285's and actual mpg

sirhuntalot1 · March 26, 2005

The question would be how much difference in gas mileage when a change is made from 245's to 285's. Truck came with 285's on it so I dont know what mileage difference was b4 that. I get 10.73 mpg consistently 1/2 hwy 1/2 city.

Im guessing and would hope that correct mpg's would be say 1 more mpg. Any thoughts guys???????

Mighty HD · March 26, 2005

I think when i calculated the difference I got about 11 corrected, and 9 un-corrected?

I don't usually pay attention anymore. I know when I pass a gas station, I'd better stop and fill up..lol

BADGERMAN · March 26, 2005

When I switched from 245's to 285's it dropped about 1 to 1 1/2 mpg.

BBSilverado · March 26, 2005

When I switched from 245's to 285's it dropped about 1 to 1 1/2 mpg.

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I gained .6 MPG going from 265's to 285's but my daily commute is 95% interstate driving.

I don't know what the difference was from the 245's to the 265's, I took the 245's off 3 days after buying the truck. I ran with 265's for 18,000 miles at 13.1 MPG and have been running the 285's for 5000 miles now and I'm getting 13.7 according to the DIC.

the speedo was calibrated after each tire change and if my memory is correct they said that the 285's was the largest they could upgrade the calibration for.

builderpaul · March 26, 2005

The question would be how much difference in gas mileage when a change is made from 245's to 285's. Truck came with 285's on it so I dont know what mileage difference was b4 that. I get 10.73 mpg consistently 1/2 hwy 1/2 city.
Im guessing and would hope that correct mpg's would be say 1 more mpg. Any thoughts guys???????

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Actual should be higher mpg because the bigger tires lowers the effective rear end ratio. (numbers just an example) Original tires 4:10 rear end with larger tires the effective ratio may be say 3:90. This is why you notice an acceleration loss with the bigger tires. The problem is most people don't change the speedo calabration so it will appear to be less.

Speedo calculates mpg by tire rotation not by the actual miles.

Larger tires move farther with each rotation so the speedo says you went 1 mile when actually you went say 1.3 miles. It is going to take more gas to go 1.3 miles than 1 mile. If you don't recalibrate the speedo for the bigger tires, any mpg calculations based on a milage will show higher mpg.

Mighty HD · March 26, 2005

I think is what you're trying to say:

The bigger tires make your speedometer go SLOWER. @ 60mph the speedo is 5mph off, so you are actually doing 65mph

Here's the formula you use I dug it up

Let's say you have 10,000 miles on the truck, you put the tires on when it had 1300 miles.

10,000miles - 1300 miles = 8700 miles x 0.07782 = 677.034 + 10,000 = 10,677 actual miles on your truck

There's a size difference from 245's to 285's of 7.782%

So you take the miles @ your last fill-up, miles at the current fill-up subtract the two to get your mileage

then x 0.07782 = difference. Add the difference into your mileage, then calculate the mpg.

Hope I said it right.

Actual should be higher mpg because the bigger tires lowers the effective rear end ratio. (numbers just an example) Original tires 4:10 rear end with larger tires the effective ratio may be say 3:90. This is why you notice an acceleration loss with the bigger tires. The problem is most people don't change the speedo calabration so it will appear to be less.

Speedo calculates mpg by tire rotation not by the actual miles.

Larger tires move farther with each rotation so the speedo says you went 1 mile when actually you went say 1.3 miles. It is going to take more gas to go 1.3 miles than 1 mile. If you don't recalibrate the speedo for the bigger tires, any mpg calculations based on a milage will show higher mpg.

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sirhuntalot1 · March 27, 2005

When I switched from 245's to 285's it dropped about 1 to 1 1/2 mpg.

<{POST_SNAPBACK}>

I gained .6 MPG going from 265's to 285's but my daily commute is 95% interstate driving.

I don't know what the difference was from the 245's to the 265's, I took the 245's off 3 days after buying the truck. I ran with 265's for 18,000 miles at 13.1 MPG and have been running the 285's for 5000 miles now and I'm getting 13.7 according to the DIC.

the speedo was calibrated after each tire change and if my memory is correct they said that the 285's was the largest they could upgrade the calibration for.

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BB when u say they were able to correct for up to 285 tire size (who were they)

I asked the gm dealership if they could do a correction on the tire size the shop said no

sirhuntalot1 · March 27, 2005

well i appreciate the info, that makes me feel a lot better because 10.73 mpg was just not going to cut it then towing a 6200lb trlr wet was getting in the range of 8 to 8.5 thats unbearable

Mighty HD · March 27, 2005

I do hope my math was right..

Mattt · March 27, 2005

The question would be how much difference in gas mileage when a change is made from 245's to 285's. Truck came with 285's on it so I dont know what mileage difference was b4 that. I get 10.73 mpg consistently 1/2 hwy 1/2 city.
Im guessing and would hope that correct mpg's would be say 1 more mpg. Any thoughts guys???????

<{POST_SNAPBACK}>

Actual should be higher mpg because the bigger tires lowers the effective rear end ratio. (numbers just an example) Original tires 4:10 rear end with larger tires the effective ratio may be say 3:90. This is why you notice an acceleration loss with the bigger tires. The problem is most people don't change the speedo calabration so it will appear to be less.

Speedo calculates mpg by tire rotation not by the actual miles.

Larger tires move farther with each rotation so the speedo says you went 1 mile when actually you went say 1.3 miles. It is going to take more gas to go 1.3 miles than 1 mile. If you don't recalibrate the speedo for the bigger tires, any mpg calculations based on a milage will show higher mpg.

<{POST_SNAPBACK}>

I think its the reverse....when you go larger tire, and dont have it calibrated it will show lower MPG and speed. They was MightyHD laid it out is correct.

GM Dealer is only allowed to re-calibrate with a Tech 2 up to 265/75-16. Dealer told me they can ONLY calibrate for a tire size that was available from the factory. No factory built truck came with 285s.

I saw my mileage drop about 1mpg going to 285s from 245s. Mine is anywhere between 10 and 13mpg. However, it may have stayed the same because if you re-calculate your total mileage per tank to compensate for the speedo being off, that could make up for the drop in mileage.

If your odo says you are getting 300 miles per tank and you have 285s(w/o recalibrating the speedo) you have actually gone roughly 323 miles. So, you went more miles on the same amount of gas, higher MPG.

Clear as mud....right?

Mattt · March 27, 2005

I typed in "tire size calculator" in a search engine, and ran the #s. Here is what it gave me.

Tire size calculator

Tire size results

Specification, Sidewall, Radius, Diameter, Circumference, Revolutions, Speedometer, Odometer, Difference

245/75-16, 7.2", 15.2", 30.5", 95.7", 662/mi, 60MPH, 10000mi, N/A

285/75-16, 8.4", 16.4", 32.8", 103.1", 614/mi, 56MPH, 9280mi, 7.8%

http://www.powerdog.com/tiresize.cgi

vansterdam · October 29, 2006

This tire size calculator is pretty good too.

redvett · October 30, 2006

Just remember that not only is the speedo off with a tire change without a calibration change but your PCM,ABS, SIR all are out of calibration. This means you engine HP/TQ, brakes,air bags dont work at the max. You need to get a calibration change to figure out correctly your fuel economy.

kcorley9959 · February 25, 2007

Where can I get my truck recalibrated? I just had 285's put on my 02 z71? I didn't know not recalibrating it would affect so many things besides fuel mileage, and slow starts.