75ZT Community

Evening all,

Just wondered if anyone had found their MPG decrease upon fitting larger rims. I've fitted 11 spokes to both mine and Dad's tourer. Previously mine would run on a combined average of 43-47 mpg, this is now down to around 37 since fitting the new wheels.

Now my understanding is that the larger wheels will make the gearing longer, but the great rolling resistance will counteract that to some extent.

Previous tyres were: 215/50/17

New tyres/wheels are: 225/45/18

Would be grateful to hear what people have found.

Cheers,

Jake

The wheels are not that much difference in rolling size, you are increasing the rim size but lowering the profile of the tyre, the end result should be around the same. BUT....read on

That said with 15" alloys my speedo was around 8 to 10% fast with 18" it is around 3% fast so obviously there is a slight difference. If you are taking your readings from the mileometer on the car, that may be the discrepancy as your speedo is now more accurate

Thanks Dave,

Someone did mention on the other side that sometimes the ZCS codes can be changed to make the car think as if it is wearing 18in boots.

That said, neither of us can decide what to do in the long term. The 18's look really nice, but cause the car to sit higher and I think roll more in corners.

Also being tight as I am renowned for being 215/50/17's would be cheaper too!

Jake - I have 19s and my satnav speed reading is 5mph less than the cars speedo. I am consistently getting over 40+ mpg ( IPK (Instrument pack (IPK from the German)) reading ) and have noticed little difference since changing the wheels, however I do have a synergy 2 fitted which Duncan says can affect the IPK reading so must do a brim to brim if i can ever afford to fill the tank, but the point is i didnt notice any difference in the fuel consumption with the bigger wheels fitted

http://www.miata.net/garage/tirecalc.html

You will see from the above that you have only increased the circ by 1.6 inches on 80. so call it 2% ... :shrug:

Thanks everyone, been chatting with home tonight and I reckon we are going to go back to 17's as it's now down to about 34, just don't get it!

Appreciate everyone's input!

Best,

Jake

Sounds like you've changed the gearbox from a manual to an auto, as I get 37 from mine...

Hehe, the auto is still off the road so god knows what that would be like on fuel if being ran every day!