The speedometer on my '94 Civic SI is off by 7% at all speeds. A mechanical engineer buddy of mine thinks that increasing my tire OD by 7% to compesate might mess up my suspension geometry. I'm not sure if he is correct about that and wonder if any of you have successfully increased tire OD for any reason or if you had problems.

My idea is to install 185/70/14's to replace the specified 185/60/14's. This will increase the tire radius by 0.8 inches. I've visually checked the tire clearances, and it doesn't seem to me like this will be a problem.

I know that offroaders increase tire diameter at will and only change the speedometer gear to compensate.

A side benefit of slightly bigger tires is decreased engine RPM and increased fuel economy.

 

Your buddy is correct. Stick with the proper size tire for your car. Don't try to fix one problem by making a second mistake.

If the speedometer error is too high (and 7 percent is unusually high, although errors of half that percentage is not uncommon), then you may have a problem with your speed sensor or gauges. However, keep in mind that there are a lot of variables that can affect speedometer accuracy - everything from differences in the size of tires marked as a given size, to treadwear (the difference between a brand new tire and one worn down to the treadwear indicator bars is about 2 percent), etc.

 

Awhile ago I looked into the speedometer error and someone quoted federal regs for new cars as +10%/-0% to be within spec. I don't know if that were true, but it does hold true to my car and would keep people from speeding unknowingly.

A side effect of course being that my particular car reads 7% more miles on the odometer than it actually has, therefore decreasing its resell value. On a positive note, it makes my fuel economy look better than it actually is.

 

I have a 2003 corolla where I switched out the shitty stock 195-60-15's to g force sport 225-60-15's ... they lower the rpm on the highway and the rest of the time they improve braking due to a wider, stickier tread. I have not measured MPG because it is always stellar.... anyway... the main reason that I switched to bigger tires is because of the horrible roads here. The extra height means that the tires are better at rolling over bumps and at a somewhat low pressure ... the ride is like a limo. Highly recommended if you have a car with normally high ride height and can fit a bigger tire. The roads here are so bad I'm considering keeping my stock suspension on my integra and bumping up the sidewall from 55 to 60 on a 195/___/15


just my two cents

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by magic94gsr &raquo;</TD></TR><TR><TD CLASS="quote">I have a 2003 corolla where I switched out the shitty stock 195-60-15's to g force sport 225-60-15's ... they lower the rpm on the highway and the rest of the time they improve braking due to a wider, stickier tread. I have not measured MPG because it is always stellar.... anyway... the main reason that I switched to bigger tires is because of the horrible roads here. The extra height means that the tires are better at rolling over bumps and at a somewhat low pressure ... the ride is like a limo. Highly recommended if you have a car with normally high ride height and can fit a bigger tire. The roads here are so bad I'm considering keeping my stock suspension on my integra and bumping up the sidewall from 55 to 60 on a 195/___/15</TD></TR></TABLE>

BAD IDEA. And this forum isn't for Corollas.