Dear Aunty. I have a Vogue and a Gazelle which have both been fitted with radial ply tyres. The Vogue’s are 165R13 83T, replacing the original 650x13s, which had recommended pressures of 25 psi front and rear, and the Gazelle’s 165/80 R15 87T, originally 560x15s at 24 psi all round.
Can you please tell me what the recommended pressures are for the radials?
Yet Another Article from the July - August 2012 Mascot
Dear Mr Wellshod. I don’t know of any set formula to work this out, but the general rule in the trade is to increase the manufacturer’s recommended pressures for the cross-plies by 5 psi when replacing them with radials. The pressures should be set when the tyres are cold. Thereafter you can try increasing them a pound or two a time to see if there is any improvement in ride and handling, then stay with the setting that gives the best results.
With regard to the Vogue, the radial size is a very good match for the old crossplies, as 6.5” = 165.1mm, so 30 psi should be fine.
But for the Gazelle, I wonder why, when 145R15s would have been an almost perfect match for the size, 165/80 was chosen to replace the 560x15s, as 5.6” = 142 mm, yet 80% of 165mm is only 132 mm, or 5.2”. So your tyres are 22 mm wider and give a 10 mm lower ride height, which should both be great for high-speed cornering. However, the smaller od will reduce the overall gear ratios - great for improved acceleration, but braking efficiency will be reduced and the speedo calibration and mileage recorder adversely affected. Also, the cross-sectional area of the radials is about 7% less than the original tyres, so the 5 psi rule may not be sufficient, and I would suggest a minimum increase of at least 6 psi.