Tire size and MPG

[SpeleoRover]

OK here's where I am.

I'm having trouble deciding between 255/70's, and 265/75's for my 2000 Disco2. I've got the OME HD 2" lift, but otherwise stock.

I keep reading about a significant MPG and power loss when folks got to the 265's. I'm wondering - will I see a MPG drop with the stock-width 255/70's? In the tire I'm looking at they're something like 30.8" vs. 31.7" for the 265/75's (and still significantly taller than stock 28-29" rubber). I'm really wanting to avoid as much power loss as possible but still fill out my newly expanded wheel wells.

Whatcha think? Gas ain't gettin' any cheaper, and I drive alot of hills.

Thanks!

[300bhp/ton]

Personally I don't think you'll see much MPG drop. There are several factors however.

1. Driving style, if you drive hell for leather then the bigger tyre will probably reduce MPG slightly more.

2. Tread patteren, knobly M/T's create more drag than A/T's. This may affect performance and MPG. In a fairly recent test in one of the Land Rover mags they tested a 90 with A/T's and then M/T's of the same size over the same jounrey.

A/T's = 31.3mpg
M/T's = 29.9mpg

3. Remember a lot of people claiming big loses forget to correct the odometer as the bigger tyres will make it incorrect. In my Disocvery 110 miles only shows as 92 miles on the odo. So over an entire tank ~400 miles it makes it look like I've covered far less ground but in reality I haven't.

The size difference you are looking at is only small so I don't think it will really make any real difference.

In my own experience my 200Tdi Discovery gets pretty much the same MPG with 31.10.50 BFG All Terrains as it does on 33.11.50 agressive Simex Jungle Trekkers. It does depend how I dirve it though.

As for power, well the bigger tyres are noticable however even when the motor was stock (111bhp) it still drove more than ok and never felt out of it's league.

Another nice bonus of the bigger tyres is motorway (freeway) driving as they let me run at the same speeds as before but with lower rpms.

[p76rangie]

Originally Posted by 300bhp/ton

Another nice bonus of the bigger tyres is motorway (freeway) driving as they let me run at the same speeds as before but with lower rpms.

Why is this a bonus?

[300bhp/ton]

Originally Posted by p76rangie

Why is this a bonus?

eh???

Thought it was pretty obvious.

But hay what ever.

Lower rpms means quieter and less fuel consumption, pretty simple really.

With the 33's on I can run at 70mph at a tad over 2500rpm, where as with the 31's it's more like 2800rpm+.

And on a diesel this makes a big difference, if I run at 3000rpm on the motorway is uses substantially more fuel than keeping it nearer 2600rpm.

[p76rangie]

Originally Posted by 300bhp/ton

eh???
Thought it was pretty obvious.
Lower rpms means quieter and less fuel consumption, pretty simple really.

So what you are saying is that having larger tyres actually gives you better fuel economy and not worse. Interesting.

[Disco Mike]

This is not correct.
Rovers are geared so high to start with for their weight that adding taller tires will change the power and torque curve for the engine causing some reduction in power.
When I switched from stock to 265's I lost nearly 2 miles per gallon and 1.5 at highway speeds along with a reduction in low end power. I regained all but .25 MPG when I switched to 4:11 gears.
Large tires will cost you in gas and performance but last longer and ride better.
Mike

[300bhp/ton]

Originally Posted by p76rangie

So what you are saying is that having larger tyres actually gives you better fuel economy and not worse. Interesting.

At a constant speed say 65-70mph on fiarly level terrain (no real steep hills) such as motorways in the UK then yes I would be willing to bet that my 200Tdi Discovery see's better (only slightly) MPG on 33" Simex Jungle Trekkers than it does on 31" BFG A/T's.

Not sure all vehicles would respond the same, but my TDI has enough power to happily cruise at that speed without too much throttle input on both sets of tyres. With the 33's it simply turns over at lower rpms.

[300bhp/ton]

Originally Posted by Disco Mike

This is not correct.
Rovers are geared so high to start with for their weight that adding taller tires will change the power and torque curve for the engine causing some reduction in power.

The profile of the curves will not change no matter what, the engine will not be affect at all, at least not directly.

If you are on a rolling road then the extra weight of larger tyres may show up slightly lower numbers at the wheels although the actual shape of the curve will be unaffected. But this the only or main reason a vehicle on large tyres may feel sluggish.

It's down to gearing, running a larger tyre has effectivly increased the gearing, it will rotate less rpms for a given speed but reduce accelaration.

[300bhp/ton]

I guess a V8 may be more prone to MPG loss though compared to a diesel.

Thinking about it, my TSI makes PEAK torque at about 1750-1800rpm's so I don't have to drive it hard to make it go.

Generally a V8 will not make PEAK torque as low (and often not as much torque as a diesel down low - yes Ian we know your thoughts on it.....). So maybe you V8 guys tend to drive a tad harder which highlights as a higher fuel comsumption.

I guess this theory would be consistant with why diesel trucks (Dodge Ram's, etc.) are so much better for towing than their petrol counter parts and why lorrys/trucks and farm tractors are diesel and not petrol.

[Disco Mike]

The power and torquescurves don't change, you are right, what changes is that with larger tires, your RPM's drop , lowering you out of the effective operating RPM range and not allowing the engine to generate what little power it has.
Mike

[KevinNY]

Worrying about the gas mileage of a discovery is like worrying about dieing in a plane crash, it sucks and there is nothing you can do about it!

Seriously, forget about it.

[SpeleoRover]

Maybe it doesn't make sense for some folks, but I'd like to keep as much power and fuel economy as possible - even if it isn't great.

Does the loss of power and efficiency come from height or width?

[p76rangie]

Originally Posted by SpeleoRover

Does the loss of power and efficiency come from height or width?

Both. The bigger tyres change your gearing and require more effort by the engine to turn them and maintain speed with them. Wider tyres have more rolling resistance and again requires more power to drive them. But if I had to have a guess, taller tyres would be worse than slightly wider tyres.

[SpeleoRover]

That's what I was afraid of...

[300bhp/ton]

Originally Posted by SpeleoRover

Does the loss of power and efficiency come from height or width?

Both, but IMO the size difference you are looking at is so negliable that it really won't matter, espcially if you are not going to an agressive off road tyre.