I have larger tires on my truck, and used http://www.miata.net/garage/tirecalcold.html to see the % change. From what I gather, when the speedo says I am running 60mph, I am going faster by around 8%. That makes sense.. But my poor brain is hurting. The truck reports that over the last 1000ish miles, I averaged 17.1 mpg. So, given the correction, would the actual mileage be as indicated, 18.2, or around 16?

 

well, have you traveled more miles or fewer miles than the odometer indicates? (more, so the mileage reading will be equally higher)

 

That means you're closer to 18.2 mpg. Cause with a larger tire you actually go further than what your odometer reads (you also go faster than speedo reading).

 

generally larger tires are wider too. this increases rolling resistance, which means less mileage. part of this is off-set by higher gearing of taller tires, which means lower RPM.

best for mileage is a tall skinnier tire like 2.35-85R-16's profile

 

2500 nailed it. Even better for mileage would be a taller, skiiny really hard tire like you get from 19.5s-- but they are so heavy that the mpg you'd get on paper gets eaten up in the real world.

jh

 

I recently replaced my Uniroyals with Michelins and that along picked up 1.7 more mpg! The new ones were less offroad and more hiway tread. Plus I think a better tire!

RK

 

Teke,

After you correct for the additional mileage, by adding miles to the total your odometer shows, your mileage will be appear higher than it was before adding those miles. But that does not mean it is higher than what you'd be getting with stock tires. With bigger tires you have more rolling resistance, more mass to spin up and more wind resistance. So any improvement you might be looking for with lower RPM will have to make up for those problems before it can produce a net improvement. Higher gearing does not automatically mean better mileage. Appropriate gearing for the conditions you are driving in is what is best.

You're probably getting 1-2 MPG less overall with the bigger tires. Once you know exactly what the true miles traveled is, you can hand calc with the fuel used and correct for the overhead error. My overhead, even though my odo is correct, reads about 1.5 MPG high. So now I just look at it and reduce it's reading by that much for a rough estimate of mileage. I use it for trends and general mileage info like trips vs city vs towing, etc.

As stated earlier, tall and narrow radial tires will have the lowest resistance but don't make the mistake of thinking a 5% taller tire means a 5% improvement in mileage. It doesn't.

 

But the OP said that he calculated 17.1 mpg. So that means that he actually got a little better mileage than that because the bigger tires go a longer distance with each rotation. Yes, maybe he lost some mpg by going to a bigger tire, but nonetheless he DID calculate 17.1.

 

Exactly, I thought about getting these until I saw how much just one tire weighs not including the rim. Putting a 19.5 setup on adds alot of rotating mass. That affects acceleration, braking and suspension.

 

The overhead computer calculated 17.1, not him. If it would have known the actual miles travelled it would have read higher. Without the correction the overhead will read lower mpg with larger tires. But it is not an accurate readout so the actual mileage is not known.

He'll have to know the true miles travelled and the fuel used before he can calc an accuratre number.

 

good thread