How do MX5s manage to be so terrible on fuel?

[The_Burg]

Light, small engine, moderate performance. Yet fuel consumption appears to be utterly dire.

Why? Is it an epically poor engine? Terrible mapping?
1999 1.6, urban allegedly 25mpg with a huge 90hp and 0 - 60 over 10 seconds.
Surely for the performance it should be high 30s with ease?

Comparably the MGF can have not far off double the power (160 on trophy), but manage 35mpg?

Very confusing. Already got an F but looking at MX5s now as it might be fun, 25mpg wouldn't, managed near that in a V70 T5.

[mattshiz]

Really old engine design I think, recycled from the 1980's 323's.

[Bonefish Blues]

We used to get low/mid 30s out of our 2.5 Sport and that was nannying it. As you say, out of all proportion to its power.

[The_Burg]

So with a modern map and decent exhaust and cone filter would it drastically improve?

[KelWedge]

I get 33 mpg give or take one or Two, mine is Mk 2.5 Navada, Not driven slow most of the time!

I cant complain as my other two cars don't get close 30mpg.

[dapearson]

Burn the non-believer!






P.S. yes my mx5 was rubbish on fuel. 28mpg despite not being thrashed.

[harleyE30]

My MK1 1.8 will do 38 mpg MAX on a motorway run.

Around 25 - 28 mpg on short journeys.

By comparison, My E30 325i would do 31 mpg max on motorways and 22 - 25 mpg on short journeys

[ExPat2B]

Low compression engine. Same reason they are so reliable.

[Herman Toothrot]

9:1 compression ratio because it was designed to be a turbo engine and old with it.

There are benefits mine ran 265bhp/230lbs on standard internals for years with regular track use.

[anonymous-user]

I often wonder this. It's even worse with the roof down and/or the lights up. I put it down to

Poor Aerodynamics
Old engine design
Very primitive EFI system
Gearing very much NOT for economy

To be honest if you're getting 25mpg in general use I'd check your O2 sensor. I was getting about that on my 1995 1.8 with it driving fine and passing it's MOT emissions test, but the diagnostic test (with the flashing LED) said the O2 sensor was faulty. Changed it and now I get 30 - 32mpg if it's not being thrashed.

[harleyE30]

Aero is not too bad on the MX5 providing the roof is up and the headlights are down! Drag coefficient is around 0.31 I believe. But with roof down and lights up this increases to 0.5 !

[The_Burg]

ExPat2B said:

Low compression engine. Same reason they are so reliable.

So a skim would improve it? Looking at the later 2.0 they still appear dire.

Is the engine related to the 1.6 in the Ford Sports Ka? Another drinker for negligible performance.
(Which was a shame as it was a good drive but st engine).

[harleyE30]

I'm unsure what the limit is to skimming, before the engine ceases to become a "non interference" engine.

The advanced timing modification may help very slightly in fuel consumption. If the timing is advanced in conjunction with CR being raised then knock may become an issue - but then again somebody I know had one on a dyno and found it extremely difficult to get the engine to knock!

[0a]

I'd also suspect that your average mx5 is far more thrashed on shorter journies than your average car

[Steffan]

0a said:

I'd also suspect that your average mx5 is far more thrashed on shorter journies than your average car

Da is absolutely correct. I regularly get much higher mileage and lower fuel costs in every car I drive than most others seem to manage. Driving style is undoubtedly a major factor in fuel consumption. I own a number of cars in total and I invariably get better consumption than others who drive them from tine to time Typically I regularly get 37+ from mt Freelander on runs and well over 30 around cities. I do think driving style does influence consumption. Having said that the MX5 has an old engine built for reliability and longevity and poor aerodynamics for a sports car. I do think a remap and rolling road set up might pay dividends. Raising the CR needs careful consideration because valve clearance and bearing stress will need assessment.