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Change in Change in Tire Size/Speedo Calib/Lower Fuel Economy

1K views 7 replies 7 participants last post by  Mr.Duck 
#1 ·
I put some Michelin LTX's on my truck just before Christmas and went up from a 245/75/16 to a 265/75/16. Since the new tires have been put on, my mileage the last two tanks has dropped from the mid 16's around town to the mid 11's. We've had a fair amount of snow since the tires were installed, so I've been in 4WD some, but I don't think it was enough to make the mileage drop 5 mpg. I checked my oil and no sign of diesel fuel. I've searched the forum here and seen a couple posts that talked about speedo re-calibration after a change in tire size; do I need to have that checked out? Is there a way to do it myself?
 
#2 ·
The actual difference between 245's and 265's is 3.9%

Stock speedo seting would read 60mph and your actuall going 62.3mph

Everything being the same your fuel mileage should drop from 16mpg to 15.6mpg

I went from 13.5mpg (towing) to 12.9mpg
 
#3 ·
FishHunter;1507286; said:
I put some Michelin LTX's on my truck just before Christmas and went up from a 245/75/16 to a 265/75/16. Since the new tires have been put on, my mileage the last two tanks has dropped from the mid 16's around town to the mid 11's. We've had a fair amount of snow since the tires were installed, so I've been in 4WD some, but I don't think it was enough to make the mileage drop 5 mpg. I checked my oil and no sign of diesel fuel. I've searched the forum here and seen a couple posts that talked about speedo re-calibration after a change in tire size; do I need to have that checked out? Is there a way to do it myself?
Remember that if your using the DIC for your mileage calculations. It will show worse fuel economy as your tires are larger
 
#4 ·
speedo might have not been correct to begin with, too.
Check your speed on the freeway with a GPS and figure out the difference between the speedo and GPS reading.
I know before I put on my new tires, my speedo was showing faster than I was going. Now its reading 10% slower. Which really sucks when you set the cruise at 75 (the speed limit) and your actually going closer to 83.
 
#5 ·
FastCR;1507691; said:
speedo might have not been correct to begin with, too.
Check your speed on the freeway with a GPS and figure out the difference between the speedo and GPS reading.
I know before I put on my new tires, my speedo was showing faster than I was going. Now its reading 10% slower. Which really sucks when you set the cruise at 75 (the speed limit) and your actually going closer to 83.

My speedo with 245's was a couple MPH high, now with 265's its maybe 1.5-2 MPH low (as checked with a GPS).

There will be some difference in diameter between different brands of tires that are the same size ie, 265's. I've figured the difference on mine to be 4% so when I figure mileage I take the odo reading times 1.04.
 
#6 ·
That small tire size change shouldn't do that to your mileage. First of all you need to hand calculate your mileage for awhile, if you are not already, second, be sure you comparing apples to apples. Are you doing the same sort of driving? How much idle time did you have on this last tank vs the other tanks? Did your truck high idle to warm up on cold days? That will suck major fuel when that happens. My old LLY would drop over 2.5 mpg just by high idling in the winter. All that makes a big difference. Idle time kills mileage big time.
You have to eliminate all the variables between tanks of fuel.

Good Luck.
 
#7 ·
Thanks for the replies; I appreciate everybody's feedback.

Here's a little more information:

A) It's been a little colder lately, but I've been using the block heater so that I don't have to drive it before it's warmed up. Mine's an '02, and I don't think the computer can do the high-idle start.

B) I checked my speedo when the old tires were on it, with a GPS, and the speedo read about 2 mph slower than the GPS. I haven't yet checked the new tires with the GPS, but I plan on doing it soon.

C) I'm thinking of putting on the GM grill cover to help keep the engine temperature up. Has anybody used the GM grill cover? I'm concerned how well it will stay on the vehicle.

D) When I check my mileage, I do it by hand using the odometer reading and dividing by the gallons.

E) I haven't been able to get up to my local Stanadyne dealer for more fuel treatment, and maybe with the change in tire size along with the colder weather and the fuel blend change, and no fuel additive, all that might have something to do with it.

F) Other than using the 4WD a couple trips into work, my driving patterns haven't really changed.

Again, I appreciate all your input. Like I said above, I have been using the block heater to reduce warm up time, but I'm hoping for some warmer weather to see if the mileage starts to go up. I live south of Denver, and we've been getting hammered with snow, and I've been having my share of fun driving around in it :driver: !!
 
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