Sam’s right that you’re not likely to save money by making a northward detour. His statement that
… only really applies in the western half of the country, as far as I know. In Ontario, which will make up a huge chunk of your west-east trip (depending on where you enter the country), there are no highways with limits over 100 km/h. In the northwest, (like, along the shore of Lakes Superior and Huron, I’m confident (not 100% sure, but close to it) that the limits on the TransCanada are 90 km/h.
The TransCanada in Ontario is called “Hwy 17”, and runs from the Manitoba border to Quebec via Kenora, Thunder Bay, Sault Ste Marie, Sudbury, and Ottawa. In the last stretch near Ottawa it becomes a four-lane divided expressway, and is thus renamed “Hwy 417”.
That’s actually useful to know for travel in Ontario. 400-series roads are the fast ones. They’re divided expressways of at least two lanes per direction. Speed limits are 100 km/h, but it’s usual to find traffic flowing at 120-130 km/h without hassle from the police. On other roads - Hwy 17, for example, you usually find one lane per direction divided only by a yellow line, with variably-frequent passing lanes and stretches of 4-lane-divided. On the two-lane sections of these roads, 90 km/h is the usual limit, sometimes reduced where the highway passes through a settlement. Traffic flow is more commonly at 100-110 km/h in good weather.
Know that ‘driving too fast for conditions’ is an offence in Ontario (not a criminal one), so exercise judgement during inclement weather. 400-series highways are mostly found in the southwest portion. If you want to use them and are coming from the northwest part of the province, you need to head south from Sudbury, then continue east from Toronto or so. There exists one toll highway in Ontario - the 407 Express Toll Route sort of encircles Toronto. Tolls are charged electronically, so there’s no stopping.
Depending on where you enter the province, here’s my recommended routes:
Entering from Manitoba
Hwy 17 from Manitoba border to Quebec (becomes 417 west of Ottawa). This simple route has more services along it than the remote and more northerly Hwy 11 that arcs north of it until meeting it east of Sudbury (at North Bay). Scenic, it takes you through the hilly country of Northwestern Ontario, the shoes of Lake Superior, the forests of Northeastern Ontario, the foothills of the Laurentians (just the view of lake Nipissing at North Bay is probably worth it), the valley of the Ottawa River, and through the heart of the national capital, within sight of the Parliament buildings. In my opinion, it’s much prettier than the 401 route in Southern Ontario, but of course, I’m from Sudbury, and likes me my forests. Waypoints include Kenora, Thunder Bay, Wawa, Sault Ste Marie, Sudbury, Ottawa.
Alternately, if you want to see Toronto, turn south when you set to Sudbury, and follow Hwy 69 (becomes Hwy 400). Take Canada’s busiest highway (the 401) east from Toronto. (There are plenty of alternatives in the Toronto area.) The 401 does not pass through Ottawa.
Entering from Minnesota
Take Hwy 11 (which follows the border, more or less) to Thunder Bay, then Hwy 17 as described above.
Entering from Michigan (Upper Peninsula)
Cross at Sault Ste Marie, and take Hwy 17 as described above.
Entering from Michigan (Lower Peninsula)
Cross at Detroit/Windsor, and take the 401 right through to Quebec. If you’re itching to see Ottawa, jog north on the 416 when you get to it.
If for some reason you’re in the norhtern Lower Peninsula of Michigan, it’s easier to cross at Port Huron/Sarnia, and take the 402 to the 401.
On preview, I see cantara has added some info. I’ll note that in the decade since s/he did that trip, services have probably improved on Hwy 17, and access to services on the 17 is better than on the northern arc of the 11, which is part of why I recommended it.
Also, by contacting the Ontario Ministry of Tourism, you can get a free paper copy of the Official Raod Map of Ontario, linked above in PDF format.
(Footnote: This is my 1000th post!)