A 245/45/17 and a 235/55/17? I found a set locally with tires for cheap. Any difference as far as speedo? I believe they are the same arent they?

 

Not the same, no. The 235's are almost 20mm taller than the 245's. Assuming I did the math right, that's about a 4% increase in actual speed vs speedo readout.

 

What about difference with stock tires and everything.

 

Also the dimension you really want to know is the circumference.

the 245/45/17 diameter is 25.681 and the circ. is 80.679

the 235/55/17 diameter is 27.177 and the circ. is 85.379.

the 4% was close, its actually about a 5.5% difference.

Also, the 235 having a larger circ. would mean the speedo would read 5.5% SLOWER then the 245 tire. larger circ. tire covers more distance in 1 rotation then the smaller circ. tire. The smaller tire has to rotate faster to cover the same distance so the speedo would be higher with the smaller diameter.

Granted that is all relative to if your speedo is accurate to begin with if it had the stock size tires.

edit:

theoretically at 65mph the 5.5% difference is about 3.5mph


edit:

yep, million is right, i forgot the width is eurometric sizing and teh sidewall is a % not a mm rating.

OOOOOOOOOOOOOPPPS haha

 

Yes but the % of the width is different

Tire height =

45% of 245 mm= 110.25 mm

vs

55 % of 235 mm = 129.25 mm

19 mm taller

 

looking around the "stock" size for a 65 is ~195/78/14 which gives you a dia. of 25.976 and a circ of 81.606.

with that the 245 tire would be closest to "stock" for the circumference.

 

Hmm ok. Thanks everyone. I knew the 245/45 would be close, but the 235/45 came by and I just wondered.

 

even then though, the 235's are about 4.4% off from "stock" so at 65mph youre only off by just under 3mph where the 245 is off by just under 1mph.

to me, its close enough considering even with the stock size you would probably be off say 2% just due to mechanical wear over 44 years

plus since it is a smaller circumference youll appear to be going 3mph faster then you are.

 

you have the value twice on a tyre (38mm ~ 1.5")
the diameter is this value above the rim and below the rim + the rim size


so the values that are standing now in 65rangoons post are as exact as you could get them. it will still slightly differ from what the manufacturer will tell you about the circumference and revs/mile

 

just use this calculator

http://www.discounttire.com/dtcs/infoTireMath.dos