I have been using JDM Alto 2015 for one and half year now and have driven it more than 50,000 km. I wanted to share my experience:
engine oil: total quartz 9000 0W20 - always changed it after 7k km. because in my opinion combined with experts reviews, using fully synthetic oil and dumping it below 5k isn't worth it.
spark plugs: alto came with altis grande spark plugs which are similar to mentioned in JDM manual. These spark plugs are considered obsolete models (NGK ILKR7J8, DENSO ZXU22HPR8) and their alternates are available which are: ILKR7D8, LKR7AIX; LKR7BIXP, LKR7AIXP; IXUH22I, VXUH22I - referenced to NGK and DENSO official websites; i have opted for NGK LKR7BIXP recently.
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tyre pressure matters a lot. i have been using 29-30 psi but when went through manual i found that recommended pressure is even higher and i have been using 32 psi pressure for a long time now.
With 0W20 oil and 32 psi tyre pressure with tyre size of 155-65-R13, i have personally experienced as following:
a) in relatively moderate season in which AC wasn't required i traveled around 650 km with average 80 km/h speed (zindagi say bezaar ho k koi bohat hi thakay huye banday ki tarah pedal pay paon rakha hua tha) and i ended up with 32.4 km/h. or hum pakistani agay foran puchtay hen k petrol wali hai ya meter wali
so, it was petrol wali in a single fill up of 27 litre tank. 32.4 is exaggerated figure but the reason to mention it is to realize the people that with recommended oils, pressure, spark plugs, routine maintenance and most importantly banday k putron wala soft foot
in Alto JDM can potentially help in achieving not exactly 37 as claimed by suzuki japan but 30+ even in pakistan.
b) recent trip from islamabad to toba tek singh which i have done with bigger tyre size (MAXXIS 165-65-13) as compared to previous one (Bridgestone 155-65-13) with full AC, 3 adults, full saaz o saman, kept speed around 120 and above throughout the journey except salt range and i got 20 km/h average at the end of the day.
Based on my personal experience, i'll always recommend to go for JDM if one is looking for Alto.