hymer1942 Posted April 5, 2009 Share Posted April 5, 2009 Hi, I have a 650 S Class Hymer on 195/70 R 15 C twin wheeled rear. Is it feasable to put slightly bigger tyres on the rear to do 2 things, reduce revs at a given speed, and to increase M.P.G. Has anyone done this??? Link to comment Share on other sites More sharing options...
dikyenfo Posted April 5, 2009 Share Posted April 5, 2009 You can use 215 width tyres as that is the recommended oversize for a specified rim but would not advise changing the profile as that could cause interferance with other parts of the chassis. Link to comment Share on other sites More sharing options...
hymer1942 Posted April 5, 2009 Author Share Posted April 5, 2009 But that would not increase the cicumference would it???????? Link to comment Share on other sites More sharing options...
George Collings Posted April 5, 2009 Share Posted April 5, 2009 According to my info that increase in tyre size would give you about 4% reduction in revs for the same speed. I doubt it will be worth the trouble and expense. It will give you an extra weight margin of about 100 kg per tyre so they do not have to work so hard. On its own that does not mean ,ore payload. At a guess 2-3% improvement in fuel economy but if you speed up as the nose is lower you blow that away. Ear plugs are more cost effective. Link to comment Share on other sites More sharing options...
hymer1942 Posted April 5, 2009 Author Share Posted April 5, 2009 Thanks for that, then how about the larger tyre all round.???? Link to comment Share on other sites More sharing options...
George Collings Posted April 5, 2009 Share Posted April 5, 2009 Differnt sizes front and rear. So what size spare do you keep? In theory there might be front to rear brake balance problems. In practice I doubt there would be much effect on braking but it would give an insurance company room to wriggle in the event of an accident. There are usually alternative axle ratios available and the cost of an axle from a breaker might be less than a set of new tyres all round. I take it your Hymer is Mercedes based. A chat to your dealer should reveal the available ratios for your vehicle and the cost of a new unit (four figures?). Link to comment Share on other sites More sharing options...
Ranger Posted April 5, 2009 Share Posted April 5, 2009 Some one went to a lot of trouble to work out the exact size of tyres for that vehicle. So what ever it says in the hand book stick to that you don't want to strain anything nor over load the brakes, nor increase the height of the vehicle. Then there is the change of tyre pressure may make the vehicle unstable. IF IT AINT BUSTED DON'T FIX IT! Link to comment Share on other sites More sharing options...
libby Posted April 6, 2009 Share Posted April 6, 2009 If following an accident the tyres are non-standard then your insurance company could walk away and leave you the bill. Link to comment Share on other sites More sharing options...
Derek Uzzell Posted April 6, 2009 Share Posted April 6, 2009 hymer1942 You need to keep in mind that what you own is no longer a Mercedes chassis-cowl but a Hymer-converted motorhome. Irrespective of whether or not there are alternative-size tyres specified by Mercedes for a 416 chassis, if Hymer has constructed your vehicle's rear wheel-arches specifically with 195/70-profile tyres in mind, increasing the tyre size may well result in wheel-arch clearance problems, particularly when the rear suspension is fully compressed. There's also the question of whether fitting wider tyres would allow an adequate 'gap' (or any gap!) between the twinned rear tyres when these are under maximum load. Additionally, if your motorhome's speedometer is currently accurate, then an increase in tyre circumference will cause it to under-read and make it illegal. The trouble with fiddling around with the overall gear ratio - either by altering tyre sizes or the axle-ratio itself as George mentions - is that (unless you know exactly what you are doing) there's no guarantee that the resultant effect will be as was originally anticipated. An increase in your Hymer's tyre circumference will impact on every gear-ratio including reverse. If your motorhome is significantly under-geared at present, upping the overall gear ratio should make motorway driving more comfortable and should help with fuel consumption during high speed cruising. However, if your Hymer's current gearing is a good match to the vehicle's power output, weight, frontal area, etc. then raising the overall gearing may mean that your motorhome will begin to struggle when faced with long motorway slopes forcing you to down-shift. (In fact, any increase in overall gearing will inevitably reduce a vehicle's ultimate hill-climbing ability.) If, after fitting larger tyres, it proved necessary to down-shift much more often than before the tyre change, then your anticipated improved mpg would go out the window. If I owned your Hymer, unless I were planning to replace all seven tyres in one go I would not contemplate altering its present tyre size. Even then I wouldn't bother unless the vehicle had proven to be horribly under-geared. I'd much prefer to have a motorhome that could trickle along delicately in traffic and during tight manoeuvrings, and climb (forwards or backwards) the side of a house, than concern myself about the odd mpg. (Incidentally, I'm a mite surprised that your 3-year-old S-650 has 195/70 tyres. Michelin's technical handbook suggests that Year-2000-onwards Sprinters using the 416 chassis normally had 225/70 R15C tyres.) Link to comment Share on other sites More sharing options...
hymer1942 Posted April 6, 2009 Author Share Posted April 6, 2009 Derek, you have hit the nail on the head, it is not the MPG which is my concern but the gearing seems really low. I know it was a different chassis, but my 640 starline did the same speed when on a motorway say 60MPH at something like 500 RPM less, with the same engine/gearbox. After what you have said I will check with as many owners I come across on thier size of tyre and take it from there. Thanks to all and will post the outcome. Link to comment Share on other sites More sharing options...
Derek Uzzell Posted April 6, 2009 Share Posted April 6, 2009 I think your Starline would have been on a 3500kg/3800kg MAM chassis with 'single wheel' rear axle wiith 225/70 R15C tyres and (very probably) a higher final-drive ratio - so it wouldn't be at all surprising if it were more long-legged on motorways. What you've got now is closer to a lorry than a delivery-van and its gearing is likely to reflect this. In MMM April 2006 there was a report on a B-Starline 660 on 416 chassis like yours and this was said to have Continental Vanco 195/70 R15C tyres. Link to comment Share on other sites More sharing options...
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