Does this sound normal after the swap?

Does this sound right?

I had 4 x new BF Goodrich AT tyres fitted not that long ago. 265 75 16 on boost alloys.

Over the last few tanks (fairly soon after i replaced the tyres) i have noticed a drop in mpg, down from average of 33mpg down to 26 -27 over the last few tanks, doing similar driving. Thats about a 20% drop! Before anyone says anything, the normal MPG for my driving and truck is about 33 average over about 10000 miles, so its a definate drop in MPG.

Also i have noticed it feels a little more sluggish and requires a little more throttle to get the same acceleration etc out of it, especially at low revs.

So my question is should these new tyres make a difference to the way the car accelerates etc and how much throttle is required etc to push it along. I have read that BFG AT are heavy tyres so would this make a difference?

I was possible expecting a little drop in fuel consumption moving from 235
General TRs to the 265 ATs, but not 20%!!

Has anyone else noticed this when changing tyres or should i investigate other things? Maybe i could check the MAF etc to see if its fooked and its a coinsedence?

Cheers

purely down to the amount of drag that your tyres are now creating over the smaller previous tyres

When I changed from 7.50x16 XCL's to 265/75x16 BFG AT's on my 90 I suffered no change in performance or fuel consumption. Constant monitoring of the fuel consumption over many years has shown that it always returns between 28-33MPG depending on journey type.

A friend of mine suffered exactly the same problem as you when he changed over to BFG MT's, admitidly that was on a Nissan Patrol and he had gone up a size but the principle is the same.
He weighed up the various options of running two sets of wheels/tyres, finding a different comprimise tyre etc but in the end went for a re-map of the ECU which while costing about £500 restored the lost performance & fuel consumption back to what they were.

It's nothing to do with drag or anything else (although these might have a very marginal effect).

You will probably find that destinations have got closer as well if you go by your speedo.

The radius of your new tyres are about 10% more so it looks as though you have reduced fuel consumption by 10% as your measured miles are now 10% longer than they were on the smaller tyres.

So it looks like your fuel consumption has got worse but actually you are going further than your speedo is telling you!

edit...
Your circumference was 1.47 and is now 1.66m. That's 113% increase. The rest is probably increased drag.

Litch said:

When I changed from 7.50x16 XCL's to 265/75x16 BFG AT's on my 90 I suffered no change in performance or fuel consumption. Constant monitoring of the fuel consumption over many years has shown that it always returns between 28-33MPG depending on journey type.

A friend of mine suffered exactly the same problem as you when he changed over to BFG MT's, admitidly that was on a Nissan Patrol and he had gone up a size but the principle is the same.
He weighed up the various options of running two sets of wheels/tyres, finding a different comprimise tyre etc but in the end went for a re-map of the ECU which while costing about £500 restored the lost performance & fuel consumption back to what they were.

Click to expand...

So should i expect this then? You said you didnt suffer any change when you went to 265? Its a hell of a drop and one i wasnt expecting after doing alot of researching and questioning etc!

Oddie said:

It's nothing to do with drag or anything else (although these might have a very marginal effect).

You will probably find that destinations have got closer as well if you go by your speedo.

The radius of your new tyres are about 10% more so it looks as though you have reduced fuel consumption by 10% as your measured miles are now 10% longer than they were on the smaller tyres.

So it looks like your fuel consumption has got worse but actually you are going further than your speedo is telling you!

Click to expand...

assuming the 235s were 235/70 as were fitted as standard size to a 90

Oddie said:

It's nothing to do with drag or anything else (although these might have a very marginal effect).

You will probably find that destinations have got closer as well if you go by your speedo.

The radius of your new tyres are about 10% more so it looks as though you have reduced fuel consumption by 10% as your measured miles are now 10% longer than they were on the smaller tyres.

So it looks like your fuel consumption has got worse but actually you are going further than your speedo is telling you!

edit...
Your circumference was 1.47 and is now 1.66m. That's 113% increase. The rest is probably increased drag.

Click to expand...

Good point, but the two different tyres should have a similar diameter so should be minimal, although this may account for some of the drop.

But surely if the tyres are nearly the same in diameter, there shouldnt be any effect on the amount of throttle that is required. I might just disconnect the MAF to see if it makes any difference.

Would a dodgy MAF account for any increase Fuel consumption. I know it might account for a little flat spot etc!

G-man said:

assuming the 235s were 235/70 as were fitted as standard size to a 90

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No, either way it will look worse than it was, irrespective of whether either reading is true. I'm guessing he's going only on what the speedo tells him vs what he puts in the tank, so it's gonna LOOK worse either way!

G-man said:

assuming the 235s were 235/70 as were fitted as standard size to a 90

Click to expand...

They were 235 85 16 general Grabber TRs standard fit from new

New are 265 75 16 BFG AT,

So both sould have very similar diameters, i think

robpenrose said:

Good point, but the two different tyres should have a similar diameter so should be minimal, although this may account for some of the drop.

But surely if the tyres are nearly the same in diameter, there shouldnt be any effect on the amount of throttle that is required. I might just disconnect the MAF to see if it makes any difference.

Would a dodgy MAF account for any increase Fuel consumption. I know it might account for a little flat spot etc!

Click to expand...

13% increase in distance travelled will increase the fuel consumption by at least this amount (taking into account your points about increased drag from teh wider tyres and the knobblier pattern, which will certainly also have a bit of a bearing)

Oddie said:

13% increase in distance travelled will increase the fuel consumption by at least this amount (taking into account your points about increased drag from teh wider tyres and the knobblier pattern, which will certainly also have a bit of a bearing)

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so is it increasing it by what i think, or is it the fact that speedo is reading slighly inaccuratly, hence throwing the figures out?

Cheers for your input

robpenrose said:

so is it increasing it by what i think, or is it the fact that speedo is reading slighly inaccuratly, hence throwing the figures out?

Cheers for your input

Click to expand...

Hmmm...tricky one!

Have distances seemed to get less with them? Is there a run you do that you know the exact distance of that you could test against (according to your speedo)? I hadn't realised they were 85's which i think are similar to the BFGs.

I think your previous ones were more of a road tyre weren't they?

robpenrose said:

They were 235 85 16 general Grabber TRs standard fit from new

New are 265 75 16 BFG AT,

So both sould have very similar diameters, i think

Click to expand...

the diameters may be the same but your contact patch on the road is nearly an inch wider than the 235s..... all 235s are not equal as not all 265s are equal,

for instance a 235 bfg track edge was narrower than a 235 bfg A/T

over 4 tyres your talking about over 3 inches of extra rubber on the floor that you are having to push

yeah, so it could actually be a fuel consumption increase. This is the reason i was unsure, as they are meant to be very similar diameter, but it appears the increase in MPG is 20%, which i thought was to high for the extra width. Maybe not?

I think i will try the MAF sensor, as its the extra throttle required. I wouldnt have thought it would be that noticable just by adding very slightly wider tyres, Maybe im wrong!

robpenrose said:

yeah, so it could actually be a fuel consumption increase. This is the reason i was unsure, as they are meant to be very similar diameter, but it appears the increase in MPG is 20%, which i thought was to high for the extra width. Maybe not?

I think i will try the MAF sensor, as its the extra throttle required. I wouldnt have thought it would be that noticable just by adding very slightly wider tyres, Maybe im wrong!

Click to expand...

the other thing is your going from a road tyre to an all terrain so you will get increased fuel consumption from that too

yeah i guess, its just when i spoke to some of the guys on here who had swapped from the same old tyres i had to even MTs, they generally didnt notice much at all. Chaps running 265 MTs on 110s were still getting 26-28 MPG, so i was expecting still around 30 in my 90.

Maybe its just lots of factors adding up, increasing the MPG. Bugger!!