Larger tires - affected speedometer and odometer

[lhrocker]

The original tires on my 97 Legacy was 185/70R14. I went up plus 2 and thought I had 205/50R16 on them now, but they are actually 205/55R16. I dd some research and found out that now when my speedometer says 60, I'm actually doing 62 and change, so speedometer is no longer correct. Will this be the same with the odometer? If so, how can I now calculate what the actual mileage is with what I see on the odometer.

 

Other than the size, everything else seems fine. I will get 205/50R16's when I replace the tires- or should I just do it now?

[Andyjo]

so your speedo is reading 3% to low, as is your ODO.

i bet if you used a GPS to track your distance, from a zero point, when your ODO hits 100 miles, the GPS will read 103 miles...

just a though

[jamal]

I have the same before and after sizes. Not really enough of a difference for me to care and it was actually more accurate according to gps.

[ccrinc]

All else being equal, I upped my Legacy Turbo tires from 195 to 205s. Those darn "your speed is" signs always show my speed as lower than my speedometer. 2-5 mph depending on overall speed.

Of course, I don't know how accurate those signs are, but every one has shown the same results. FWIW

 

Emily

http://www.ccrengines.com

[2X2KOB]

A 3% error is probably within the accuracy specification for the speedometer. In any case it's probably not enough to worry about.

[chef_tim]

Now when you go from 185/80/13 to 28/8.5/14s the 20% difference is quite substantial

[rverdoold]

I can notice some small difference since i changed from 185/65 on 14" to 205/55 on 15" but it is only 1.2% or something. I also did the GPS track and my odo gives little more (speedo as well).

[idosubaru]

odometers aren't very accurate to begin with, so it was probably wrong before anyway. US vehicles can easily have %5 error (speedometer and yes odometer will read lower) and it's even higher in Europe. so maybe your speedometer is accurate now instead of being wrong to begin with. psychology/marketing....."Wow, i'm going 75 but it doesn't feel like it, this thing is so solid and smooth i have to buy it".

 

it is annoying trying to get legitimate numbers. GPS or checking highway marker signs the next time you're on a road trip are your best bet. check what your odometer reads against 100 highway miles per the little side marker signs on the interstate.

[OB99W]

Here's an on-line calculator if anyone wants to see how differently-sized tires theoretically affect indicated speed:

http://www.net-comber.com/tirecalc.html

 

(I said "theoretically" because the assumption is made that the speedo is accurate when tires of the manufacturer's suggested size are used. Besides calibration errors, of course tire wear and inflation pressure play a role in this as well.)