Stacked vs. Single-Limit UM Coverage in New Mexico

By the FigureMyTax Editorial Team · How we calculate

Stacking adds the uninsured motorist limits of several vehicles or policies together. New Mexico's statute does not mention it. Its courts have required stacking where separate premiums are charged, unless the insurer obtained a clear written rejection that explained stacking and disclosed the premiums.

What stacking means

A household may have more than one vehicle or policy with UM. Stacking adds those limits after one injury, so $100,000 and $50,000 become $150,000. Single-limit coverage pays only the one limit that applies.

What the statute says

NMSA 66-5-301 requires UM, includes underinsured coverage and lets the named insured reject it, and says nothing about adding limits across vehicles or policies.

What the courts say

Decisions we read only in part, available on the court's official opinions site, describe stacking as a judicially created doctrine. They report that stacking has been required where separate premiums are charged for UM on each vehicle, that in Montano v. Allstate (2004) the Supreme Court said an insurer can limit stacking only with a clear written rejection that explains stacking, discloses the premiums and invites questions, and that an ambiguous or invalid rejection leaves stacking in place. A July 2025 Supreme Court decision, Garcia v. Allstate, enforced a signed waiver that met those requirements even though the declarations listed per-vehicle premiums. The court did not select it for publication, and court rules restrict how unpublished decisions may be cited, so treat it as a sign of direction, not a rule.

A worked example

Suppose you are insured on two cars with $100,000 and $50,000 of UM, another driver carries $25,000 of liability and paid it, and your damages are $250,000. If your limits stack, UM could pay $125,000 (shortfall reading) to $150,000 (on-top reading). If you signed a valid rejection of stacking, the single $100,000 limit pays $75,000 to $100,000. Try your own numbers in the New Mexico UM/UIM stacking calculator.

What to check

Look for a signed stacking rejection form in your policy papers and at the premiums on each declarations page. See what the statute says and leaves to the courts and how New Mexico UM covers underinsured drivers.

This is general information, not legal advice, and the state comparisons above are illustrative, not an exhaustive survey. Confirm how New Mexico's rules apply to your specific case with a licensed New Mexico attorney.