Mitsubishi Delica Owners Club UK™

[offroad]

thinking of towing a caravan to france. i have bigger at tyres on and i don't get great mpg as is. do you think changing to smaller diameter tyres so engine has less load and more rpm at lower speeds would be of much benefit?

cheers lee.

[Google]

[spectrum]

Unless I'm missing the obvious here - I always thought that the higher the engine rpm the more fuel is used, regardless of the actual road speed of your vehicle :?

So if your proposed scenario was accurate, I would have thought that you would actually end up using more fuel?

However engine rpm and wheel rotation speed aren't directly linked as the gearbox sits between the two and effectively alters the rotational speed of the wheels for any given engine rpm depending on what gear is selected. So to my thinking, all that would happen if you fitted smaller diameter tyres would be that the wheels would rotate faster for any given road speed - giving you a false speedo reading (the speedo would read faster than you were actually travelling).   I can't see how this would actually affect fuel economy one way or the other  :?

I think more benefit would be gained by changing from AT tyres to 'standard' road tyres of the same size as these would have less rolling resistance, thereby requiring less engine power to just get them rolling, thereby saving you a bit of fuel.

I'm sure somebody more technically minded will be along some time soon to explain this more clearly and either tell me I'm completely wrong, or at least explain how I'm partly right.  

Cheers,

Kev

[offroad]

Generally the engine will use less fuel at less rpm but in reality is not always the case. The engine is designed to have a sweet spot which is a balance between power and economy. Let's say it's at 65 mpg with std gearing. Increase the tyre size and now with the engine in the same rpm sweet spot you are now doing 70 mph. Now slow your speed to 60 mph to allow for towing and you end up out of the sweet spot. You need more throttle position to maintain the given speed so end up using more fuel. We've proved this by over gearing Vw Tdi converted camper vans in the past. It would be nice to have someone confirm it's the same with the Delica 2.8. Cheers lee.

[spectrum]

Interesting concept, and not one I've thought of before.

Was the VW camper manual or auto?

With the auto Deli (I'm assuming yours is auto - most are), I would think your best chance of increasing fuel economy would be to look at ways of keeping it in OD/4th gear as much as possible?

Going through the first 3 gears, some power is I believe always lost (and therefore fuel burnt for no gain) through the torque converter.  It is only when OD is engaged that I believe the torque converter locks up and All the power of the engine is transferred to the road wheels. (By the way - I don't profess to understand the inner workings of auto boxes, it's all a black art to me! :D )

So I would think the first question to answer would be whether or not changing to smaller wheels would encourage the Deli to stay in OD more?

Cheers,

Kev

[offroad]

You are right about the torque converter fuel waste and with smaller wheels the speedo would over read so in theory shift up and converter lock up would happen sooner too.
I have bigger than standard wheels so the opposite is occurring ATM. My Tps was also set correct. Lock up occurs at about 45mph although I sometimes feel it happened earlier when the engine / gearbox is cold.

[andyman]

If your tc locks at 45mph ,  is that under hard or gentle accelleration?  You may find when towing a caravan, you need to hang onto lower gears for longer , so you get fully into the max torque band in the next higher gear. If the wheels are bigger, you are going to be hanging onto 3rd longer, so wasting fuel . As you are restricted to 60 anyway with a caravan, I'd go for smaller tyres, the Deli will be relaxed enough at 60, and your transmission will be less stressed. Tow in 4h so you have differentiated 4WD to distribute the stress and tyre wear.
_________________
Andy C L300 Super Exceed - over 100 smiles per gallon!

[andyman]

PS., the last thing you want is the gearbox wavering between 3rd and o/d when towing, you will quickly wear out the auto clutch/ brake bands. If you find it doing this, switch out of o/d until you can go a bit faster.
_________________
Andy C L300 Super Exceed - over 100 smiles per gallon!

[offroad]

45 mpg lock up is with very gentle throttle input without towing. With average throttle it's closer to 50 / 55 mph I think.

[spectrum]

see .... I knew that somebody who understood all this black magic stuff would be along soon.    :D  :D  :D  :D  :D

Cheers,

Kev

[andyman]