Posted: 2/13/2007 2:09:50 PM EDT
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I have a 2001 Chevy 1500 ext. cab 2wd pickup. I've been disappointed in the gas mileage since I purchased it. I just discovered that while I'm running on P255 tires, the manufacture plate says it should have P235. I guess this creates odomoter/speedometer to read incorrectly. My question is this: Is there a way to tell the computer that I'm on a different size tire so it can adjust the gauges accordingly, or is this purely mechanical and uncorrectable (as long as I keep the P255s)? |
So my mileage issue is probably not an odometer misread, but an actual issue... |
The height number is a ratio. The width is a straight up measurement. I could be wrong, it's happened before (once). |
Right. So let's assume 255/75/16 and 235/75/16. 255 x 75% = 191.25mm 191.25 x 2 = 382.50mm = 15.091 inches + 16 inches for the wheel = 31.091 inches overall 235 x 75% = 176.25mm 176.25 x 2 = 352.5mm = 13.878 inches + 16 inches for the wheel = 29.878 inches overall |
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Tire size calculator To change speedo calibration, find someone w/ a hypertech: www.hypertech-inc.com/gmfeaturechart.html 
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Dude the height of the tire is based on the width in a metric tire. 255 is the width 75 is the height in percentage form 16 is the rim diameter |
Yep that's how I have always figured it out. I have changed tires sizes on my truck a couple of times. I went from 17 inch to 18 inch wheels and never had to adjust the speedo because I figured out, using the math above, what size tires would be the same height. Now when I regeared that's a whole other story. 
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Ok, maybe I could clarify I think the tire size was 255/70/16 and you could get a 235/75/16 in 2wd. I know mine were 265/75/16 for a 4wd. First number being width in mm, second being ratio of sidewall height to tread width (meaning the sidewall height is X% of the tread width), third being rim diameter in inches. Many of the Mercedes I work on have staggered width tires, and a popular size combination is 225/45/17 in front with 245/40/17 in back. This keeps the tire height nearly identical front and rear. |
You can't use someone else's Hypertech or any other brand of programmer. Once used on on vehicle they are vin locked until you return the vehicle back to stock. You can't do any kind of tuning or adjusting unless it is the vehicle it is vin locked to. The only thing they can be used for on numerous vehicles is for diagnostics, reading trouble codes. |
Can a mechanic with do this then? |
Or lets assume 235 X 75.............. vs 255 X 70............... The difference is.......................... 2mm |
Or it could be a 255/85/16 and it could be 4 inches bigger. All I am saying is that you just can't make a blanket statement that there is no difference. My only point is that there could be a large difference. |
There could be a big difference, or there could be virtually no difference. I believe many car makers set their tires so that multiple configurations all have about the same diameter, so they don't have to adjust the speedo because of the standard wheel/tire combos available. |
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5.7 or 5.3 liter engine? There are a number of things you can do to improve milage. New plugs and wires, cap and rotor, synthetic oil, throttle body spacer, cold air box, cat-back exhaust. Make sure you have the right thermostat in it. My (older) 5.7 Suburban will get anywhere from 17-20 on the highway (depends on weather, road conditions, speed, etc.). |
5.3 liter. I'm averaging about 15.5 |