255/55x18 tires hurt gas mileage

[2005rtmag]

I switched from 235/55x18 Michelin MXV4s to 255/55x18 Michelin MXV4s recently. I checked my mileage today on a trip on the interstate today. It was a pretty constant 80 mph most of the way with some 75mph. It checked 21mpg. I've never checked it and found less than 24mpg before after installing the Superchips program. I used the program to change the tire diameter to correct the speedometer. I'll check it again a few times to make sure, but it ain't looking good.

Otherwise, I am happy with the larger tires. I like the looks and the handling. Both sizes are quiet and smooth.

[Popeye's Hemi]

Do you use the EVIC to calculate the MPG? Try using good ole Math and see if theirs a difference. I did not use a tire size calculater to see the diff in the two tires diameter but if there is it could be the cause. Also a wider tire has more contact area and will cause more rolling resistance. Air pressure along with tire material. Remember a shorter diameter tire will increase numerically your effective gear ratio and in turn effect the MPG.

[TB107]

I would first go for a nice drive on an interstate and make sure that the mileage travelled corresponds to the mile markers alongside the road. I remember when I had my Superchips, the number I had set it to did not correspond to the mileage travelled using the method above. Once that is all settled, then reset your EVIC and recalculate.

As well, maybe your tire pressure is set a tad too low. Try increasing it a bit and see if that makes up for the difference. Not sure it will make for a 3mpg difference but my guess is it more a result from the larger diameter...

[InferAl]

Sometimes you don't realize it but traveling in on direction may be a slightly uphill grade and the other is slightly downhill, maybe not even enough to notice but it will effect MPG

[FloridaRT]

Check your tire pressure, I run 36 PSI in my 255x55x18 tires.

Bob

[2005rtmag]

Tire pressure was 35 psig in the 235's and 255's. With both sets, I adjusted the tire diameter with the Superchips using the measured diameter and also made sure the speedometer agreed with my Garmin navigator gps. There was an elevation change since where I started was at 20' above sea level. And, I don't have the electonics package, so I don't have the mpg readout. I used the old fashioned method. Checking the mileage on about 1/2 a tank is not but so accurate, so maybe that and the elevation change explains some of it. I'll check it again on the way home Thursday night.

[redinorange]

I just replaced BFG KDWs with new Sumitomo HTRs and got a 3MPG bump in addition to a quieter and smoother ride. I'm not surprised that there is a difference...

[formerice]

You might have got some bad gas with more ethanol than the 10% allowed by law. I've heard of folks getting gas tested and finding up to 30% ethanol.

[Ron380]

X2 on checking the tire pressures. I've found some tires to be quite picky about what pressure you put in them- in terms of mileage, comfort and grip! Experiment with them for a tank or so, and see what they "like" the best! ;)

[2005rtmag]

The gas I bought was from Sam's Club, 93 octane. I'll check it again a few times and confirm. And, I drive it to work every week, 340 miles from home, so I check air pressure every couple of weeks or every week. The max pressure rating is 51psig, so maybe I should try a higher pressure. I don't want to go too high with only a 7.5" wide rim. Don't want the tires to wear in the center.

[Fondy]

what are miles per gallon....??? LOL

I didn't spend this kind of money on a car to be picky about MPG. don't get me wrong I want good MPG but with a HEMI comes some compramises.

call me stupid I am. LOL but I guess I know I should get around the 20-24 range but I have never checked to see how I am doing.

good luck with all your calculations.

[blackmagnum06]

ill take the tires if you dont want them lol i can trade for my stock tires.

when i had a set i never noticed the mpg

[2005rtmag]

what are miles per gallon....??? LOL

I didn't spend this kind of money on a car to be picky about MPG. don't get me wrong I want good MPG but with a HEMI comes some compramises.

call me stupid I am. LOL but I guess I know I should get around the 20-24 range but I have never checked to see how I am doing.

good luck with all your calculations.

I understand what you are saying. But, the handling was very good with the 235/55x18 MXV4's. There is improvement but not enough improvement to justify 3mpg. Everything is a compromise. If performance was the most important thing, I'd trade it for an SRT Challenger.
That is what I like about the 5.7 hemi. It's a great compromise between haul butt performance and economy.
And the Magnum will haul my family in great comfort, carry lots of cargo, and it pulls my 3k lb boat. Can't beat it.

Just wanted to inform everybody what to expect if they put the big tires on.

[2fast mag]

Those tires are about 1" bigger in diameter, so you are travelling almost 3% farther than on the old tires, should account for almost 1/2 of it.

[2005rtmag]

Those tires are about 1" bigger in diameter, so you are travelling almost 3% farther than on the old tires, should account for almost 1/2 of it.

Evidently, you didn't read the whole thread or you would know that I adjusted the speedometer by changing the tire diameter in the computer with the Superchips program so that it agreed with my Garmin GPS with both sets of tires. So, the change in diameter was not the reason. I need to check the mileage several more times to make sure I have an exact reading. After that I plan to bump the pressure up to 40 psig to see if that helps. Up to this point, I,ve inflated both sets to 35psig which was the maximum for the 235s. The 255s are rated 51 psig max.

[jg]

If you're looking at highway FE, the width of the tires/wheels has a significant effect on aero. The wider the wheel/tire, the larger hole you're punching in the wind. Also, tire compound, and the actual shape of the section of the tire have effects on aero. If there's any sharp edges and or a lot of bulge in the section to the wheel face, it's not going to help FE.

An inch in the O.D. of the tire is changing your gearing also. Not good for FE upon acceleration, nevermind the added weight.

3mpg is a big difference! When I swapped my factory 18's for SRT8 20" wheels/tires, I didn't experience a 3mpg difference!

[RT NOMAD]

IMO, don't rely on the EVIC mpg's accuracy. I measure gas mileage the old fashion way. To be as accurate as reasonable, you have a few variables that you need to minimize their variation.
1. Different tire size - different diameter, how accurate is the odometer? then vs now. It may be difficult to verify with the previous tires.
2. What is the accuracy of the Superchips electrical wheel diameter adjustment? + or - ?
3. Gas quality - same grade of gas (octane), same gas station should minimize the variation
4. Gas quantity - fill up to "click" - same gas pump, same time of day, minimal ambient air temp difference.
5. Winter gas blend vs summer gas blend - mileage will vary greatly depending on whether you're running on winter blend or summer blend (I found about 1 to 1.5 mpg difference - worse in summer. note: I did not use my AC during the summer - 2000 Olds Intrigue)
6. Driving habits must be consistent, and driving routes must be consistent
7. And check your tire pressure often - keep them at a set pressure

Complying to all these will be impossible for most all people "with a life".
Calculate average mpg per fillup: trip odometer miles divided by fillup gallons
The more data points (full tanks of gas), the greater the confidence in the calculated average mpg having minimal error. If after 10 fillups your average mpg varies no more than + or - 0.5 mpg, you probably have a reasonably accurate mpg for the conditions you adhered to. A statistical program would give you an empirical confidence interval for the average mpg.

If all this garbage seems unreasonable (which it truly is), disregard this post.

[2005rtmag]

Thursday night I drove back from VA and checked my mileage again. I was coming down hill to some degree and I didn't use the ac for a while. I averaged 21.6 mpg driving the whole time mostly in 75 and 80 mph zones. I was running 10 over as much as possible. There were a few miles of 55 and 60 mph zones. I topped off the tank to make sure it was full since I immediately started the car and got back on the road. I checked the speedometer reading against the Garmin GPS again and it is right on.

I need to pump up my tires to 40psig from 35 and see what that does. I know the difference is mostly the increased rolling resistance.

[Fondy]

That is what I like about the 5.7 hemi. It's a great compromise between haul butt performance and economy.
And the Magnum will haul my family in great comfort, carry lots of cargo, and it pulls my 3k lb boat. Can't beat it.

I could not agree more!

[2005rtmag]

Today, I pumped up the tires to 45 psig (51max rating) from 35 psig and drove back to VA. Gas mileage checked 21.6 mpg. Same as at 35 psig. Tires took 3 mpg away.

Everybody should know this when they are considering buying 255 tires.

[bazetthound]

But lets not forget the LCF (Look Cool Factor). 255 tires simply look way better than stock. :beerchug: