Hey guys,

I pulled the trigger and bought a 2018 E53 AMG Wagon 6hrs drive from home, WOW what an amazing car...the Turbo 6 is pretty incredible compared to my old 2014 E350 V6.

I noticed when on the highway using cruise and Android Auto that at 120kmh (65mph) the the Speedo is out quite a bit due the tyres diameter being lower than ideal so it's reving harder than I'd like compared to if it was running taller tyres then it would be equal to the GPS speed or better.

It's running the factory 20" staggered wheels and has 245/35 front and 275/30 on the rear Yokahama Advan Sport brand.

I wanted to know can I go up is Series to a 40 or 45 front and 35 or 40 rear series to increase rolling diameter.

 

usually, high performance summer tires will have better/lower rolling resistance than high performance all season. So, if you are to look for this, then you may want to consider the Michelin PS 4S or the new S5 tires.

 

Those tires appear to be the correct size. Obviously as tires wear, their diameter decreases, and same goes for if you are not running the correct tire pressure as indicated on the fuel door sticker for the current load of the car. In what direction is the speedometer off? Speedometers in cars deliberately show a higher speed than actual, as by law they are never allowed to show less than the actual speed. The indicated speed can be up to 15% higher depending on the region. This is so that even when the tires are completely worn, the speedometer still meets the law. Also, keep in mind that even GPS devices may not show exact speed in order to give you a buffer and minimize the chances of getting a ticket. To know what the actual speed is, you'd first have to measure it with a calibrated device that you know shows true GPS speed, but even GPS has a tolerance. Consumer-grade GPS is accurate to within 1-5 meters, so that affects the accuracy of the speed it displays. Worst case your position constantly fluctuates between -5 to +5 meters of your actual position and those 10 meters will result in inaccurate speed calculations. Millitary-grade devices can be as accurate as 1 meter. Best way to accurately determine the speed of a car would be a precisely measured distance with a highly accurate timing device, or a radar or lidar gun. Otherwise the speedometer and as said even GPS devices are only good enough approximations of your actual speed.

However, if you are curious about how increasing the sidewall will affect the relative accuracy of the speedometer, here's a good online calculator that lets you compare different tire sizes.

https://tiresize.com/calculator/