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235/70r16 vs. 245/70r16 gas mileage
http://www.lostjeeps.com/forum/phpBB3/viewtopic.php?f=8&t=32081		 Page 1 of 1
Author:  kayadog [ Thu May 01, 2008 10:45 am ]
Post subject:  235/70r16 vs. 245/70r16 gas mileage
235/70r16 vs. 245/70r16 gas mileage

Anyone have real world (not hypothetical) experience with this? With gas prices nearing $4/gallon every little bit matters.
Author:  tjkj2002 [ Thu May 01, 2008 10:51 am ]
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I can tell you the difference between 225/75's and 245/75's MT/R's,about 1-2 mpg's in town and zero difference on the highway.Used those 2 sizes on my pre-lowered KJ with no lift or any mods besides the CAI and Optima battery.
Author:  tommudd [ Thu May 01, 2008 10:57 am ]
Post subject: 
mine came stock with the 235-70s and then went to the 245-70s in a Firestone Destination AT and really could not notice enough difference in mileage to even talk about. Ride and handeling was so much better though
Author:  DryBones [ Thu May 01, 2008 12:23 pm ]
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I went 235/65/17 to 245/65/17 with no difference.
Author:  kayadog [ Thu May 01, 2008 12:54 pm ]
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If I go with the Grabbers again there is only .3" difference in diamter so I guess it won't make much of a difference. Thanks for the input.
Author:  scoobysnax [ Thu Jul 03, 2008 1:08 pm ]
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I did the exact same thing in the spring of this past year and yes I have noticed a difference. Here's my Gassavers log.

http://www.gassavers.org/garage/viewgaslog/1080

I have tracked every tank of gas since the end of Dec last year. You'll be able to see where I changed over from the very worn out and crappy 235/70/16 stock tires and put on 245/70/16 Destination AT's.

I noticed most of the mpg loss in city driving. I actually picked up a little in highway driving. I lost at least 1.5 mpg maybe 2 mpg in city driving. On the highway in steady driving, especially with cruise control I have gained as much as 1 - 1.5 mpg. I guess overall it's almost a wash at 1/2 a mpg loss.

The biggest factors for this (my opinion) is the added weight. These tires are (I think I can't remember right now) 2 - 4 lbs over the stock tire weight. For each 1lb added to the tire weight it's an extra 10lbs of rotational mass that the engine needs to power to accelerate and the brakes need to handle when slowing down. I do a lot of city driving so it effects me more. I keep the tires at 35 psi. On the overhead computer thing it will read 35/36 when cold and 39/40 at highway speeds warmed up. I'm thinking of trying to bring it up to 38 psi to see how they do. Driving 65 mph is really 66mpg on the gps.
Author:  kayadog [ Thu Jul 03, 2008 2:30 pm ]
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I didn't note much of a gas mileage change (I haven't kept track) but I did lose some power. I'm glad I went with the 245/70's though since it actually looks like a Jeep now, handles and corners better on road, and grips better off road and in snow.
Author:  Liberator [ Tue Jul 08, 2008 3:27 am ]
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Sounds like you may want to get a ScanGauge II to do a comparison.

One of my hope to do projects is to run the various tires I have on the SG and analyze the data...
Author:  sleeve84028 [ Tue Jul 08, 2008 8:00 am ]
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What are the weight differences in the tires? That's were the real impact will come from.
Author:  QuestMan [ Tue Jul 08, 2008 5:13 pm ]
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When I added the Frakenlift, I noticed about a 2mpg drop. I drove that way for about 5 mos., then changed to 245/70s. I lost about 1-2 mpgs more, since I added the 245s. I don't know if the loss is due to the tire change, or if there is another issue I haven't thought of. Last winter, I replaced the O2 sensors, (due to an engine code), but that didn't seem to help the mileage.

I also have noticed a drop in power, since changing to 245s. When I drove my camper up to Mackinaw City, MI, last summer, I was getting about 15 mpg. This summer, I took the same trip. The only difference, is that I have the larger tires. THIS time, I only got 11 - 12 mpg. (Driving the same speed - about 65mph).

Again, I REALLY don't know if it was because of the tires. I suspect that there might be another problem, but there is nothing obvious..............(Oh, I changed the plugs out too, for the record, just to be safe).

The best mileage I've been able to get since changing my tires is between 20 and 21, at 55 mph. Stock, I regularly got 24 - 25 mpg at 55. I always check my mileage when I put gas in the tank, and I rarely have more than a 1/4 tank when I fill it, so I'm not just filling it after 10 or 20 miles, and saying Ha! I got 25 miles to the gallon, all down hill!

Hope this doesn't scare you too much
(I forgot: I had 235/70s originally, in case you were wondering)
Author:  tjkj2002 [ Tue Jul 08, 2008 5:20 pm ]
Post subject: 
QuestMan wrote:
When I added the Frakenlift, I noticed about a 2mpg drop. I drove that way for about 5 mos., then changed to 245/70s. I lost about 1-2 mpgs more, since I added the 245s. I don't know if the loss is due to the tire change, or if there is another issue I haven't thought of. Last winter, I replaced the O2 sensors, (due to an engine code), but that didn't seem to help the mileage.

I also have noticed a drop in power, since changing to 245s. When I drove my camper up to Mackinaw City, MI, last summer, I was getting about 15 mpg. This summer, I took the same trip. The only difference, is that I have the larger tires. THIS time, I only got 11 - 12 mpg. (Driving the same speed - about 65mph).

Again, I REALLY don't know if it was because of the tires. I suspect that there might be another problem, but there is nothing obvious..............(Oh, I changed the plugs out too, for the record, just to be safe).

The best mileage I've been able to get since changing my tires is between 20 and 21, at 55 mph. Stock, I regularly got 24 - 25 mpg at 55. I always check my mileage when I put gas in the tank, and I rarely have more than a 1/4 tank when I fill it, so I'm not just filling it after 10 or 20 miles, and saying Ha! I got 25 miles to the gallon, all down hill!

Hope this doesn't scare you too much
(I forgot: I had 235/70s originally, in case you were wondering)
Going to bigger tires gives you a higher final drive gear ratio,the reason for the sensation of lost power and lower mpg's.The added weight from the larger tires is a factor also,5lbs is nothing big and probally not going to see any change but a 15-20lbs heavier tire you will have a massive drop in power,mpg's,and braking ability.

I went from a 45lbs stock wheel/tire weight to a now tipping the scales at 90lbs wheel/tire rate,can't wait till I mount my 110lbs 35" tire/wheel combo on(at least I'll be geared for them).
Author:  Pote [ Tue Jul 08, 2008 5:38 pm ]
Post subject: 
kayadog wrote:
If I go with the Grabbers again there is only .3" difference in diamter so I guess it won't make much of a difference. Thanks for the input.


just keep in mind that Grabbers are about 25-35 pounds heavier per tire than a stocker or my AT for that matter at 245. (an extra 125-175 pounds extra with 5 new tires) that WILL change your MPG. the size of 235 to 245 wont make the big difference.
Author:  QuestMan [ Tue Jul 08, 2008 5:48 pm ]
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I'm simply observing that others who have made the change to 245/70s, have NOT noticed a significant loss of power/mileage, while I AM noticing both. So, I'm wondering if there is another problem, that I have not realized, i.e., do I have a valve going bad. (That happened on a vehicle I owned before: I am 'light' enough on the gas pedal, that it wasn't obvious to me, until it had gotten very severe).

If you're light enough on the gas, you can actually get almost the same mileage as a vehicle with full compression: to a point. After a while, I passed that point......... :shock:
But it took a while.

Later this week, i'll see if I can dig up a compression checker, for the heck of it, and see. After all, I've got about 7k left on the power train warranty, so it would be a good idea to make sure the engine's OK, before my warranty goes bye bye....... 8)
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