Fuel Economy On 33's

[Snorkel]

Your Logic hurts my brains

Don't worry it's hurting mine too, lol. Please feel free to prove me wrong, I'm happy to be proven so, but for the minute I'm not seeing it.

Circumference is calculated by 2*Pi*r, so increasing the tire wall increases the radius (r) and therefore by definition increases the circumference.

By definition the circumference is the distance covered by a fixed point on the outside of a circle during one total revolution. So increasing the circumference increases the distance covered in one revolution.

I agree.

Soooooooo as the speedo works by translating revolutions per minute of the axle, and each revolution is covering more distance then it did before...

I also agree, HOWEVER, with a smaller size circumference one has to travel faster to get to the destination in the same time. Indeed the speedo would be wrong, but the OP didn't mention his 'speedo', but that the 'counter' was out.

I think it's just another case of misinterpretation. With bigger tyres I guess he'll actually be going slower than his speedo says, but the 'counter' should still clock the same distance.

[TonyN]

Your trip meter and speedo will be going round slower for the same physical distance covered. -10% is the number bandied around from stock 30ish" tyres to 33's. But never been sure.

[nero279]

http://www.dakota-truck.net/TIRECALC/tirecalc.html will calculate various things to do with changing your tyre size.

[KERRSURF]

Carl, just keep your random MPG thoughts to yourself in future.

[andycook]

ive just swapped old 30" diameter tyres for 31's and noticed a difference on mileage - and using one of the online calculators suggests a 3% difference in speedo and mileage recorded on the truck

so speedo may say 58mph but actually doing 60mph
i need to check with gps, as it used to be the case if speedo says 60, but gps said lower
so i maybe spot-on now!

3% equates to 10miles in 300, and i have noticed this

[Karma Supra]

I also agree, HOWEVER, with a smaller size circumference one has to travel faster to get to the destination in the same time. Indeed the speedo would be wrong, but the OP didn't mention his 'speedo', but that the 'counter' was out.

The counter is driven by the speedo

I just started working things out as I intend to run 33s... Worked out that on a gen 2 it is a 10% increase and on a Gen 3 its 6.5%.. which seemed to tally up with what Tony said, but not what the caclutor said..

At this point I decided life is too short, and my speedo over reads anyway, (5-10%) so it'll probably cancel the speed difference out.. (but not the gearing)

My head hurts now too... It amazing what being bored at work drives you to do!

[stormforce]

Since running my btruck on 33's i have checkerd my speed against sat-nav and my speed is more accurate with 33's then it was with the 31's which were nearly 5mph slower according to satnav

[Snorkel]

Life is too short, lol, but indeed, I AM pretty bored, so just to show you all how sad I actually am - here are some figures, and I have every confidence some of you will use them too (even if you won't admit it ).

30's - circumference is 94.2 inches (2.39m)
33's - circumference is 103.62 inches (2.63)

30's - rotate 418.41 times per km
33's - rotate 380.22 times per km

If someone gives me the diameter of the axle, I'l work out the speed differential.

oh dear

... that makes me remember ... to add...

the percentage difference of circumference between 30's and 33's is 9.1%

[TonyN]

Since running my btruck on 33's i have checkerd my speed against sat-nav and my speed is more accurate with 33's then it was with the 31's which were nearly 5mph slower according to satnav

Linda's truck is the same, we borrowed Robs GPS at the first RTTS, and her truck on standard gears (4:10's) was bang on at 70mph with the 33's.

Time will tell how what difference the 4:88's gears will make to the speedo.

[cataclysm]

My 2 cents:

If the Surf was a nice aerodynamic shape, the bigger wheels should give better economy, as you're effectivly in a slightly higher gear.

But it's not. It's a big ol' brick. Most of the engine's work is fighting the air resistance, so with a higher gear, the RPM goes down but the engine load goes up.

What effect this has on your milage is mostly a factor of:
- Engine breathing
- Injector condition
- Air filter condition

There's also 'sweet spots' to consider. Most engines have a few RPMs that they're best at. If you tyre size has pushed you toward one, you'd expect better milage. If you've been pushed away, worse.

End of ramble.

[PDR]

At this point I decided life is too short, and my speedo over reads anyway

That should be OK then…. mines the other way round, an indicated 30, 40, 50 & 70 is really 35, 45, 55 & 75 so I have to drive at an indicated 25,35, 45 & 65 so as not to break any speed limits… there is one advantage, the cruise control set at its max turns out to be a true 70 mph.

[Karma Supra]

All the calulations... My notepad at work is covered with similar Numbers to Snorkels.

I totally agree With cataclysm about drag too!

Now, really... I'm going to shut up

[cataclysm]

I totally agree With cataclysm about drag too!

Now wait a minute! Oh, right I get you, never mind...

[Snorkel]

yep, I got onto the second sheet!

[Snorkel]

- stood up quickly and nearly blacked out haha

*Note to self* - brain needs fuel when working too!