My recent post was complaining about how my mileage dropped after upgrading from 255/70/17 to 265/70/17 tires. According to gear and mph calculators my speedo is off anywhere from 1mph to 1.5 mph depending on speed. I am actually going faster than the speedo reads. Is this little difference enough to make a difference on calculating gas mileage?

 

It is off 1 to 1.5 now.....so was off at least that before in opposite direction. That means as much as you are decreased now, you were closed to that inflated before......

 

Your speedo was already off by about 10% from factory... If you are only exhibiting 1mph to 1.5 mph variances, then you are closer to reality than you were at stock...

 

Does it really matter?

I can understand keeping it in mind that you might be traveling a little faster than what your speedo reads ... especially when the law is watching. But your gas mileage is your gas mileage ... and that's that. It's not like you're going to trade in your truck for a hybrid just because you're getting 13.5 instead of 14mpg.

 

No matter how accurate it was from the factory... It ill still be faster now. So your calculated mileage will be worse... Even if the milage you are calculating now is more accurate.

lets say that average speed is 40mph... your speedo is off an average of .7 MPH... 1.8% according to the tire calc i used.

so... if you go about 400 miles on a tank your odometer reading will be 7 miles lower than it would have been on the old tires. It could be diffrent depending on average speed... because the faster you are going... the more the speedo is diffrent than it was before.

so if you went 80 for the whole tank it would be off by 14 miles..

So to answer your question... no... it doesnt make much of a diffrence.

also keep in mind that the tire calculators are not that accurate... tires of the same size from diffrent manufacturers can actually be diffrent by quite a bit.

A more accurate way is to compare the revs per mile from the tire manufacturers specs... but that is getting a little bit ****...

-stephen

 

No. That is not an accurate means of measurement. The only true way is to measure the tires themselves when they are inflated to the correct pressure while on the vehicle.

 

correct, but if you have your speedo recalibrated by ford, they want the revs/mile for your new tires to update the pcm...