Anyone with the 195/50/15 s03 on the stock rims find that their speedometer is a little off? I could have swore that before, when the rpm was at 3, I would be right on 60mph and when th rpms were on 4, i would be going 80mph. Now it is a few mph ahead. Maybe I am just imagining things but can someone help? Thanks!

 

That's correct--you're using the wrong aspect ratio.

 

what does that mean?

 

you changed the gear ratio with smaller diameter tires, stock size is 195/55/15 and you made a smaller diameter so the gauges will be off a bit.

No biggie, just deal with it

 

It means the rolling circumference of the tire you are using is less than the tires that the car came with i.e. 195/<FONT COLOR="red">50</FONT> vs. 195/55

Another way you could put it is say you roll the car forward 100 ft.

Your tire would do more revolutions in that 100 ft than the 55 series tire would.

The upside is that your acceleration will be better

 

The downside is your mileage accumulates faster . . .

 

Say I did 100 miles....how many miles would it be with these tires?

 

195/50-15 / 195/55-15 = 1465mm height / 1484.5mm height = 0.987

100 actual miles x (1/0.987) = 101.3miles on 195/50 speedo.

This, of course, assumes the wheel/tire height is exactly as stated. A variance by a few mm is normal, anyway from the manufacturing process.

Don't worry about it.

 

thanks!

 

ACtually by the calulator I used you are 3.3% off and that makes 100 miles 3.3 miles extra.



Now figure you drive 20,000 miles on a set of tires. THat is 660 EXTRA miles you would not have put on your car had you used the right size.

I am NOT advocating using the correct size as I use all sorts of different setups on my R ju8st pointing out the downfalls of the 195/50/15 . . . .

 

Its funny you should mention this topic. I have S03 pole positions (hence my username) 195/50/15 and just drove from Key west to Miami.

I wasnt really measuring for tire/gear ratios but just to track my miles.

From mile marker 0 (lowest point in continental US) to mile marker 100 I showed 0.9 miles over 100 miles. IN other words 100.9 miles

But you really shouldnt be that picky about that. Im not.

LIke the tires?

 

In an Euro ITR means -11km/h (from 195/55 to 195/50). Confirmed by GPS.

 

What do you mean by -11 km/h? At what speed? 100km/h?

 

That's strange, because if your gearbox/final drive is not changed your tach and speedo should indicate the same value as before. Only your real speed will change. Your speedo shouldn't indicate a higher value @ 4000 rpm.....

 

changing the tires to a smaller aspect ratio accomplishes the exact same thing as changing the final drive. That should then make the speedo read faster

 

Yeah, I know, but shouldn't the indicated speed remains the same at a certain rpm like 4000 rpm?

 

Mean in top speed. 250km/h in tach equals 248 in GPS using 195/55/R15.
The same ITR (Darkteg's) using 195/50/R15 makes minus 11km/k in top speed.
(Top speed means fuel cut in 5th gear at about 8700/8800 in tach)

 

at an indicated speed you will be doing more rpms than you previously had on a 195/55. Trying to make sense of the physics in my head and it is just too early here.

 

Was the indicated 250 km/h still achieved at the same rpm (8800?) in Darkteg's ITR?

 

No, that's not the indicated speed but real speed!!!! The ITR measures the rpm of the axle to get the speed, right?

 

I'm pretty sure it measures speed off of the transmission (speedometer gear inside the tranny). Since nothing between the engine and tranny changes if you put on different sized tires, then the indicated engine and road speeds should remain the same relative to one another.

 

Yeah, I'm wondering how Brownman noticed a difference in the relation between the indicated rpm and speed....