nfb: tire guys inside.....

[LoudLT1]

ok,
i replaced my daily drivers(95 chevy lumina v6) tires about 2 weeks ago. went from 205-75/15's to 215/75/15's. they are a taller tire. i went from getting 475 miles a tank full to 400. what gives
eddie

[94 formula]

ok,
i replaced my daily drivers(95 chevy lumina v6) tires about 2 weeks ago. went from 205-75/15's to 215/75/15's. they are a taller tire. i went from getting 475 miles a tank full to 400. what gives
eddie

How much taller are they. A taller tire causes your speedo to read lower than you are actually going which would cause less miles put on your odo.

[Trash man]

y didnt u drop the sidewall to a 70 series? woulda been about the same height i.e. 215/70R15 = 205/75R15 tire

[LoudLT1]

just by luck ran accross the set of four new tires for $70 dlls. couldn't pass them up
eddie

[Trash man]

well then i stand corrected sweet heart deal

[Joe K. 99 Zeee!!]

You might want to calculate how far your speedo is off or else you could inadvertantly attract the attention of the highway patrol. Trust me, I've been there. Officer said he wouldn't have pulled me over except that I didn't slow down when he got behind me. I was clueless!! I had miscalculated my tire diameter when I reprogrammed my PCM!!

[wolfdogs]

its your odometer reading because of the taller tire. im at home and dont have my Tire & Rim handbook or i could give you the demensiona diff. got ot www.tirerack.com, they have a chart there.

[94 formula]

If you take it to the dealership they can change your speedo for you.

[LoudLT1]

how much can the speedo be off ?
really dont need a ticket right before the Holidays
eventhough i really never speed with my lumina...
peolpe tell me it looks like a sop car
eddie

[Joe K. 99 Zeee!!]

Here's a good estimate. Multiply the width by the height ratio (ex. 215*.75). Double that number, convert it to inches and add your wheel diameter (15 inches). That's a good estimate of the diameter. Divide the current tire diameter by the previous one and you should get a percentage.

That percentage is your correction factor. Multiply that percentage by the speed your traveling and you'll get your actual speed. For example if you got 1.10 (110%) than your actual speed is 10% greater than your speedo. If you're traveling 60 on the speedo than your actual speed would be 66mph.

Someone correct me if I've gone awry at all here, but this is how I did it till I got my speedo corrected till I fixed the tire height in the PCM.

If you have a gps than take that with you and see how far your off. Or you could time how long it takes to go a mile, but you'd have to keep a constant speed...very difficult with hills or traffic. After the cop left, I timed for a mile, but it took a few tries to get a good run. At 60mph you should finish a mile in 1 minute.

[ReBop]

I converted my CK1500 pickup tires to 33" Coopers and my speedo was off by 10%. That means when it read 70, it was going 77! That'll get you a ticket in my state. So, I just rain 63 mph and had no problems. If you don't reprogram, just do the calculation and keep tabs on the speedo when you're driving.

Eventually I returned to stock tires.

[RangerBob]

determiining your speed

[Injuneer]

The difference using the calculation is 2.13%. At a true 60mph, your speedo will read 58.7mph. When you drive 1,000 actual miles, your odometer will only indicate 979 miles. The "error" is not enough to account for your mileage drop. The 400 miles you got on the tank is actually 409 miles.

In general, "miles per tank" is not very accurate. Miles driven between refills / gallons in the tank for the refill is the only correct way to calculate it. Then you need to look at the results over several tanks, to allow for variations in how "full" the tank was. Then allow for the "highway" vs. "city" miles driven, any significant changes in ambient temperatures and possible changes in gasoline composition - e.g. increased oxygenates for winter blends, which lower mileage.

Are the new tires correctly inflated..... that's worth an MPG or two? Do the new tires have a more aggressive tread, or do they have a poor temperature rating, wich would indicate excessive "squirm" (maybe Ken can correct the terminology on that one)?