mad tyres prob

[Diesel28]

Hi all ,i recently bought new tyres as was onlast legs wiht old ones and was running the old style BFG 245 /75 R16 and have bought some Maxxis Buckshot Mudders which are 245 / 75 R16 .i measured the tyres b4 the new ones went on and th overall height was 750mm and the new ones are 780mm high.the first things i noticed was is that the revs have dropped by about 250revs when driving.But My fuel cnsuption went thru the roof and my speedeo seems likes it kinda counting up slower.When on old tyres i was getting about 500k from the spout to 1/4 of a tank left.But now i am only getting about 230k's out of it.
Does my speedeo need to be re-callibrated or Not .Or am i using more fuel due to the new tyres .
i appericate any help or ideas on this as i dont know whats going on at moment .

[Graham]

Sounds like something isn't quite right to me too.

Going from 750 to 780mm is only about a 4 % change, so all your readings should be out by about 4% as well.

If your driving habits are regular, ignore the numbers and consider how often you used to fill up compared to now, is there any change?

G.

[mudgrip4]

When I bought my bighorn it ran 245/75/16 tyres (A/T's) at a height of about 755mms. I have upped the size over the years to to 265/75/16 which are 810mms high - a much bigger change than you have just done. The effect is to drop the revs a bit, and my speedo is now 9% slow - a great way to save on road taxes.... But there is no real increase in fuel consumption - maybe 2-3% on average.

Your change in tyre height is half mine, so I don't think you will have too much problem. There should not be 250rpm difference at 100kph. As noted above, difference should be more like 4% or about 100rpm less at 100kph which should not affect performance or fuel consumption greatly.

For the moment consider the speedo to be reading about 4% slow and drive accordingly. Also try driving at your normal speeds for another 2-3 fills and see if your problem is still there. If it is, then I suspect it is another problem that just happens to have showed up at the same time as the tyre change. Mike.

[Dave]

I ran 33's on my 92 Bighorn, and had better fuel consumption than that!

[geeves]

30mm differance is only 15mm a side. As these tyres start with 14mm tread there isnt realy a differance at all. Were the old tyres an AT or MT tread? This can have a bearing on things although 250k from 3/4 tank is not right How many liters did it take to fill and what are your tyre pressures. Also keep an eye on the tread for strange wear paterns as there might be a wheel alignment issue Also after a moderate run check the hub temperatures to see if theres a binding brake.
The only time I ever got through 3/4 tank in 250k was when the gauge was faulty (40 liters empty to full not 80 like it is now)

[Diesel28]

the old tires wer BFG A/T and the new ones are Maxxis buckshot mudders which are the M+S tires.the tread pattern is alot bigger then the bfg i had on before.
i had the fitted thru tonys tire service and they said they put the tires up to 37psi.i nt think i have a faulty uel gauge as this prob only came on when we got the new tires.so as you can see im absoutly stummped as to whats going on and when it takes $65 to fill up its costly each week.Am doing a fishing trip this coming weekend into the canty high country so will see what happens on a long run.(with a spare of fuel in the boot lol )Oh and my truck is a 93 bighorn T.D intercooled auto .maybe my speedeo is out of sinc.i still fill up the same time each week so only putting in gas once a week so cant be over using the gas.when i put gas in it last night it was just above empty and it took about 63 litres and that was form near E to the spout .

[Diesel28]

i did check out on the BFG website that the tires i had b4 when brand new they were a overall dia of 760 and i had them near bald on 2 of them so that would of taken them to say about 745ish and the new tires are a dia of 780 so would that 35 mm make the speedeo be out enough to show the k's wrong ?
thats all i can think of is the k's arent counting up correct .

[Diesel28]

just been thinking ,i havent changed the fuel filter for about 8 months so could that be something to do with excess fuel usage ?
only $20 so may be on the cards to do this thur night .changed air filter yesterday .

[geeves]

Unless you have a leak my moneys on the air filter Mine would work fine till very near use by date then suddenly loose power on hills and develop a huge thirst. One tank would go 700 k the next under 500.back to 700 with a new filter. Bad fuel filter will only rob power.

[Harry]

When my fuel guage is on E for enough there is still about 20l in the tank.

[Pissy]

This may sound funny but what about filling the car up to FULL and then refilling it after 100kms has been done, do that a few times and that might give you a more accurate idea on fuel consumption per 100kms. Upgrading to larger and especially mud tires will usually increase fuel plus incorrect tyre pressure.

Cheers

[geeves]

how did the fishing trip go and the consumption?

[Pissy]

I actually stayed off the sauce as we were off shore boat fishing everyday, and believe me the last thing you ever want to experience is a hangover at sea. Plenty of fish and plenty of diesel too (160L for 800-900kms), but I was well and truely loaded and with the roof top bag fully packed (See photo in Aust Member profiles), the MU drunk heaps.
(Sorry guys Off the topic.)

Cheers

[richie]

So to make this short, putting on bigger tiries makes the car drink more fuel and drives slower?

[geeves]

somthings wrong but beats me. If it had only been measured on one tank I would be asking if the boy racer down the road now has a smoky exhaust but that obviously not the cause. Look for leaks around the injectors and pump

[Diesel28]

Geeves, fishing trip went well but Bloody bloody cold up in canty high country that weekend.Caught 2 trout and lost 2 and a medium hangover.
As for the mileage it was a huge improvement by changing the air filter.got one form supa cheap for $30 and the milage is now doing 430 up to 470ish .dont recall exactly how far i got but is a huge improvement from the 230 i got the week b4 i did the filter.
with the maxxis mudders on truck i went past one of them speed mobile thingies on the side of the road( No not the Revenue gather in the van ) the wee trailer thing and my speed is still corect so tires havent put speed out.What sort of tyre pressure would you put into my tyres as guys at tonys tires service said 37psi but would like your opinion ?
Your views much appercated
Neil

[geeves]

I run 32psi and get good wear etc but you watch the discusions start flying on this one As a general rule the higher the presure the better the ecconomy unless you get to the point that the tyres loose traction to easily I would say 37 is too high but lots of people run this sort of presure with no issues

[Stu]

I drove to Ohakune last weekend on the 32inch simex's and did notice a bit of difference in the fuel economy. I guess its partially due to the larger rotating mass and could also be due to the aerodynamics of the tyre itself - although that could be just sepeculation. Those big knobblies wouldn't be the best for slicing thru the air compared to the normal road tyres. The speedo was out by 6km/h at 100k - thats if the gps is to be believed. Sat at around 95 the whole way as the simex's get really loud if you go any faster - its hard enough hearing the engine at 95 haha. Ran them at 32psi on the road and 10psi off.