Changing tyres affects gearing and speedo?

[gezmedinger]

I just had a thought as I put some Goodyear Eagle F1 205/50/R16 tyres on the rear of my MCS to replace the Dunlop 195/55/R16s runflats that were on previously. You can quite clearly see that the effective size of the wheel is now much smaller on the back than the front (due to the difference between the 50s and the 55s). Should I put the same tyres on the front, wouldn't this be equivalent to shortening the gearing? Won't this mean I'll spin higher RPM at 80mph on the motorway, read a higher mph than true anyway as the wheels will be spinning faster, and also probably increase fuel consumption?

Any thoughts anyone?

[British SD]

For safety reasons and handling, should have the same sizing & tires on all 4 wheels. Its not a F1 car!

[frylock]

I agree with British SD--for safety's sake keep all 4 the same. Also, you're probably correct that larger tires would increase fuel consumption. On the other hand, all of these things might turn out to be barely noticable. If you like the look of large wheels on the back, keep it. Just drive carefully for the first few weeks.

You might also find that wider tires are more subject to hydroplaning--especially in a small car like the MINI.

[MINIAC]

Gearing change will not be that significant:

[obehave]

I run 205-50 16s in autocross and they make a nice difference.

I heartily agree with the others that mixing tires like you have mentioned is a bad idea..

[PmP]

I do alot of motorway driving to and fro to work and ive driven on my 17's and 18's and there is a sizeable difference in the circumference of the two sets. So it stands to sense the larger ones will take you further for everyone revolution of the driveshaft. And if the police put a hairdryer on me and I sat at 60mph past them, I would be going slightly faster on the 18's.

I swear that at 70mph on motorway on 18s the rev counter needle sits slightly lower than with the 17's on. Weight difference of certain rims may also contribute to differences in speeds and mpg

[eltel]

This website may help you determine the effects of wheel and tyre changes.
http://www.miata.net/garage/tirecalc.html

[Electric Blue]

or this one:
http://www.powerdog.com/tiresize.cgi

Specification Sidewall Radius Diameter Circumference Revolutions Speedometer Odometer Difference
195/55-16 4.2" 12.2" 24.4" 76.8" 825/mi 60MPH 10000mi N/A
205/50-16 4.0" 12.0" 24.1" 75.6" 838/mi 61MPH 10155mi -1.5%


Oh, and as long as you have the same size tyre on each axle you are ok as far as the law goes. I have ran wider rims on the back of RWD cars for years (so do manufacturers). I can't see any safety problem with running undriven wheels with different tyre sizes to driven wheels.

In answer to your question, theoretically the difference is 1.5% as shown above, BUT, your speedo will be a touch out anyways (mine is about 2 mph out at 70mph), the only time to worry is when you are changing to a LOT bigger rolling radius. The 205s will actually reduce rolling radius so technically you should accelerate a touch faster (probably not noticeable, they would also have to be on the front)

[Suzrider]

Hang on there! Different sizes on front and rear is bound to affect the ABS system and the tyre pressure warning system because they both work on the rotational speed of the wheels. A difference in the effective circumference of the wheels is bound to affect the systems.

Probably affect other systems as well especially the braking force distribution systems and possible the traction control. Anybody?

Not sure I'd feel safe thinking the ABS was there and working when it might not be, in a crisis.