Better fuel econamy with bigger tyres

Post by **fordy1** » Mon Jul 28, 2008 12:40 pm

Hi All

i have just fitted 225/75/16s to my 04 Grand Vitara and i knew the fuel would go down a little but i now baby the car to get 430Ks from a 60L tank and before i drove with the foot flat to the floor and got 500Ks from a tank.

i also have found the old girl to be very slugish of the line.

i want it to feel better and bring back a little fuel econamy, i dont have the time or money to do gear swapping and wanted to know of any other ways to help the problem in a gv

thanks

tim

Post by **joeblow** » Mon Jul 28, 2008 12:55 pm

did a couple of gv's where i changed the diff ratios to 5.1's and installed airlockers. was quite a job but resulted in the owners gaining 100-150 k's out of thier tank. oh, the extra power was a plus too.

Post by **Gwagensteve** » Mon Jul 28, 2008 12:59 pm

#1 have you corrected your speedo? (or corrected your results?)

What's happened to your gearing? (i.e what have your cruise revs dropped to?) - sometimes going up in tyre size will result in the car not locking the converter, for instance, in an auto, and therefore using a lot more fuel. Modern cars that are geared for maximum economy at a given speed won't like the added load and lowered revs larger tyres impart.

The aerodynamic changes and the rolling resistance should only we worth a couple of % by themselves.

Steve.

Post by **fordy1** » Mon Jul 28, 2008 2:34 pm

i have a manuel and i have not corrected the speedo but i have been watching the rpms and try and stay away from 3000 as it sucks the juice and

I also now have to drop agear going up hills which also uses more fuel.

lockers and gears would be nice but i dont have the time or funds.

im looking at other ways i can change it

Post by cj » Mon Jul 28, 2008 2:50 pm

Put your stock tyres back on

Post by **fordy1** » Mon Jul 28, 2008 3:11 pm

mmm unroadworthy tyres... good idea

Post by **Gwagensteve** » Mon Jul 28, 2008 3:38 pm

you need to add 5% to your readings - so 430 = 450 straight away.

If you're driving all below 3K I reckon you're probably lugging the motor. I assume you're a V6? They're not that keen to chug.

With the added load of the taller, more aggressive tyres There's going to be a lot more load than stock.

Checked inflation pressures?

Did you do anything else at the same time like more lift? This will play a part too.

Steve.

Post by **fordy1** » Mon Jul 28, 2008 3:46 pm

i have not checked tyre pressures as they were just fitted!! but i am just trying not to acelerate as if the car had the old tyres as the further i put the foot down the more fuel she will use.

i did lift the rear 50mm befor adding the tyres

Post by **joeblow** » Mon Jul 28, 2008 4:13 pm

yep, same rubber as the gv's i did.

Post by **Gwagensteve** » Mon Jul 28, 2008 4:31 pm

so the new lift went on just before the new tyres?

Did you ever drive it with the lift before the new tyres?

You will have lost economy with the lift - increased aerodynamic drag and angle on uni's/CV's will be sapping some power.

Check tyre pressures for sure. Those tyres are going to have a LOT more rolling resistance than stock.

Steve.

Post by **alien** » Mon Jul 28, 2008 4:45 pm

welcme to owning a modified 4x4 =P haha

i dont know about the V6s, but the 4cyls like to rev... if you baby the engine your fuel economy will suck... take off fast in first and keep the revs up after that. i try to cruise between 2500-3000rpm, 3400 is 100k km/hr on mine.

Post by **GRPABT1** » Mon Jul 28, 2008 6:11 pm

Like alien said keep the revs up, this doesn't mean you have to floor it but rather just hold the gear longer. Try keeping below 4000rpm or even 4500rpm and then see what happens.

Post by **david123** » Wed Jul 30, 2008 7:23 am

You will never get fuel economy equal to road tyres with agressive off road tyres

The coopers are 80% off road on the car, if you use them, sweet, forget fuel milage, if they are just there for looks, sorry, very bad choice.

Post by **alien** » Wed Jul 30, 2008 9:12 am

I run the STTs on mine too - and theyre awesome offroad an on... they grip better than my skinny road tyres used to.

Post by **david123** » Wed Jul 30, 2008 9:48 am

I used to run them on my F150, yea good tyres, but they are 80% off road, and quite chunky, not really a daily driver tyre.

If you want fuel economy, get a road tyre.

Post by **fordy1** » Wed Jul 30, 2008 10:21 am

i need these tyres but losing 100ks from a tank even when babying is hard, just trying to get some back!

i know it wont be the same as before but just an extra 30 - 50ks will make a big difference and i would be very happy with that...

the next experiment is i fill her tomorrow and drive normally and see how we go!!!

Post by **david123** » Wed Jul 30, 2008 10:35 am

fordy1 wrote:i need these tyres but losing 100ks from a tank even when babying is hard, just trying to get some back!

i know it wont be the same as before but just an extra 30 - 50ks will make a big difference and i would be very happy with that...

the next experiment is i fill her tomorrow and drive normally and see how we go!!!

What pressure u running, with the bajas on the road, suzis recomend pressure is a bit low, they say 19psi, i run them about 25psi.

When im at 5psi on the road, crikey duz she get thirsty

Post by **GRPABT1** » Wed Jul 30, 2008 11:36 am

fordy1 wrote:i need these tyres but losing 100ks from a tank even when babying is hard, just trying to get some back!

i know it wont be the same as before but just an extra 30 - 50ks will make a big difference and i would be very happy with that...

the next experiment is i fill her tomorrow and drive normally and see how we go!!!

Like it's been said your odometer is probably out by a bit so it may not be 100kms.

Post by **fordy1** » Wed Jul 30, 2008 12:34 pm

mmm should be about 8.1% as noted from friend with the same gv and size tyres - 430Ks = 464.83 not bad , i will recalculate when next tank is used

the main thing is not being able to fill up just once a week and the lack of right foot when she who must be obayed drives

thanks all for your input

Post by **monkeyplex** » Wed Jul 30, 2008 6:29 pm

I've recently put 31's on my 4 cylinder, and yes, you will use more fuel if you regularly go up hills, and do stop start driving.

Just a quick google says your engine's max torque is at 3300 rpm, that rpm is when your engine is running most efficiently, so theoretically try and cruise at around 3300rpm...

Also, try drafting behind semi trailers

Post by **przooker** » Mon Aug 04, 2008 8:15 pm

I used to run some 30" chunky muds, 9.5 wide with a 16valve 1.6efi. I used to get 9km's per litre. Now with standard sized tires im getting nearly 12km's per litre. These figures are not exact, but pretty close and i did no other mods. And it has heaps more accelleration and i dont have to put the accellerator down as much to go somewhere.

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Better fuel econamy with bigger tyres

Better fuel econamy with bigger tyres

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