75ZT Community

Over the Christmas period I got very tired of the extremely poor handling my 75 had in the snow.

Knowing my previous 75 had been excellent in the snow on 195/65/15 Goodyear NCT5's I thought I would dig out my scruffy set of Crowns from the garage and fit a set of part worn winter tyres.

The grip, from the Goodyear Ultra Grip 7's, has been excellent but, an added bonus would appear to be that, I am getting 41MPG (over approx 800 miles) at the moment as opposed to my average just below 35MPG when I had the Unions on with Nexxen Summer Tyres (Approx 5k miles).

Had noticed this 75 not being as economical as my last, but had put it down to it having done 60k more miles than the old, and possibly injector wear and tear.

What are your thoughts? Could just changing down to narrower tyres on the 15's account for the better fuel economy? Nothing else has changed.

hi chris,l popped in to see russ last week.russ said he had put his crowns back on instead of the straights an he noticed an improvement on his mpg too

jon

Narrower tire = smaller footprint = more weight per square inch = better grip + less rolling resistance..

Narrower tires are better?..
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James.uk wrote:Narrower tire = smaller footprint = more weight per square inch = better grip + less rolling resistance..

Narrower tires are better?..
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No question about it.

I changed me 225/45/17s for 195/70/15s (winter tyres) and everything improved.
Mpg 10% better, ride a lot better too.

I'll be selling my 17 meteors in the summer - they may look nice, but in every other department a set of 15" crowns or contours etc. beats them.

It is interesting isn't it.

In these times of rapidly rising fuel prices, it would seem better to have a set of 15's on.

Now do I sell my Unions complete with tyres with only 5k on and use the money to fit a full set of new tyres to the Crowns or just put the Unions back on again for the summer??

Tires and wheel sizes make a big difference to grip and in-car comfort, so why not use the tyres that suit the car best? i.e. the 15 inch they came with..
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As many will know I normally do mega mileages,
I found that changing from 15" to my 16" forked alloys improved mpg by around 2mpg on average.
Slightly different to above, mines a automatic so gear ratios may help?

James.uk wrote:Tires and wheel sizes make a big difference to grip and in-car comfort, so why not use the tyres that suit the car best? i.e. the 15 inch they came with..
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Mine came with the Unions on James.

I had always liked the look of Unions and had always wanted to buy and fit a set to my old 75.

Luckily I had a set of scruffy Crowns in my garage, left over from my old 75, that I could fit winter tyres to. 2 of the Crowns on my old 75 were scruffy and 2 were good, I bought a set of four off eBay for next to nowt and put the best 4 on the 75 and one in the boot as a spare. My old 75 was sold with the best 4 Crowns on and the space saver back in the boot.

Have you checked whether the rolling radiuses of the tyres are different?
I read somewhere that the later wheels and tyres have a slightly larger rolling radius and so higher gearing. The comment at the time was that the speedometer accuracy had improved with the higher rolling radius, which suggested that it was the size originally intended for the 75 by MG-Rover.

Kearton

My present tyres are 215/55.16, on union alloys. Within the next month,or possibly two months, I will need 5 new tyres. Can anyone tell me if I replace my present tyres with. 205/60.16 tyres will this affect my speedomeeter in a significant way.

shy talk wrote:My present tyres are 215/55.16, on union alloys. Within the next month,or possibly two months, I will need 5 new tyres. Can anyone tell me if I replace my present tyres with. 205/60.16 tyres will this affect my speedomeeter in a significant way.

There are lots of tyre size calculators on the internet, this one here http://www.alloywheels.com/tyrecalc.asp will give you the comparisons you need.

The tyres themselves can have a significant impact on economy. Both tread pattern, and compound, as well as the width all play a part.

If you look at the economy versions of some popular models (Greenlines, blue motions, eco, ecomotive or whatever) then the tyres are very particular. They are narrower, have a lower resistance compund, and special tread patterns. However I'm told the trade off is grip and handling, though I've never found a problem with the ones I've driven

When I first started at Austin Rover (yes, that old), my first training placement was in the performance prediction department, and we were just starting to do some basic computer modelling of vehicle performance and economy. I ran a few combinations, and was surprised quite how much diffference the tyres made, over and above some of the other variables like gear ratios, final drive, powertrain losses and so on.