In answer to the question:

Can anyone quantify in real numbers the savings by installing GPS on service vehicles?

The article "Measuring the Benefits of GPS Tracking" highlights a case study where a company achieved fuel savings of 20% through optimized routing and reduced idle time with GPS tracking.

Here is the link: https://www.fieldtechnologiesonline.com/doc/measuring-the-benefits-of-gps-tracking-0001

I work for https://www.cleargps.com and some of the most popular problems solved by telematics are stolen vehicles and service tracking.