Head wrecking speedo issue...

[adrian]

Hi all,

OK....kinda of a question to make you think (well, it made me think anyway).

I got the jeep in at the weekend and I have 20 inch alloys on it. When I put the old alloys/tyres (15 inch with very deep tyres) up against the new ones the older ones are higher meaning that they would have a greater turning circle.

This is effecting my speedo... when it tells me I am doing 70 MPH my GPS tracker is telling me I am only doing 62. This GPS system is accurate.

Does this mean that I am actually clocking up more miles than I am doing?
Does this mean that I appear to be getting better MPH than I am?
Does this mean my engine is actually having to rev higher to turn the "smaller" wheels faster than it would have to turn the "bigger" ones to get the same speed?
Can I get the speedo re-calibrated to be accurate?

I wonder next time could I get the tyres a bit deeper to make up the difference?

Thanks a million as always

Adrian

[Kace]

Does this mean that I am actually clocking up more miles than I am doing?
>>> Yep - you're running smaller wheels than the car is set up for, therefore the car thinks you're doing more miles than you're actually covering

Does this mean that I appear to be getting better MPH than I am?
>>> If you mean MPG then no you're getting worse MPG than you should do because you're having to burn more fuel to turn the wheels more times to travel the same distance.

Does this mean my engine is actually having to rev higher to turn the "smaller" wheels faster than it would have to turn the "bigger" ones to get the same speed?
>>> Indirectly yes, because if you 'really' want to travel at 70mph, you will need to drive your car at nearly 80 on your speedo to get you to that speed - burning more fuel than you would if your wheels were the right size.

Can I get the speedo re-calibrated to be accurate?
>>> Probably requires some specialist electronics - may not be the easiest thing to sort out.

You're correct - you just need to get deeper tyres for your 20" alloys - that would create the correct rolling radius and solve all your problems.