high enough, forget taller tires.

Many mfg's offer different gears for the speedo drive, I use the term "gear" as I cannot remember the term for modern speedo pickups
I suggest a visit to the dealer for this as the vehicle needs to know the correct speed.

For max fe, use the smallest and most narrow tire you can stand..

Also, use the max tire pressure and have the tires wear evenly.

A pal of mine owns a large tire wholesale house, he told me tire pressure limits are more related to the WHEEL than the TIRE.

Most factory "Aluminum" rims have the max pressure cast into the inside of the rim.

Tire Contact Patch
This is figured by this method

Vehicle Weigh divided by Tire Pressure equals Contact Patch in Square Inches.

Wider tires have more "Lateral" traction
Also note drag racers use extremely low pressure resulting in sidewall "Wrinkle"

Hope this helps, someone
DrVette

As said before, "taller" tires will slow your speedo even more.

A "taller" tire will in effect gear the car higher, lower rpms ,, you must consider the terrain and if you're already geared high enough, forget taller tires.

<snip>

For max fe, use the smallest and most narrow tire you can stand..

<snip>

DrVette

I'm not sure how a smaller outside diameter will get you better mileage. Due to it being lighter?

Since a smaller diameter tire makes your gearing a little more aggressive, doesn't that have a negative impact on mileage? I can see it'll improve perported mileage, since the car's odometer thinks you've travelled further, all due to the smaller O.D.

..........For max fe, use the smallest and most narrow tire you can stand......


Sorry, but it seems larger = better for RR:

http://www.barrystiretech.com/rrandfe2.html


..........A pal of mine owns a large tire wholesale house, he told me tire pressure limits are more related to the WHEEL than the TIRE......


Sorry, but EACH tire has a maximum inflation pressure written on the sidewall as required by law, but not only is there no law concerning wheels, many wheels don't! That would say that tires are more of a problem.


..........Tire Contact Patch
This is figured by this method.....


Sorry, that's an old wife's tale:

http://www.performancesimulations.com/fact-or-fiction-tires-1.htm

As others said-You are backwards.

Your car-according to GPS- is doing 64 mph, but your speedometer is showing 60 mph-RIGHT?

If you put on bigger tires-you might be doing 66 mph-GPS- your speedometer will still show 60 mph.

Like Mendel said- IT IS VERY UNUSUAL FOR A CAR'S SPEEDOMETER TO READ TOO LOW.
Manufacturers usually set them up to read 62 mph when you are actually doing 60 mph.

I wonder if your tires are actually stock sized?
Check your tire size!
Maybe the PO was a MPG fanatic and put taller tires on it.

Charlie
PS Are you sure your GPS is correct?
You can find mile markers on interstates. Maybe do a straight 5 mile stretch, and check your GPS against it.
I don't think GPS's need calibration-they work by using calculation based on speed of light and distances from various satellites-geometry and Trig I guess.
Maybe there is some sort of calibration, but I would guess it is automatic-nothing you have to fool with.

Check your tire size. If someone already mentioned this-sorry-didn't read entire posting.
Charlie
PS I would also get a buddy with a different car to drive 60 mph and you see what speed you show with speedometer and GPS in your car.

PPS I've gone to a bigger than stock height tire once-2001 Prizm(Corolla) -Didn't seem to change mpg enough to measure, but it wasn't as buzzy (2800 RPMS vs 2650 rpms) at 60 mph.Tires did last longer and more ground clearance for our crummy potholed streets-NOLA

Here is a very good site (http://www.miata.net/garage/tirecalc.html) for figuring tire size/speed, etc.

...You can find mile markers on interstates....

I haven't seen any such mile markers in Canada in years, if ever. Wish there were though. I seem to recall some from when I was a kid, maybe it was on a trip into Washington State? I guess you could sort something out using a straight run between two highway junctions with google maps, but not sure how accurate that would be.

Mendel Leisk-wow no KM markers??
Our interstates have lots of mile markers-heck, maybe they have mile markers for every mile??

It is odd-the OP getting a much higher reading from his GPS than from his speedometer.
Like you said earlier(think it was you) usually speedometers intentionally optimistic to avoid speeding.

OP- let us know what size/brand/ your tires are.

Maybe you can actually measure their height?
Charlie