Large tyre considerations fuel consumption not as bad as you thought

[Pradosaur Member Find all posts View Profile Close Pradosaur]

Just something to consider when discussing larger tyres and the impact on fuel economy.

Your fuel consumption calculations you need to consider that the circumference of the large tyre is greater that the factory tyre (say 33 is 104 inch versus 31 is 97 inch) so the odometer will be out .ie a a larger diameter tyre will be turning the odometer over slower than a small diameter. Meaning that the larger tyre will have to travel say an extra 7 inches before it will complete a full 360° rotation.

Based on the above numbers it is about a 7% variation, so whilst the torque definitely takes a hit and will influence gear selection particularly for city/suburban driving, the economy will be affected but probably not as bad as calculations using odometer suggests, (particularly for highway driving.)

In this case you’d need to multiply the distance travelled by 1.07 first to work out a more accurate economy figure. Hope my maths are correct, I’ve only had one coffee..ps this assumes all other gearing remains constant.

nb the larger diameter will also impact on the speedo.. so watch your GPS.

[Leethal Advanced Member Find all posts View Profile Close Leethal]

The theory is correct however what is difficult to calculate is the extra throttle/fuel required to turn the larger tyres at a set rpm, I work in a field where we can easily change the engine load by fitting different wheels (okay they are propellers but has the same effect) and measure the increase/decrease in engine load and therefore fuel rate at the same rpm and correlate that with speed.
I think larger tyres with an otherwise light load will probably have minimal effect on consumption but the same setup with a heavy load will suffer a greater effect on fuel consumption. So consider all the other factors as well and decide on what is more important to you.
For me, in some areas I have travelled changing to 75 profile (replacing 70 profile) was better because it was a more common size along with a greater range of tyres to choose from, so peace of mind was also a factor. I reckon tyre size is up there with the diesel vs petrol, BFG vs Cooper love'em or hate'em debates.

Lee

[Atherton Senior Member Find all posts View Profile Close Atherton]

I have torque pro hooked up in an android headunit in my car with the correct PID's for fuel level (in litres) in each tank. I calculated that on my run from home to the University of the Sunshine Coast I average around 10.5L/ 100 kms. I do this drive about 4 times a week and that reading has stayed fairly consistant so I'm pretty confident of it's accuracy. The drive itself is about half 110 km/h highway driving, however I've got about 20kms worth of tight gravel road between my place and my nearest main road as well. I was pretty impressed when I calculated this, I'm running larger tyres (265/70's), a roofrack and awning as well as a 2inch lift. Car says I'm doing about 11.5-12L/ 100 according to the trip computer but I know this to be incorrect as the speedo has been corrected with the larger tyres. Torque and power have both taken a noticeable hit, but there's no such thing as a free lunch.

[JohnLynn Senior Member Find all posts View Profile Close JohnLynn]

Do you guys have an opinion of whether fuel economy would be affected in the same way on a manual transmission? I ask because I am guessing that you are talking of automatics which I know have a much higher 6th gear than a manual. I have often wondered why manual 6th is so low and if a higher gearing would actually improve economy?

[Pradosaur Member Find all posts View Profile Close Pradosaur]

I feel the principal would be the same although it would come down the the thing mentioned above and how often and how long your staying in topgeA

[Razorback81 Advanced Member Find all posts View Profile Close Razorback81]

Do you guys have an opinion of whether fuel economy would be affected in the same way on a manual transmission? I ask because I am guessing that you are talking of automatics which I know have a much higher 6th gear than a manual. I have often wondered why manual 6th is so low and if a higher gearing would actually improve economy?

i to have the 6sp D4D, i have now started driving it more up the rev range and am getting better response doing 100km/hr in 5th at around 2750rpm verses doing it in 6th as i am always loaded for the trips (3+T)
i use 6th gear when doing 110km/hr (if im in a hurry & loaded) but takes a hit in the economy

if no roof rack 110 in 6th is great

aero is your big killer for fuel & then tyre size plus you need to know the sweet spot in your engine which is up around that 2700 to 2800rpm , they are not like to old school diesel and lug from idle (although they do) they need the cam to open them up and let the air in

just my observation after our 12,000km Cape Trip

when i got my car the previous owner ran 70 series mickey Thomson couldn't get more than 1350km out of the full tank, i switched back to 65 series BFG KO2 and get 1700 from the full tank without anything on the roof
watch your speed and rev range and it will do more
same thing when loaded i can get 1500km out of the tank or as low as 1300 pushing harder and head wind

also fuel type has an effect vortex diesel & Bp ultimate diesel seem to be the pick , i wasn't sure about the Puma up in QLD so stuck with the caltex, but noticed the difference when we has to use reg diesel (still caltex or BP) only used shell once as that was the only fuel available in Weipa