Last weekend i bought some new tires for my 94 Lude. I just got it in January and it had 205/55/15's on it, and I changed to a 205/50/15 size tire. While driving on the freeway i noticed that my speedometer seemed like it was off, as it said i was going 80 mph though it didnt really seem like i was going that fast. Is there any chance that by switching tire sizes it could cause the speedometer to not register correctly?

 

i doubt its that much of a diff. maybe 1-2 mph at 80. i went from 205/50/16 to 215/45/16 and the speedo doesnt seem to be much of an issue.

 

going down one size won't make much difference, if any

 

first off there is a link to a tire size calculator in the FAQ

second your speedo and your odometer are 3.4% to fast......so it's reading wrong and you are puting more miles on your odometer than you are actually driving

 

yeah, it's off. Figure if you're running a 205 (which for the rookies means it's 205mm across), then a 50 instead of a 55 means your sidewall is now 10.25mm shorter.

This means your original radius from the center of the wheel (that's half of 15, for the noobs) is 190.5mm (7.5"), plus the sidewall height (205mm X 55%) of 112.75mm means you have a total radius length of 303.25mm.

Your new tire size means you have a radius of 293mm (303.25mm-10.25mm).

multiply those numbers by two and you get the diameter. multiply the diameters by Pi and you get the circumference:

1904.41mm old tire
1840.04mm new tire

this means that your car is moving 64.37mm (about 2.5") less for every rotation of the wheel. Therefore your your wheels have to rotate faster to give you the same mph you had before...thus your speedometer is now off.

If you want, i can tell you by how much...but I don't feel like doing the algebra right now.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by DTPrelude &raquo;</TD></TR><TR><TD CLASS="quote">i doubt its that much of a diff. maybe 1-2 mph at 80. i went from 205/50/16 to 215/45/16 and the speedo doesnt seem to be much of an issue.</TD></TR></TABLE>

that's because the difference in cicumference between your two sizes is only 36mm (1919.8 minus 1883.7) versus the 64mm difference between the OP's...your difference is only barely noticeable whereas his is almost twice as bad

was this a long answer, or what?

 

So what is a possilbe solution for this? Are there any negative effects of the dieffernce in sizes, other than my speedometer being off?

 

Speedo/odometer will be off. Your city milage will probably go up a tad, while your highway may go down. It'll probably look funny too

 

SO in the past week ive notcied that my gas milage has decreased by a signifigant amount. Is there any way that I can increase the gas milage back up, aside from replacing the tires.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Bok Choi &raquo;</TD></TR><TR><TD CLASS="quote">Is there any way that I can increase the gas milage back up, aside from replacing the tires.</TD></TR></TABLE>

short answer: No. not unless you can figure out a way to save fuel elsewhere (e.g. lightening the car about 500lbs)

 

sober that was quite the analysis.....conveniently theres a miata website with a built in calc for questions like this....

ill be back with the link soon......EDIT: http://www.miata.net/garage/tirecalc.html

you got a date wednesday baby....

 

DID YOU MISS THE PART ABOUT YOUR ODOMETER BEING OFF?

lets say you drive 10,000 miles a year....thats a low average...i drive more, but this is just for simplicity.

You are putting an extra 340 miles on your odometer each year, that you are not actually driving.....

you should probably take the tires back and say you are dissatisfied....most places are able to offer you some kinda of warentee or satisfaction claim....and get yourself the right size

 

what exactly is the right size? I am about to buy some 16" rims and put 205/50/r16 tires on there, am I going to be doing the same thing? I have a 4th Gen. ?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by mgags7 &raquo;</TD></TR><TR><TD CLASS="quote">sober that was quite the analysis.....conveniently theres a miata website with a built in calc for questions like this....

ill be back with the link soon......EDIT: http://www.miata.net/garage/tirecalc.html

you got a date wednesday baby....</TD></TR></TABLE>


nice calc!!! I know what to do when I lease a new car... keep the payments low, and the mileage low too!!!
hahaha

 

yeah 205/50/16 or 225/45/16 are close enough....i'd rather be on the + side if i was off.....both of those are 0.8% and 0.4% to slow respectively....so really you are saving about 80 and 40 miles lol

yeah if you are leasing a car and drive a lot, swap out some tires with a bigger diameter until you give it back....lol

 

If you ain't cheatin' , You ain't tryin'!

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Silver Surfer &raquo;</TD></TR><TR><TD CLASS="quote">yeah if you are leasing a car and drive a lot, swap out some tires with a bigger diameter until you give it back....lol</TD></TR></TABLE>

 

at that 3.4% that guy is off right now.....if he was on a 30,000mile lease he could only drive 29,000miles

think of it the other way around and maybe go slightly bigger