Although I'm a fairly new member, I've been visiting here for quite some time. I've always been puzzled with the increased tire size and decreased MPG question. So here's my question.

Given that your engine has enough horse power and torque available in the base to get the truck moving without over revving why would a slight increase in tire size decrease MPG. If you assume that the transmission ratio is constant, Increasing tire size would decrease your overall gear ratio, which would mean lower RPM's per tire revolution. Shouldn't this in turn increase your MPG???

I know this doesn't apply when making large changes, due to the extra power required to get going. I'm just talking about minor changes like 32" to 33".

Keep in mind I'm a mechanical engineer, so I'm sure I'm over analyzing this and missing some pratical details. Thanks for the help.

 

Easiest way to picture it is to think of pedalling a 10 speed bike. If you start out in 1st gear its easy to pedal, now try it in 10th gear, your legs move slower but work much harder. Same thing applies to truck engine, its turning slower with bigger tires but having to overcome a taller gear ratio. Hence it works harder and uses more fuel.

 

Unless the tire increase is substantive, MPG will likely INCREASE slightly with larger diameter tires.

 

You're right except when they go larger, they usually also go wider. The increased width increases rolling resistance and results in decreased mileage. Very high mileage vehicles generally go with a very skinny tire for decreased rolling resistance. Look at the tires on a racing bicycle compared to a regular bicycle.

 

Gear Ratio Guide for Larger Tires

 

Unless the computer is reflashed for the larger tires, you are traveling further than the odometer reads. Hard to tell how much further unless you have a GPS & compare. This makes your MPG appear lower than it actually is.

 

larger tires= lower gear ratio. Lower gear ratio will give you better mileage on long highway trips, but in city/country driving, you will loose tons of fuel efficiency.

 

all generalizations that don't always work. Same reason some people are getting better fuel efficiency with their 5.4 than I am with my 4.6.

A bunch of different factors determine fuel efficiency. In regards to gear ratio, the engine working at optimum efficiency is what leads to optium fuel efficiency - and that doesnt necessarily mean moving slower or having a lower gear.

Fact of the matter is, I went from STX 245 tires to FX4 285 tires and my fuel light comes on at 250 miles whereas it came on at 300 before. There's an obvious speedometer difference, but it certainly isn't 20%...maybe 3% tops.

THe engine bogs unless I tack it out. And if the engine is struggling because the gears are too low, than I don't need to be a mechanical engineer to understand why my fuel consumption is worse.

Also, the rims are significantly heavier and the extra 40cm accounts for more drag.