Need Advice from Tire Gurus: Best Mod turned into Nightmere

[Vette_Fan]

Guys, I'd been wanting new tires cuz run craps just ran out of life. So anyways, after all the reseach and considering a good price I went with the Nitto Invos. Let me tell you, it gave me INSTANT smiles. No road noise. I could actually hear the engine, exhaust (stock), and hold a conversation without yelling. Grip of the tires is fine, nothing spectacular, since it rained here, they would loose a little grip if I stepped on it which I thought was normal for any tire. Anyhow, here's the side. I ordered stock size for C5. Invos don't come in 245/45/17. So, they put the 245/40/17 in front, and stock in rear. I told the guy that it might have issues with speedometer, change the rake angle, and maybe some braking issues. He said, if anything, with the lower profile it will give better steering response (handling), and won't change much "CUZ its in the Front". He also called Chevrolet so ask if it was unsafe, and they said the difference would have to be 10% to be unsafe. Well, I took the car home. Later at night went to see a friend, drove about 60 miles or so altogether. And my half tank gas was all GONE! I noticed my gas milage dropped quite a bit. Also, if I stepped on the brake a little firm, I got the "ABS" sign in the Dic, which I'd never seen before. Also, noticed the speedometer totally incorrect. While driving in the rain - slow - same speed as others, the meter says, 80 Mph. No way I was doing 80. The front wheel-well looks more empty due to the smaller tire, which I can live with cuz of how well these tires ride. But I can't take chances with the ABS brake, or gas milage dropping, and speedometer out of wack!! I don't get it though, is this NORMAL going one size Smaller tire ?? Thanks in advance...

[Mr. Lucky]

The speedometer is controlled by the VSS (Vehicle Speed Sensor, an optical sensor which reads ring gear revolutions in the differential). It's impossible to affect speedo accuracy by changing front tire size.

As for the ABS issue, the 245/40-17 represents a 3.9% difference from the stock front size and a net difference of 7.3% between it and the stock rear size. I tend to think that is not the cause of your issue. Try it when the roads are dry and see if you still have the problem.

[c4cruiser]

If the VSS has gone bad, that may be the reason for the incorrect speedo display and the ABS light coming on. Gas mileage computation as displayed is figured in part with vehicle speed.

If the speed displayed was say 10-20 over what you were actually doing, it's entirely possible that the gas mileage computation was in error. Check gas mileage manually to see how far off the display may be. But if the indicated speed is off, the odometer may be off too.

It would be a PITA, but you can fill up at a station near a freeway on ramp and drive say 100 miles. Pull off at a freeway station and top off the tank and compute mileage using the amount of gas that went into the tank and the distance between the two stations (use exit numbers at the on and off ramps or the mileage on a map)