Tyre sizes

[robo1968]

Am I right in thinking it's possible to have different tyres on the same wheel (within a certain bracket)? For example I have 7 x 15 wheels with 225 50 15 on the rear and 205 50 15 on the front. It's quite heavy steering and I only intend it for road use so is possible to change the fronts to say 195 50 15's on the same wheels? Even if this is possible would that mean steering geometry gets altered in any way?

[peterg]

195/50 15 tyres will fit rims from 5.5in to 7in  

rim width calculator here

[blankczechbook]

will that not lower the front even more? - 195/55r15 might be closer to keep ride heights from altering much?

[robo1968]

Thanks for that. Useful calculator!

Also I've just worked out that the car originally had 195 50 15 tyres (as that's what size the spare is). The speedometer is not accurate - I was thinking maybe that's because the tyres had been enlarged. It's 8-10 mph more than the real speed (according to my sat nav). Trouble is though, when I use the calculator from the above link, by going from a larger back to smaller tyre I would make my speedometer problem even worse than it is I think?

[peterg]

Depending on what speedometer you have and wher it takes the reading from (gearbox/wheel/prop. it may need re-calibrating... most are inaccurate to a greater or lesser degree so go by satnav speed    

[stephenh]

If you fit a smaller diameter tyre then for a given gear and engine speed your true road speed will be lower, or for a given true road speed you'll need to rev the engine higher.

[pistonbroke]

If you fit a smaller diameter tyre then for a given gear and engine speed your true road speed will be lower, or for a given true road speed you'll need to rev the engine higher.

and accelleration will be quicker