Hey Guys,

I just put some larger than stock tires on my 04 F-250. I went from 265/75/16 to 285/75/16. As a result of this tire size increase my speedometer is actually spot on now as I expected from previous posts, however I noticed that my gas mileage when calculated by hand (by dividing miles driven from the odometer by gallons of gas) is dramatically decreased.

While this is not unexpected I have dropped 2-3MPG and I suspect it's because the odometer is not reporting the accurate distance traveled is that correct? Another reason I find that odd is this past weekend I just went on an all highway road trip over flat terrain and I would have actually expected better MPG given that my final drive ratio is now lower than it previously was (Cursing at 1800 RPM's vs 2000RPM's the entire way).

Am I way off base, or whats the deal?

 

Mike i just put 285 70 17 on my 05 250 410 ls auto with the 5.4. Was running goodyear silent armers in the stock 265 70 17 size. 2 days latter i had the dealer adjust my speedo and it is now dead on and so id odermeter. I think the truck is burning more fuel just by pulling the bigger heaver meats around. they are the goodyear duratracs wich are almost as agressive as the mtrs. by the revs per mile i went from 410 to about 400 which should help on gas but big meats will hurt u in the millage department. Its only gas and the world has way to much of it.

 

Mike, two things are going on, here.

1) With the previous tires, the odometer was clocking more miles than you were actually driving. This makes it seem like you're getting more miles for every gallon.

2) With the current tire, it's reporting accurately (apparently...did you check it against a GPS?). If your speedometer is dead-on, then your odometer is necessarily dead on as well.

Normally this is the other way around, in that the speedo is correct with factory tire sizes but then under reports with larger tires, so that if you're actually going 70, say, it's only it's indicating 65, which makes it seem like you're not covering as many miles as you really are (and therefore are going fewer miles per gallon).

If the speedo checks out as now exactly accurate, then let the computer figure out your gas mileage, as it will be much more accurate than trying to figure it out from fill-up to fill-up. Whatever it says should be the correct mileage.

 

FireKite,

Thanks so much thats the exact answer I was looking for although I can now say I'm not happy with the results, but none the less it's correct.

To answer your question yes I have verified it with my GPS unit and it's dead on accurate with the larger tires now. I've read several accounts of 99-04 trucks running 265/75/16 tires upgrading to 285/75/16 tires and having the same thing happen. I knew my speedo was reading high with the 265's too and that was verified by GPS.

That said I am now officially disappointed with my V10's mileage.

 

If your spedo was off before you changed the tires, were you taking that into consideration with your mpg calc? If you didnt adjust the calibration and its dead on with the larger tires, then with the original tires you would have got an inflated MPG figure unless you reduced it by how much the odo was off.

On my 150 I went from 265/75/16 to 285/75/16 and even with an underpower and overworked V6/3.55gears my MPG stayed the same after I calibrated it. I picked up 20 more pounds per corner too. Everyone always reports a lot more loss with the SD's which surprises me seing how the change would be more insignificant since the SD already weighs so much more, the engines are so much more torquey they dont have to work much harder, and a lot more rotating mass in the driviline already... but I guess I'll find out since I plan on putting on some 35's.