Tire size change to give better final drive ratio?

potzman · Post by **potzman** » Sun Jun 19, 2011 8:04 am

Hi Guys,

I currently have some 15 X 6.5 aftermarket alloys on my 1.6 NA manual and the rear tyres are due.

Now I want to try get a bit more fuel economy out of the beast on open road trips and with the current tyre size of 205/50/15 at 100km/h im doing around 3500rpm which I think is a tad high. I have tuned the timing up to 14 degrees so she has alot more torques and think if I lower the rpm at 100kph it will make a big difference to open road fuel economy ( currently getting the same economy open road as around town, 450km per 35L tank). My partners Astina auto with the same engine in it gets over 600km but has an rpm of 2700 at 100kph.

The idea I have is to increase my new tyre size to 205/55/15 to give me a larger overall diameter. Trouble is this seems to be an odd size so tyres are more expensive

What are your thoughts??

WideOpenThrottle · Post by **WideOpenThrottle** » Sun Jun 19, 2011 9:24 am

For a while we had 195/55/15 tyres on our 1800 NB and while we got a smoother ride i would have to state that performance was sacrificed in the sense that nippiness was down along with the fact that we needed to change gear earlier going up hills....we never noticed any fuel savings and i put this down to the fact that it takes slightly more revs to get the car going & to maintain the bigger diameter rolling circumference & inertia.
My 1600 NA with 205/40/17 tyres was a sluggish pig until i turboed it but i have to admit the rims are'nt the lightest!

We so much prefer the 195/50/15 so went back to them.
If you have a standard motor then maybe just try the two rear tyres.

Anthug · Post by **Anthug** » Sun Jun 19, 2011 9:26 am

I find using this http://www.miata.net/garage/tirecalc.html helpful when calculating tire sizes.

m4vr1k · Post by **m4vr1k** » Sun Jun 19, 2011 2:42 pm

I went from 185/60/14 to 195/50/15 and have noticed a big drop in revs at motorway speeds, went from nearly 4k at 110kph to just over 3500rpm. I have noticed a lack of "nippiness" through the revs too, but the added traction makes up for this IMO

Donovan · Post by **Donovan** » Mon Jun 20, 2011 1:24 pm

I went from 185/60/14 doing 3,500RPM at a GPS confirmed speed of 100kph (106KPH on the speedo) to 205/40/17 doing 3,200RPM at a GPS confirmed speed of 100KPH (speedo was dead on 100kph as well).

Made no difference to economy (maybe a bit worse if I remember rightly), but then again my calculations were based on the trip counter, which of course was 6% different than before.

Pity the 6 speed doesn't have a better final ratio, otherwise you could have always just swapped that around. Wonder how hard it would be to put the Diff from a NB in there

http://www.miata.net/garage/KnowYourCar/S8_Gears.html

ham · Post by **ham** » Tue Jun 21, 2011 8:41 am

Is the MX5 not a sports car and so tuned and geared accordingly? If we want fuel economy maybe another vehicle would be better. I have heard some can get down to 4ltr per 100. I seem to remember getting more than 100k from a litre on my honda 50 stepthru

sprsta · Post by **sprsta** » Tue Jun 21, 2011 2:54 pm

ham wrote:Is the MX5 not a sports car

i get about 200k per tank

i just put a set of 195 45 15s on
they are even smaller diameter than the 175 70 13s that it was rolling on

anyway this should help

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

Donovan · Post by **Donovan** » Tue Jun 21, 2011 2:58 pm

I was getting about 400km on 30 litres, ranged from the best of 6.9l/100km to the worst of 8l/100 and that one was with some track driving (not much as the little thing was cooking itself after 3 laps). Driving was mostly rush hour, never really took it for a long trip before it went under the knife, so don't know if it would have been better or worse.

aidwano · Post by **aidwano** » Tue Jun 21, 2011 6:59 pm

I am lucky to get 400km from 40L. But then I do make the most of driving haha

m4vr1k · Post by **m4vr1k** » Tue Jun 21, 2011 8:48 pm

I managed a...ahem "spirited" drive from Silverdale to Wellington. Filled at Silverdale, put in half a tank at Taumaranui, then about 3/4 of a tank in Wellington CBD. That was on stock wheels and tyres, average speed was 100kph from a TomTom GPS. Although at a track day at Pukekohe, I get 150km of racing to a full tank of 98.

brownie · Post by **brownie** » Tue Jun 21, 2011 11:45 pm

3500rpm at 100km is ok ,Thats about where the power band starts,bearing in mind power will peak out at about 6200rpm,On a 1600cc NA,standard tuneish
You are on oversize tires already.
With Standard 185/60/14 tires ,195/50/15 is a little bit lower and 205/50/15 is a little bit higher..
At least it's not a Rotary or a big V8 ,Then you would have something to complain about with fuel prices the way they are

.
Of course you are running 95 unleaded.
QAre driving with top up or down ?,Aerodynamics will be a factor in the Astina getting good economy,I would get about 400km out of my 1800 Astina,But then i was not afraid of the accelerator pedal either

potzman · Post by **potzman** » Thu Jun 23, 2011 11:52 am

Ok so turns out my very worn and dodgy no name branded 205/50/15's on the rear were a smaller overall diameter than they should have been anyway as i found out when i swapped front to rear ( near new tyres 205/50/15 were on front)
So keeping those on the rear, gives me a nice 3250rpm at 100kph, and have gone for some new MY01 195/50/15's for the front at a very good price. This narrower width should help with fuel economy along with the lower rev's, they look the biz too!

Had some alignment issues tho, see new topic started.

Tire size change to give better final drive ratio?

Tire size change to give better final drive ratio?

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