808subaru

iTrader: (16)

Ok, go easy this is not for my suby, but can help people understand my dilemma. I was unable to get help on the car specific forums. Ok, so my OEM tire size is 185/60/15. The wheel and tire weight is 33lbs.
The new tire (19lbs) is 195/50/15, and the wheel is the same, so 34 lbs combined. Now, my speedometer change as calculated was 4.5%. Many have said that going from a 60 series ap. ratio to a 50 series is a "no-no". The revs per mile goes from 880 to 915. Now my question comes in 2 parts: 1)Can you calculate the change in MPG, and
2)How screwed up is my choice in tyre size?

Thanks peoples!

4080wrx

iTrader: (38)

Chris not to sound like a smart *** but do you think it would be easier to just have your speedometer re-calibrated?

as for the calculations Burt might be able to help you on that one.

808subaru

iTrader: (16)

Forgive my noobness on this subject, but if I left the car to re-learn the axle spinning as with more revs/min., by itself, would it work?

Ross

iTrader: (12)

1. The change in MPG is impossible to calculate. You've reduced your tire's overall diameter by 5%, so you've effectively lowered your final drive ratio. Acceleration might be a tick better (probably not) and your car is revving higher than before for a given engine speed. If you do a lot of stop-go in town driving I'd say that your mileage would probably stay the same. If you do a lot of freeway driving, I'd guess that your mileage would stay the same or get slightly worse. The whole mileage issue is probably moot anyway, especially for a less than 5% change in tire diameter - your driving habits will play a bigger part. There have been times that I've spent most of a tank driving to and from town during peak traffic hours and only got 180 miles out of the tank. Other times, I've driven almost exclusively on the freeway and got 275 miles before I needed to fill up. So, how and when you drive can make way more difference than the 5% change in tire diameter.

2. A closer size tire choice would have been 225/50-15 but that tire is over 1.5" wider than the stock tire, so likely wouldn't have fit on the stock rim. In the end, for the minor increase in contact patch and the reduction in sidewall height, I would say that you should just stick with the stock tire size. I don't think that 185/60-15 is a tire that is installed on a performance car anyway, so any decrease in aspect ratio and increase in tire width will probably not make any significant gains since the tires are likely not the weak link. I don't think your choice in tire size was necessarily "screwed up", but I don't think you did yourself any favors either.

808subaru

iTrader: (16)

Good info. Do you think that the 4.5% difference in speedometer reading would calculate to a 9% difference in odometer reading as well? Or is that still impossible to calculate?

Ross

iTrader: (12)

The 4.5% difference in the speedometer reading will translate to a 4.5% (high) reading on the odometer as well. There are so many factors that come in to play, like tire growth at speed, that it's not possible to say exactly how much your speedometer and odometer are off. Even the tire diameters are approximate, just using the numbers and assuming that everything is perfect. On top of that, your speedometer reads a couple of percent high from the factory to begin with.

I guess what I'm trying to say is that the small difference is not worth worrying about or worth going through the cost and trouble of recalibrating your speedometer. Just go back to the stock size when it's time to change tires, as you probably didn't gain much by going to the 50-series tires in the first place.

808subaru

iTrader: (16)

Actually, I went from Goodyear Eagle LS: http://www.tirerack.com/tires/tires....nSpeedRating=S
to Hankook Ventus RS-2
these: http://www.tirerack.com/tires/tires....el=Ventus+R-S2

I did a fair amount of research before purchasing a tyre that i felt fit my needs both on and off the track. For the range of sizes that were available, i felt i purchased the best option.

bpang1

iTrader: (25)

Chris,

Normally, I just go to Tire Rack and look up the tire I have...then go find that table that shows all the specs for that tire and all it's sizes.

Then, I look at the column that says how many revolutions in a mile...then i calculate some stuff and come up with a correct number.

Ross

iTrader: (12)

Sorry, I didn't mean to insult you or question your choice of tires. I was just thinking that the original size didn't seem like a performance tire, so I was looking at your selection in that light (daily driver, non-track use). Sorry if you took my post the wrong way, I wasn't trying to be a d**k (though my wife says that I don't have to try very hard to sound like a jerk).

808subaru

iTrader: (16)

Yeah, the tyre provides better low-end gearing, but lacks oomph up top

I'm the same way probably, so we'd get along well (my wife says the same about me too). I appreciate all the insightful posts though. You seem to know alot about the subject.

I tried to use the mile markers today, but it didn't really help, maybe my odometer is off by 5-10% it seems, but i guess the only way to tell is a long drive, maybe one weekend I'll test them out. I also filled up today, and without calibrating for the size difference, got within 2-3 MPG of what my old tyre was doing, so maybe the ECU has already adjusted?