Neil and I can attest to the following.  We have noticed an increase of 50 miles per tank of diesel and our tyre pressure has maintained from installation. 

Pro-Edge Limited is an authorized UK distributor of Puncture Safe Gel tested 66,000 miles round the world, by Tim Dennis.  Increases tyre life, helps maintain correct pressures maintaining good road handling, improves MPG, helps prevent tyre walls overheating, helps prevent blow-outs.  Reduces down-time, reduces running costs, protects drivers and passengers, improves road safety. Seals punctures as-they-happen, repair is permanent Ideal for vehicles without a spare. Will not damage or corrode tyres or rims. Punctures don’t wait for the ideal location, road conditions, or weather. A cost saving solution for fleet vehicle operators Puncture Safe is approved by the Royal Mail and is installed into their fleet of 32,000 vehicles. Also used by Balfour Beattie and Tarmac.