Whichever route you take to reach Cornwall a look at the road map will show that there are three roads which are important for travelling in Cornwall.
There are four places in Devon which might be important to your travel plan - Barnstaple, Exeter, Okehampton and Plymouth.
If you are coming from or plan to arrive in Cornwall via North Devon then you will need to head towards Barnstaple and then Bideford and thence down the A39 to Bude.
Barnstaple to Bideford is 7 miles, Bideford to Bude is 27 miles.
For the majority of travellers Exeter is the linchpin in route planning. All roads lead to Exeter, as it were, and here there are two options in your decision making (other than North Devon).
It is probably wisest to travel via Plymouth - A38.
: Exeter to Plymouth is 45 miles, Saltash (Cornwall) is 3 miles from Plymouth.
An alternative route is to Callington (for Liskeard and area), from Okehampton via Tavistock A386 which takes you across Dartmoor.
: Okehampton to Callington is 23 miles.
Travel via Okehampton A30 which will take you to Launceston.
: Exeter to Okehampton (Sourton) is 26 miles, Okehampton to Launceston is 15 miles.
The 'length' of Cornwall: from the Devon border to Land's End is approximately 75 miles. Cornwall varies in width dramatically: from coast to the coast, Bude to Torpoint is approximately 40 miles, Newquay to St. Austell is 15 miles, Godrevy Point to Lizard Point is 21 miles and Hayle to Marazion is 4 miles. From the high points of Bodmin Moors, at the widest point of Cornwall, and on a clear day, you can see the coast on both sides of the county.
Google maps is one of the best resources available for travellers. You can zoom in to the exact location and learn distances, alternative routes and view by satellite.
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It is valid to point out that once away from the main roads things change dramatically and interestingly. Narrow country lanes become tunnels of greenery and colour in spring and summer. Behind this foliage often lurks a centuries old stone hedge, beware contact with a cornish 'hedge'. For every linear metre of standard highway 'hedge' there is approximately two tonnes of stone and soil, all hand built into place and designed to stay there for a very long time. Work out the width of your car, its tonnage and then calculate the equivalent tonnage in the cornish 'hedge'. Not an argument to involve your vehicle in. Coastal roads will often awe you with the magnificence of the scenery you come across and the back lanes can have a similar affect on you.