I've been doing some research trying to find out if I get 245/75/16 BfG AT K/O tires (bigger than stock) will my gas mileage go down. So far I have found a lot of conflicting information.

Some say that the increased tire weight will throw the ratios off and therefore cause a decrease in gas mileage as the engine needs more torque to move the truck. Furthermore, I've read that since the truck will be higher off the ground, it is thereby less aerodynamic. Thus, the bigger tires will decrease MPG.

However, others say that since the tires will be thinner than the stock tire then there is less rolling resitence. Also the tires have a bigger diameter, and therefore you move further per revolution than the stock tire. As a result, you would get better gas mileage.

So which one is it? The truck is my daily driver in addition to my weekend toy so this is actually really important to me. Thanks for the input.

 

My opinion is that the taller tire gives you better mileage. It would be like adding taller gears. If you think that withextremelysmall tires,at the same rpm you won't be going anywhere.So with a taller tire you would get more bang for your buck so to speak. Go too big and your engine will not be able to handle the load though. A wider tire would just add more weight and resistance too. So, tall and skinny tires; just like a rover should have. I had the BFG AT's on my Jeep and went up in size with the same tire. I think my mileage got better, but I can't confirm it. Remember, most people with bigger wires also have a much more aggressive tread, which affects the mix too.

 

Data point for you. I just put on 245/75/16 BFG T/A KOs 2 weeks ago after having the OEM Michelins on there since I bought it. I've only run one tank of gas through (filled up right when I had the tires put on) and it looks like I lost about 2-3 MPG overall. I went from an average of 16 to 13.5 on this last tank. It's one small data point and I had my foot in it a bit more than normal to make sure the tires were balanced for up to 85 MPH, but it's the lowest mileage I've recorded since I've owned the truck.

For me, it's a fair trade off. I wanted the taller tires so I can have a tad more clearance on the trails, and a more aggressive tread. They also wear like iron.

Cheers,
Dave

 

Dave, how are you measuring your mpg? How do you come up with a figure as precise as 13.5?

May sound like a dumb question, but I honestly want to know--I'm trying to figure out what sort of mpg I'm really getting...

-Andrew

 

Since the truck needs more torque to move the bigger diameter tires, m.p.g goes down.

But alot of people forget to multiply the recorded mileage by the percentage of the tire difference. 245/75-16 are about 5% bigger than the original 29 inch tires. Therefore, you need to multiply the recorded mileage by 5%. So if you've gone 300 miles on a tank, you really went 315.

With this said, I have found that when you upsize your tires, using sport mode on the tranny allows the engine to rev higher before gear change, and somewhat mimics a lower axle ratio ( numerically higher)from 1st to 2nd, from 2nd to 3rd and from 3rd to 4th.

From a dead stop to 1st gearthe engine has a harder time to move the truck since the bigger tires augments the final ratio (numerically lower). But on the other end, in 4th gear on the highway, having a higher ( numerically lower) gear ratio help the engine by not revving as high and therefore is better on gas.

 

I think the easiest way to record how many miles you are getting is to fill her up making sure record the mileage on the truck or reset the trip counter. Then run the truck down until the gas light goes on and fill her back up again recording the amount of gallons that have gone into the truck and also the mileage at that point. Then all you have to do is subtract the beginning mileage from the end mileage and divide that number by the number of gallons you put in. This will be your average over the course of that tank. (it is probably a good idea to do this a couple of times and average those numbers to get a more acurate reading).

Hey Geotrash I was just wondering if you factored in the miscalculations on your odometer in your data? As far as I know the tire size can affect your speed by more than 5mph on the highway, so in effect, creating a wrong reading with the car's mileage count (meaning you went farther on the new tires than the truck believed you did). I think a GPS would help identify any miscalculations. Let me know what happens.

Thanks for the help

 

I've heard that before Darover............ I would be nervous running sport mode all the time however............. do you think it puts added stress on anything?

 

UOer the years and being a member of many Rover clubs, I have seen this come up many times. Bottom line, the 245 will cost you between 1/2 and 1.0 MPG depending if you measure your mileage in town or at highway speeds.
I lost a under 2 MPG in town and 1.5 MPG at high way speeds, when I installed my 265's.
Needing the taller tires, I switched to 4:11 gears and regained all but about 1/4 MPG in town.
The up side of 245's is that larger side wall that will give you a much better ride and more support off road.

 

Mike, do you have more pictures of your truck somewhere online? Your avatar picture looks great, and I'd love to get a better look at the whole rig...

 

E-mail me and I'll be glad to send you some.
mikeljacobs@comcast.net