Buck01

I KNOW that adding bigger tires affects gas milage.

I just put 33x12.50's on my truck and the gas milage went down in a hurry.

I just cant understand why?

I know the tires are heavier and have more rolling resistance...using more gas here.

But with the bigger tires you are changing your gear ratio to a higher ratio correct?

Doesn't the higher ratio make you run in lower RPM's...saving a little gas here.

I know the answer already I just don't understand the reasons why.

Can somone explain this in little words so I understand?


Thanks

TNRabbit

Two major things here are responsible for the increase in your fuel usage:

1- Bigger cross-section of the tire; it catches more air! The tires also add weight, a constant enemy of fuel economy.

2- Increased throttle required to get you up to speed, also when you go up a hill you're having to give it more throttle to keep your speed up; makes your engine have to work harder.

These will more than outweigh the increased ecomony you might realize from the taller effective gearing.

AH64ID

When changing gears with bigger tires you put lower (numerically higger) gears in, like I took the gears on my 92 from 4.10 to 4.88. This brings the RPM's back up to where the were with stock 225's. This will actually help mileage, even though your running more R's. since you get back to the normal power band. Mileage is about lower R's but only so much, too much and the engine isnt efficent. I beleive I read once that our engine is designed to be most efficant cruising at 2500-3100 RPM's. I usually cruise at 3K plus, but thats 75 mph.

The other reason you mileage dropped is you speeds is not longer accurate. Do you have a GPS, or access to a GPS; I have found that to be the most accurate and easiest way. Drive 50 on the truck and read the speedo. Take the different mulitply times two and your reading that % off, It will be the same % regardless of speed.

If you dont have a speedo you can use the approx math method, but rolling radius and mfg heights are not all the same, but it will be close. For example, you truck came calibrated for 225/75/15 a 28" tire, your 33 is 32.7". 16% larger, so at 50 on your speedo you doing 58, same on your trip odometer. If it says 250 miles, and it took 14 gals, you would think 16.7 mpg, add the 16% mileage and 290 miles is 20.7 mpg.

You will loose a little mileage for the reasons TNRabbit said, more windage and higher unsprung weight and rolling resistance....but you biggest drop was from your speedo being 16% off.

Hope this helps

RustBucket

iTrader: (1)

Yes, the rotational interia of the tires will hurt city driving especially. The larger patch of rubber connecting you to the road will also mean more rolling friction, like AH64 said.

If you don't regear you have to be on the throttle harder to get going, which means more lower vacuum in the combustion chambers and therefore more gas being sucked in!

Buck01

I knew that the mileage would go down and know I need to regear I just never understood why it affected the truck like it did.

Thanks for all the info.

BigWavePaul

iTrader: (1)

Your odometer is off, Thread Resurrected! Just like your Speedo is off, your Odometer is off too. I found this thread while looking for the chart that shows you how to calculate your mileage after switching to bigger tires. I think that I saw a table conversion showing you what percentage to multiply by to get the real mileage traveled. You've gone farther than your odometer reads, Man. I have too and I want to know by how much. I could've sworn that I saw it on TTORA somewhere years back...

ffryno13

there is a great chart at 4lo.com or something like that and there is another one. i just can t remember where its at that you can put in your gear ratio and other stuff. the windage is a pretty bigg deal i am running 37 14.50 with SAS 5.29 gears and 22re. wind slows you down more then you think it does. i was driving 70mph on i 84 in oregon and a wind gust made my 5th gear absolutly useless