Mpg down

So I have mentioned a few times on here about mpg being lower with bigger tyres. 

I've checked back and I did a run to Westcost Off-road last year to get the winch bumper fitted. Lots of motorway. I recall I set the speed limiter at 72mph which worked out to be about bang on 70mph gps. 

I got something like 38.2 on the way up and 37.6 on the way back. This was on standard tyres and my original transfer box (before being replaced under warranty, see my build thread for details). 

I've never seen more than high 39mpg. But this was commuting last Feb/March when I first got it and was running it in and driving very gentle. COVID and wfh changed these habits and mpg. So it was hard to compare. 

As the mpg with the new bumper was almost the same coming home as going. I'm not inclined to believe the weight of the bumper had any real impact. 


Recently I had need to do a similar motorway journey and distance. I ran 2mph slower this time by setting the speed limiter to 72 instead of 74. So gps speed was about 68mph. Which seemed bang on 3500rpm. 

On the way up I got 32.4mpg. This did involve some gentle dales driving. So on the way home I stopped and filled up and reset the trip. Before leaving the motorway the best I saw the average at was 34.6mpg with an average speed of 58mph. The total trip home saw it drop to 34.1mpg. 


Significantly less mpg than I had seen previously.

There are 3 changes to the vehicle between the two runs. 

1. some side bars from Jimnybits. But I really can't fathom these being the cause. Ultimately I know weight when accelerating will reduce mpg. But cruising the weight difference of the bumper and rails is just like having a passenger. And I simply don't subscribe that driving 2 up instead of just the driver would have such an impact on mpg. 

2. new transfer box. I noted the new transfer box makes more of a whine than the old one. Could this be causing the mpg drop somehow?

3. fitting of 215/75R15 Maxxis Wormdrive All Terrain tyres. I had these fitted fairly soon after the transfer box swap. I did say about feeling a performance drop once the tyres where fitted. But due to only doing short journeys at the time I struggle to recall how much of impact this was or the mpg drop vs when the transfer box was done. 


I know bigger more aggressive tyres can impact things like mpg. But the Wormdrive are only marginally bigger and less aggressive than a mud terrain. At best the mpg drop on a comparable run is 3.4mpg less. At worse it 6.1mpg less. Which all seems quite a lot. 

I don't have any other tyres to try to see if they are the sole cause. And I guess there is no way of proving or disproving if it is the transfer box. But a shame none the less. 

On my old Cherokee when we bought it it had bfg Mt on it which were kept for about a month before going to ht. The mpg went up by 5.

I appreciate you're more into your motoring than I am, any vehicle I've had has cruised at its best when the revs are held at half the rev range.

On the tyre side of things Gadget posted an interesting thread about it:

www.bigjimny.com/index.php/forum/7-jimny...-fuel-economy#219595

Not sure about how significant it is but larger diameter tyres have to rotate less to cover the same distance. So the car thinks its gone a lesser distance than it really has so it will calculate a lower mpg. Anyone know the relative circumference of the 2 tyres being compared? 

lt will be your new tyres. The side steps won't make any difference, neither will the new transfer box.

On the motorway in our 2020 Jimny l can just about keep the MPG above 40, if l sit at 65mph.
This is with the manual transmission. lt's not anything near our VW Up GTi where l could get over 60mpg at the same speeds.

The tyres fitted as standard will be designed for economy, so that Suzuki can claim 40+ mpg. lf you fit more aggressive tyres it's going to impact on that by a few % which is what you've found.

Bill Portland wrote: Not sure about how significant it is but larger diameter tyres have to rotate less to cover the same distance. So the car thinks its gone a lesser distance than it really has so it will calculate a lower mpg. Anyone know the relative circumference of the 2 tyres being compared? 

In theory the difference is only small. By the numbers the new tyres should be 10.5mm taller (diameter). Of course this would vary by brands and tread styles to how accurate they are to the numbers.

In reality I think the rolling radius is almost identical however. Using GPS to compare to the speedo, I see exactly the same results on the new tyres as I did on the old. So they must be less than 1mph difference. Not enough to cause mpg calcs to show any significant or noticeable change.