# Need help figuring new MPG

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#### Ramblin Fever

with the new tires.

Stock tires were P225/75 R16 = 29.6 diameter

New tires are LT265/75 R16 = 31.8 diameter

That equates to a 8.1% difference.

Any examples would be great.

Ramblin',

I was going to post the smarta\$\$ answer, but decided against it, but you don't have enough information to do the calculation. What you'll need is a graph showing the fuel consumption vs rpm curve at various part throttle conditions.

But in some respects you might not experience any better fuel economy, because the larger diameter tire also has more mass and therefore more rolling resistance. Along with the more mass also comes more inertia, which means that more energy is consumed during accelleration.

Hope this helps.

it may not affect your mileage at all (but most likely will drop it for reasons stated above.)

what it will do is affect your speedometer and odometer accuracy. The change is going to be relative to the change in circumference of the tires. which is for all practical purposes relative to the diameter change, or about 7%. Both your odo and speedo will read lower than you are actually travelling.

It's a 7.43% increase in tire diameter if the diamaters you gave are accurate.

Kinda what CR and Ken said. My guess is that there will be a -small- improvement in fuel economy. Whatever happens, the difference won't be big.

If you don't compensate for the new odometer error in your calculations you'll think you have a loss.

Use the mileage signs on the side of the road to get actual mileage, then compute MPG and indicated vs actual. Good math problem.

quote:

Originally posted by Ramblin Fever:

So, basically before yesterday's fill-up, I logged 202 miles (I fill up at the half-way mark) - can I just do 202 x 7.43% = 15.0086

So, actual mileage would be 202 + 15.0086 = actual mileage??

Thank you!

You speedometer will be far enough off that you should be thinking about it when you are might be around traffic cops.

Multiply 1.0743 times your speedometer reading, like 65 speedomoter x 1.0743 = 69.8 which is basically 70.

If you just add 5 mph at highway speed and 2 mph at city street speeds you won't be far off of your old readings.

quote:

Originally posted by Ramblin Fever:
So, basically before yesterday's fill-up, I logged 202 miles (I fill up at the half-way mark) - can I just do 202 x 7.43% = 15.0086

So, actual mileage would be 202 + 15.0086 = actual mileage??

Looks like everyone missed the actual question!

Yes, that's the way to do it.

quote:

Originally posted by rpn453:

Looks like everyone missed the actual question!

Yes, that's the way to do it.
[/QUOTE]

Great - thank you much

Yes, the diameter's are approximate via the two sets of tires from Tirerack.

1st set was BFG Long trail; this set is Yokohama Geolanders.

So, basically before yesterday's fill-up, I logged 202 miles (I fill up at the half-way mark) - can I just do 202 x 7.43% = 15.0086

So, actual mileage would be 202 + 15.0086 = actual mileage??

Thank you!

quote:

Originally posted by rpn453:

quote:

Originally posted by Ramblin Fever:
So, basically before yesterday's fill-up, I logged 202 miles (I fill up at the half-way mark) - can I just do 202 x 7.43% = 15.0086

So, actual mileage would be 202 + 15.0086 = actual mileage??

Looks like everyone missed the actual question!

Yes, that's the way to do it.

Yep, that is correct.

More than likely you'll note a drop in mileage with that big of a jump in tire size, and also with the change to an LT tire. The larger diameter doesn't offset all the extra weight the bigger heavier tire brings along with it.

well, the question was cleverly hidden.....

Actually I got a 2 mpg increase this week; however, I'm seriously thinking about going to the 265/70's instead.

BigOTire & Yokohama give you free 30 day test drive.

I love the height of these 75's, however they weigh 50# each versus 38# of the 70's. More leary of wear/tear on the transmission although there are LOTS of people running the 75's on the 3.2L V6.

quote:

Originally posted by Ramblin Fever:
Actually I got a 2 mpg increase this week; however, I'm seriously thinking about going to the 265/70's instead.

BigOTire & Yokohama give you free 30 day test drive.

I love the height of these 75's, however they weigh 50# each versus 38# of the 70's. More leary of wear/tear on the transmission although there are LOTS of people running the 75's on the 3.2L V6.

Don't worry about wear because of tire weight. You have gained 48 lbs of tire, which worst case would be about the equivalant of 100 lb weight in the truck. About like picking up a real skinny girl.

Unsprung tire weight adding the equivilant of 8 times unspring weight is just unfounded urban (maybe rural) legend.

one thing you may have to worry about with the trans is it lugging. with a taller tire you are numerically lowering you effective gearing which puts a little more strain on the trans. if you have a manual dont worry about this but if you have an auto you may be better of with a little shorter tire. with the 265/70/16 you will have about a 1.4 in height increase over the stock 225/75/16 tires compare to a 2.4 in increase with the 265/75/16. it may not be a prob at all all.

quote:

Actually I got a 2 mpg increase this week; however, I'm seriously thinking about going to the 265/70's instead.

This isn't surprising. Because as Racer put it,

quote:

with a taller tire you are numerically lowering you effective gearing which puts a little more strain on the trans.

Thus you are turning lower RPMs than before at the same speed. This can result in a MPG increase, of course this depends on many factors such as the additional load placed on your engine as it now has to operate in a lower RPM range than was intended. Sometimes this can offset any MPG increase gained from lower gearing.

Just drove my 1st 100 mile trip with the new tires - very comfortable, but they're placing too much load against the gearing of the truck. It had a hard time staying in Overdrive on fairly mild inclines - something this truck's never had a problem with before.

See I ran 225/75's for the 1st 75k miles, then 245/75's for 71k miles, thought I'd like to switch to 265/75's - but it's not gonna cut it, think I'll resort to the 265/70's instead.

I like the wider tire, gives it better stability, and I like the looks of the taller tire; however I live in the Rocky Mountain region and if it can't hold overdrive on a mild incline, it's REALLY gonna bog it on Loveland Pass with a few hundred pounds of camping gear and a boat, even in 3rd gear, I can tell that it would have to work too hard.

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