Hey folks, I've looked through the forums, done Google searches and read many articles, but can't get a definitive answer. I want to do the right thing here. I have a 23' Stingray with about 2700 miles on it. I have the FE1 suspension, which as you are aware is not height adjustable and I HATE the wheel gap. The easiest fix was bigger wheels and tires, which is what I did, however, the tire circumference is 4.8% larger in the front (255/35/20 fronts, 325/30/21 rears) where I understand the measurements come from? Based on what I Googled I think the odometer and speedometer will be the same 4.8% off, meaning when the car hits 10k miles it will actually have 10480 miles on it. I did read an article that says the newer digital odometers in modern cars do not read from the front wheel, they use a different calculation so it will still be accurate, but I don't know the right answer? I don't want a car that has inaccurate mileage so if that's the case I can lower the profile of the tires to get within less than 1% of stock like everyone else or go back to stock wheels and lower the car with lowering springs to get rid of the wheel gap. Obviously getting lower profile tires is another $2k I have to suck up.........the stock wheel option and lowering springs (I've already ordered them) are the much cheaper option. However, if odometer mileage is unaffected and accurate then I'll stick with the current setup (car drives absolutely perfect, zero issues). Any technical info on how the odometer/speedometer actually works (please no guessing) will be extremely helpful. Thanks.

 

There are numerous "speed sensors" in the car. There is something called a "B115 Vehicle Speed Sensor" VSS. I think, based on a statement attached from the Service Manual, that the speedometer reading comes from the VSS. And, think, based on another statement attached, the VSS is actually coming from the crankshaft speed, then it must be calculated based on the overall ratio to the rear wheels. There are also Wheel Speed Sensors WSS, on on each wheel. These, I think, are primarily for the purpose of stability control. There is even a DTC that can be set if there is a discrepancy between the VSS and WSS sensors.

Different sized front tire should not affect the speedometer and odometer readings, but could potentially result in a discrepancy between the expected wheel speed and the vehicle speed. No one has reported any issue with that. It boggles the mind. But say the rear tires have not been changed, but the front tires are larger. Then the for a give speed (which is determined correctly by the rear tire which have not been change yet), the front tires will appear to turn slower than expected. But now, make the rear tires larger too. Now for a give vehicle speed as calculated from shaft speed, the car is actually going faster than expected. But now the front tires are spinning a little faster too, so it might be a wash when all the tires are increased in size proportionally.

As for actual vehicle miles - thats another interesting question. Yes with larger tires the vehicle will have traveled further than calculated when based on the engine RPMs. But as far as all the internal shafts, gears, every mechanical part, they "traveled" the same distance. As in your example, if the car reads 10000 miles, but the distance it went because the tire circumference was larger was actually 10480, all of the internals still went 10000 mile (though at maybe a higher load). Does that make the mileage inaccurate? Does it affect the maintenance intervals? I do not know.

Now, I mentioned there are other speed sensors in the car. There are a some internal to the transmission and the output shaft - probably more that I overlooked. But regardless, the speed of the car is going to be calculated based on an assumption about the circumference of the rear tires. The larger tire will cause the car to go further than calculated, and its actual speed will be higher than the speed on the speedometer.

 

Go through one of those radar things they set up on the road and see what your actual speed is compared to what they say your speed is. Or if you have newer radar detector you can view your actual speed by it versus your speedometer
Or take it to drag strip and run a quarter mile. lol

 

GPS speed is not bad either as a way to check it. On my older cars, where I have replaced the bias tires with radial tires, the circumferences are different. I use the speed indicated by a Nav app on my cell phone to check the resulting speedometer/odometer error. I can use it in my C8 to validate that it is reading the correct vehicle speed. In fact, in the C8, with Waze on the screen via CarPlay, I can see the match between by GPS speed and the DIC speedometer. So, a disrepancy with larger tires should show up that way.

 

Great info folks, appreciate it. I kept asking Google the same question different ways and it came up with this: The C8 Corvette calculates its odometer reading based on the entire vehicle’s movement, not specifically from the front wheels.

This jives with what Andybump posted. Side note regarding ratios. The "ratios" front to back are within 1% of the stock ratios, meaning the size upscale ratio front/back from stock to my setup is less than 1% and the difference in size front to back is also within 1% of the stock size.

I can't find a single instance of anyone having an issue with the car with larger wheels/tires, but it seems like 98% of folks with 20"/21" setups have the lower profile tires to match the stock size. As I stated previously the car runs absolutely flawless, has never hiccupped ever, never thrown a DTC, zero rubbing and looks great with the bigger wheels/tires. Am I the only one that has this setup? There are LOTS of FE1 cars out there, I can't imagine it's just me?

Thanks again, this is why I subscribe to this forum!

 

With regard to the front and back tire circumferences (which are proportional to the 19 and 20 inch wheel sizes), the car would expect that front tire rpm= 1.05263 times the rear tire rpm.
If the tires are changed to 20 front and 21 rear, the actual difference between front and rear rpm is 21/20=1.05. This amounts to an "error" of less than 0.25 percent with respect to the expected rpm. that is (.00263/1.053)x100%= 0.25%. Pretty small, and apparently not enough to trigger a DTC. And no-one has ever reported an issue with that. And, you are right, if a different profile is used, the circumferences themselves change and the difference might be near 0. I think you can find the actual circumference for most tires. Sometime expressed as rotations per mile. So you don't need to guess about that.

 

Awesome, I did the calculations based on the actual circumference of stock tires vs current tires (Tire Rack shows rotations per mile for all tires) and the "error" is 0.55%. Thanks for all the help, it's been very educational and I now better understand what I'm dealing with.

 

Side note, this is what Google came up with for the speedometer indication: The C8 Corvette determines the speed displayed on the speedometer using information from the output shaft speed sensor or by calculating it based on engine RPMs and the current gear.

 

Yes. The service manual seems to say its crankshaft rpm but its not real clear. And there is an output shaft speed sensor as well. With either source, its calculated based on gearing to the rear wheel, and includes an assumed circumference for the rear tire.

 

Find a straight stretch of Interstate highway and use the mile markers for a few miles. Don't use any near an interchange, they may be shifted if near a ramp. Use these to determine your speed.