Graham Thomson Posted November 19, 2007 Share Posted November 19, 2007 My 110 has 235/70s on 16 inch wheels. If I fit the correct tyres (235/85s - but please stop me if I've got that wrong!!), will my fuel economy be better? :rolleyes: Quote Link to comment Share on other sites More sharing options...
Milemarker Type S Posted November 19, 2007 Share Posted November 19, 2007 Er... not sure it is quite as simple as a yes/no answer... The 235/70 tyres work out as being about 29" in diameter (excuse the rounding- and using imperial for that matter!) which gives a circumference of about 91 inches- ie every time the wheel turns once you move 91 inches forward (or indeed backwards ) The 235/85 tyres work out as being about 32 inches in diameter giving 100 inch circumference so you move forward 9 inches further every time the wheels go around... but it takes more engine power to do so... So the conclusion is... No idea! Sorry if this is not overly helpful but I think the way in which you drove the vehicle fitted with the respective tyres and the type of driving you do (ie. round town or motorways) would make more of a difference to economy. Shrek Quote Link to comment Share on other sites More sharing options...
Graham Thomson Posted November 19, 2007 Author Share Posted November 19, 2007 Wow, thanks for that. Imperial is fine by me........ So an extra 10 percent distance for each revolution.....compared with how much extra power to turn the wheels.... Hmm... Quote Link to comment Share on other sites More sharing options...
Milemarker Type S Posted November 19, 2007 Share Posted November 19, 2007 So an extra 10 percent distance for each revolution.....compared with how much extra power to turn the wheels.... Err...??? Ask me one on real ale... Mmm Old Peculiar Shrek Quote Link to comment Share on other sites More sharing options...
Ex Member Posted November 20, 2007 Share Posted November 20, 2007 In general, larger tyres will increase fuel consumption, although you may not be able to tell. What you need to be careful with is that the speedometer will read differently. If you use the odometer to calculate mileage, your numbers are all wrong. A 110 with those little tyres must look goofy. Quote Link to comment Share on other sites More sharing options...
Guest diesel_jim Posted November 20, 2007 Share Posted November 20, 2007 A 110 with those little tyres must look goofy. There was a 110 station wagin in town a few years back, with 205x16's on..... funnier still, was the (lack of ) ground clearance under the rear salisbury! Quote Link to comment Share on other sites More sharing options...
Lars L Posted November 20, 2007 Share Posted November 20, 2007 205-16 and 235/70-16 both have more or less the same diameter. And are too small for a 110, both the size and most likely also the load carrying capacity. 235/85 is the standard metric size. Quote Link to comment Share on other sites More sharing options...
paintman Posted November 20, 2007 Share Posted November 20, 2007 Vast amounts of info at http://carbibles.com/tyre_bible.html page 2 also has a calculator for the effect on indicated speed of changing tyre size. Quote Link to comment Share on other sites More sharing options...
BogMonster Posted November 20, 2007 Share Posted November 20, 2007 And are too small for a 110, both the size and most likely also the load carrying capacity. Probably not on the load capacity. IIRC the rear axle weight on a 110 is something like 1800kg so any tyre rated at 900kg or more will be fine, which would be most of them I think. Quote Link to comment Share on other sites More sharing options...
jules Posted November 20, 2007 Share Posted November 20, 2007 the biger the tyre is the more the engine has to turn = more fuel I dou't you would notice with that small amount and I would say your tyre is a little small anyway when you fit massive tyres most people lower the transfure gear or diff ratio to compansate but thats when the tyres size is much bigger than standard. I run 35" tyres and have a 4.75 diff ratio not the standard 3.5 (having said that its stilla bit too low now) but for a V8 its very good on fuel. Quote Link to comment Share on other sites More sharing options...
Top90 Posted November 20, 2007 Share Posted November 20, 2007 I went from 31" to 33" tyres on a 110 a few years back. (300TDi) Reduced the acceleration a bit but did make it more economical as I did a lot of runs on the motorway and the engine was not working so hard. It was something like a 7% percent change, but it did correct the speedo as they always overread from the factory to cover you. (there is legislation that they can't under-read but can over-read) Overall I probably only got 1 or 2 mpg more, about 20 to 30 on a tank. I don't think bigger tyres = more fuel unless you overgear it. Richard Quote Link to comment Share on other sites More sharing options...
Ex Member Posted November 20, 2007 Share Posted November 20, 2007 the biger the tyre is the more the engine has to turn = more fuel Ummmm, actually a larger tyre will make the engine run at a lower rpm for the same vehicle speed..... The main reason for increased fuel consumption is the higher stance and thus increased wind drag. A smaller contributor is the larger, heavier tyre increasing rolling resistance and power requirements when accelerating. Quote Link to comment Share on other sites More sharing options...
jules Posted November 20, 2007 Share Posted November 20, 2007 Ummmm, actually a larger tyre will make the engine run at a lower rpm for the same vehicle speed.....The main reason for increased fuel consumption is the higher stance and thus increased wind drag. A smaller contributor is the larger, heavier tyre increasing rolling resistance and power requirements when accelerating. not much to do with the tyre weight its to rotational moments (leverage) on the gearing and the same gearing is all to pot so the engin is labouring more... EG. same as why a Ford escort 1.4 will only do 26mpg but a 1.6 will do 30 mpg its because the engine is not labouring so it uses less fuel. (I use this example as when I was between LR's I have a 1.6 and my mum had a same year 1.4) We couldn't undestand at first why the 1.6 was alot better on fuel The more you make somthing labour regardless of reves the more fuel it will use. It will not show till you get to a proper tyre size increase Also in my D2 I was driving 200 miles aday for over a year same route every day I fitted 265/75 r16 and my fuel econemy dropped massivly as I was fitting to get up hills it was fine once I go it up to speed but it was struggling to get up to 70-80 mph to cruise. Afetr two week I took them off and sold them. There was nothing wrong with them and if I could be bothered to drop the transfur gears it would have been fine. Quote Link to comment Share on other sites More sharing options...
Ex Member Posted November 20, 2007 Share Posted November 20, 2007 Sorry, but your understanding of physics is a little lacking. The power requirements have nothing to do with gearing, but wind and rolling resistance only. With larger tyres giving higher gearing, there is no more power required. It makes the engine run at a lower speed. Whether or not that affect mileage deends on the engine. Engines have an efficiency curve and provide maximum efficiency at certain speeds. For most diesels, higher gearing on the motorway reduces fuel consumption as the higher gearing put the engine closer to its maximum efficiency point. This is why people fitting overdrives find a reduction in fuel consumption. For most engine, maximum efficiency is found near the peak torque point. The problem with the OP's question is complicated. The gearing increases (which is good), but the wind resistance increases at the same time. In addition, the power required to accelerate the tires increases (slightly). Where most people get confused is that they use the odometer to check mileage. If you change tyres, the odometer reads incorrectly and it appears that mileage changed quite a bit. Larger tyres make the odometer slow down, so it seems that the fuel consumption increased. If you adjust the mileage figures for the odometer error, you'll fnd little difference. Quote Link to comment Share on other sites More sharing options...
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