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By Train
Since there are no direct trains to Manali from Gujarat as the Major railway station near to Manali on Indian Railway Broad Gauge network is Una in Himachal Pradesh. A Narrow gauge line also run between Pathankot to Joginder Nagar. But, the best way to reach Manali by train is to leave the train at Chandigarh Railway Station and catch a volvo bus from there.
Alternate option is to catch an overnight bus from Delhi to Manali. So, you can take a train from Gujarat either to Chandigarh or to New Delhi and then take a volvo bus from there to Manali.
Distance between Chandigarh and Manali = 300 Kms
Distance between Delhi and Manali = 550 Kms
It will be around 30 hrs of journey from Ahemdabad to Manali via Chandigarh using Train and then bus. This road trip may cost you around INR 1,100 per person. This will be good option if you get your booking and confirmation prior to your travel and you have friends to accompany you. You don't need to worry about the driving, directions and distance.
By Air
Also, if you want to travel by air then you may land at Bhuntar airport which is located at a distance of 10 km from Manali, but direct flight from Gujarat to Bhuntar may not be available, so you again need to take a flight to either Chandigarh or to New Delhi and from there you need to travel by bus and again this travel may cost you 4-5 times the travel by Train + Bus.
By Road (Car)
Now, if you have friends to accompany you and you guys like driving and stopping at places in between your way from Gujarat to Manali then you can go by your car (but don't rent it as it will be more costly). Preference should be for diesel car, as the road distance is around 1500 Kms via NH-8, so if you have diesel car providing mileage of 20 Kms/liter then your onside trip will cost you
1500*48/20 = INR 3,600 i.e. per person = 3600/4 = 900 (for 4 friends)
(considering average price of diesel to be Rs. 48/liter, although different states have different prices), but you can divide this amount among your friends and time for the journey will be around 22 hrs (excluding your in between stops and taking 70Kms/hour average speed). If you have a petrol car then you have to pay more. But, for diesel car with specified mileage and 4-5 friends this tour will be cheaper and more fun giving (considering you have to pass through mountains and sharp turns on your way).