i notice my speed is ~3% slower than actual speed (speedo shows 80 actual speed 82-83) and the odometer is ~6% off (0.6 mile is lost for every 10 miles)

very weird

original tires are 215/55/17 and new tires are 245/35/20

it should not be that much off....anybody whats up?

 

The speedo will be off if you incrase the size of the wheel. However its in your favor.

 

^^^Christ^^^ I want to kiss those panty's .

Sorry,Canadian beer.

 

You should be 1.7% off, not 3%.

It is not the wheel size but the overall tire diameter that determines how many revolutions per mile.

215/55/17 = 767 revs/mile (diameter 26.3")
245/35/20 = 754 revs/mile (diameter 26.8")

So you should be 1.7% slow.

 

yeah thats why i was wondering, 3% and 6% seems too much

 

It is.

 

 

minh - I have the same situation as you, as I'm running 245/40/19's...not that big a deal IMO (you don't need to go get recalibrated, unless it truly is bothering you that much). You just need to remember when you drive or set cruise control on the highway, that you're traveling 2-3 MPH faster than what the speedo says. As mentioned above though, it is in your favor, as your odometer readings will calc. less mileage than you are actually driving....so come trade-in time or selling your car, the odo. will show that % less that what's actually on the engine. Not a huge discrepancy, but every little bit helps.

 

The police around here set up those radar screens a lot that show your speed on their radar and mine is within 2% at all the speeds I have tried and I have the same size tires as you.

 

Good point, though not sure how accurate those speeds screens are. I have a NUVI navigation and it tells me my speed to the nearest tenth, so that's how I know it's 2-3 mph off at 80 MPH.

 

http://www.miata.net/garage/tirecalc.html

 

Try this too. Works really well. http://www.web-cars.com/math/tiresize.html

 

Also
http://landiss.com/files/mphvrpmG35.xls

Lets you plug in the gear and axle ratios and get a graph of the speeds in each gear vs. RPM.

 

Uh, huh huh huh (beavis laugh)...that's way too technical for me ....and I'm a pretty technical person.

 

Try it this way:

1) Enter your desired tire size in the yellow zone
2) Read the results below as a percent error, and on the graph

Un-technical enough<G>?