How much does tire size affect MPG???

[One Awesome Inch]

A friend of mine has a great deal on some 16 inch tires he can hook me up with. The problem is my rims are 15 inch and they won't fit my van unless I buy new (used) rims. I'm debating whether I want to keep the ones he is going to get for me and buy the rims etc.

I read somewhere that larger tires will get you better MPG. How much difference will 16 inch tires give me versus 15 inch tires in terms of MPG?

 

[LeeRevell]

A quick Google search turned this calculation method up. Of course, tread design, rubber composition and tire width (contact patch size) will be variables too.

http://www.gitout.com/wpblog/bigger-tires-and-gas-mileage-how-to-determine-actual-gas-mileage/

 

[One Awesome Inch]

Thanks Lee.

Upon further reading I have found that larger tire sizes are actually worse MPG. This is because taller sizes are often wider and therefore have more rolling resistance. Also, a smaller tire takes less power to turn in stop and go city driving. Also, a larger tire is heavier thus worse on MPG.

I actually thought a larger tire would have increased MPG because if you look at different bicycle wheels, roadbikes have taller tires and less rolling resistance than the shorter mtn bike tires. I was wrong.

Larger tires are better in MPG only for highway driving where the momentum of the vehicle is more constant and therefore the engine does not have to burn higher amounts of fuel to turn a heavier tire.

 

[highdesertranger]

just a little tip here the sizes you are referring to are rim sizes 15, 16 inch are rim sizes and have nothing to do with the tire size. you can have a 15" that is larger than a 16" and vice versa. it's how tall a tire is that affects your gear ratio which in turn affects your mpg. it all depends on your final drive ratio(tires included) and how you drive that will determine your mpg. highdesertranger

 

[One Awesome Inch]

Agreed.

 

[masterplumber]

There are many factors to consider. if the tires were extremely short to begin with, or the vehicle very low geared, then going slightly larger can increase mileage. but if the gearing is already high then the opposite can be true. Also, be aware, that when you change tire size it affects the speedometer reading, so the milage you measure may not be accurate. Like most things mechanical, you have to take the system as a whole, and balance needs - ground clearance, efficiency, towing and hauling power, etc...

 

[Canine]

Don't just consider mpg, there's load capacity and handling too. Most 15 inch tires are P series while you can get LT or "truck" tires that are load range D (8 ply rated) or E (10 ply rated) with your 16 inch rims vs a passenger car tire that is 4 ply rated (or a load range B).

The stiffer sidewalks on the LT 16 inch tires can improve handling (roll) considerably. If all you ever do is run on pavement and you aren't pushing the weight limits (GVW) on your van than P or PXL is ok, however the heavier (stiffer) sidewalks found on the LT tires are beneficial, especially for anything off smooth pavement. Lots of factors to consider...

 

[Canine]

Use what the Automobile mfg say to use.There is a reason they have hundreds of engineers on their payroll.

 

[Canine]

Oh, and for some tire aspect rations vs height, you need to see if you have fender well clearance too. A tire shop can tell you pretty quick.

 

[SternWake]

I have a slightly larger tire than stock.

30x9.5x15 vs the stock 235/75/15 so diameter when up 3.8%

My around town MPG took a noticeable and obvious hit 10 to 15%. Highway appears to be about the same, but it is hard to really tell as the odometer/speedoometer was fast but now reads dead money with GPS and radar signs.

Most Speedos read fast. by 2 to 5mph

Bigger tires also reduces your braking as the lever is larger the brakes must fight, not to mention the tires are heavier too.

http://tire-size-conversion.com/tire-size-calculator/

 

[akrvbob]

You're all missing the point, rim size does not control total tire size. You can put on 16 inch rims and still keep the exact same height. You can put on 20 inch rims and have the same height, or shorter.

The reason to do it is load carrying capacity. Very few 15 inch tires can carry more than 2500 pounds while many 16 tires go over 3000--and yes, they will be the same height!!!

Chances are you have 235-75 15 inch tires and they are rated at 2100 pounds each. That limits your rear axle to 4200 pounds, that's very low and you may very well be over it. Your tires may be overloaded right now

I'd get them and upgrade to 16 rims.
Bob

 

[Canine]

Now now Bob, we didn't all miss the point. "Don't just consider mpg, there's load capacity and handling too." That was my first sentence. Bob, I have to consider "load capacity" in everything I use on the van... You are right on profile heights though. They make crazy tires these days, like 18 inch rims with a 45 series tire...

 

[One Awesome Inch]

Interesting. In my case, I will be in a 1999 E150 in a couple months. I am 100% urban so no worries about offroad ability. As a minimalist my van will have less weight than most other vandwellers, I'm guessing. Ideally, I would like to have the height on the low side to fit under underground parking garage restrictions, but I have a feeling once I install my fantastic fan vent bubble, that will be moot.

MPG is my number one concern, after safety of course.

 

[SternWake]

I'd prefer having 16 inch rims with tires of the same diameter as I have now, simply for the greater load carrying capacity of tires which fit on 16 inch rims.

But I'm never going to bother as my load range C max PSi 50 are fine at 44PSI with my loading.

OAI, use that link for the tire size comparison to see if your great deal tires are close in circumference/diameter as the tires currently on your 15 inch rims.

Tires with a higher load rating will be heavier, so all other factors being equal, you will still take somewhat of a MPG hit.

 

[Optimistic Paranoid]

Use what the  Automobile mfg say to use.There is a reason they have hundreds of engineers on their payroll.

I'm sorry, Bob, but I'm going to have to respectfully disagree with you.  The mfgs design for "average" drivers driving on "average" roads with "average" loads.  As soon as you deviate from any of those design parameters, it doesn't matter how many engineers the mfg employes, their compromise choices are now sub-optimal and can be improved on.

And don't even get me started on how the engineers are often overruled by the damn bean counters . . .

Regards
John

 

[akrvbob]

On the 90s, the GVWR of 1/2 tons were so low that 15 inch wheels were okay. But everyone overloaded them because everybody thinks a 1/2 ton is a real truck and should be able to do real work. In the 90s and before they weren't!!

Remember, the vans are just like the trucks!

In the 2000s the GVWR went up on all trucks and they all use at least 16 inch wheels (or bigger) because the 15s simply can't carry the weight.

The only way to know is to load the rig up just like it will be when you live in it with everything you own in the world and take it to the scale and get it weighed showing the weight on both front and rear axles. Divide the rear axle weight in half and see if your tires can handle that weight.
Bob

 

[LeeRevell]

You may be able to have a shop install a new speedo gearing or adjust the current speedo reading to be accurate for new larger tires, depending on the technology. These newer computerized systems may have that as a programmable option?

 

[masterplumber]

You may be able to have a shop install a new speedo gearing or adjust the current speedo reading to be accurate for new larger tires, depending on the technology.   These newer computerized systems may have that as a programmable option?

You definitely need to on the newer vehicles. I went from 245/75/17 to 285/75/17 on my truck. A difference of roughly 1 1/2 inches. The truck would hardly run, and the speedo was more than 10 mph off if I recall. The dealer was able to reprogram the "pinion factor" and get it really close. Be aware, they can only program to a diameter that actually came on the vehicle due to EPA regulations. You may be able to find a speed shop that can do other sizes though. Basically you have to hack the computer and make it believe gears are still turning the same speed at a given RPM and transmission gear. 

On the older vehicles where the speedo is gear driven off the tailshaft of the transmission, it was real easy to change out the dive gear on the end of the cable.

 

[Canine]

Here's an oversimplification, not to be relied on if there's a great sale on a specific tire set from a reputable retailer...

PXL (extra load) tires should work for your E150 as a minimalist. They will ride softer than a stiff sidewall LT tire.

LT tires are better suited for anything off road, towing a trailer (the tongue weight) or a high top van that gets wind loads from the sides. You get too much roll in a soft sidewall P tire.

PXL is generally cheaper to buy, volume sales. All that changes if you find some real "deal" one way or the other.

A great majority of the cargo e150 vans I see that contractors use are riding on P tires. That's what came standard.

I don't care a whit about mpg in town with a V8 van. I personally NEED the higher load carrying ability and safety factor. 3 guesses why...